Materials

CONTENTS

CONTENTS

Chapter 1 Classification of Materials

1.0 Introduction 1-1 1.1 Metallic Materials 1-1 1.2 Polymeric (Plastic) Materials 1-2 1.3 Ceramic Materials 1-2 1.4 Composite Materials 1-2

1.5 Electronic Materials 1-3 Chapter 2 Mechanics of Materials

2.0 Normal Stress and Strain 2-4 2.1 Stress-Strain Diagrams 2-8 2.2 Elasticity and Plasticity 2-10 2.3 Creep 2-12 2.4 Linear Elasticity and Hooke's Law 2-13 2.5 Poisson's Ratio 2-14 2.6 Shear Stress And Strain 2-15 2.7 Allowable Stresses and Allowable Loads 2-18

Chapter 3 Atoms, Molecules and Crystals 3.0 Introduction 3-21 3.1 Chemical Bonding of Atoms 3-21 3.2 Electrovalent Bond 3-23 3.3 Metallic Bond 3-24 3.4 Covalent Bond 3-25 3.5 Polymorphism 3-25

Chapter 4 Crystal Structure of Metals

4.0 Introduction 4-27 4.1 Body Centred Cubic 4-27 4.2 Face Centred Cubic 4-27 4.3 Close Packed Hexagonal 4-28 4.4 Dendritic Solidification 4-28 4.5 Effect of Impurities 4-29 4.6 Influence Of Cooling Rates on Crystal Size 4-30

Chapter 5 Casting Processes

5.0 Introduction 5-31

CONTENTS

5.1 Ingot Casting 5-31 5.2 Continuous Casting 5-32 5.3 Sand Casting 5-32 5.4 Die Casting 5-33 5.5 Centrifugal Casting 5-33

Chapter 6 Mechanical Testing

6.0 Properties of Materials 6-34 6.1 Strength 6-34 6.2 Stiffness 6-34 6.3 Elasticity 6-34 6.4 Plasticity 6-34 6.5 Toughness 6-34 6.6 Brittleness 6-35 6.7 Ductility and Malleability 6-35 6.8 Hardness 6-35

6.8.1 Tensile Test 6-36 6.8.2 Hardness Tests 6-37 6.8.3 Brinell Test 6-38 6.8.4 Vickers Pyramid Test 6-39 6.8.5 Rockwell Test 6-39 6.8.6 Impact Tests 6-40 6.8.7 Izod Test 6-40 6.8.8 Creep Test 6-41 6.8.9 Fatigue Test 6-42

6.9 Some Other Mechanical Tests 6-44

6.9.1 Erichsen Cupping Test 6-44 6.9.2 Bend Test 6-44 6.9.3 Compression Test 6-45 6.9.4 Torsion Test 6-46

6.10 Significance of Mechanical Properties 6-47

Chapter 7 Non-Destructive Testing

7.0 Introduction 7-48 7.1 Tests for the Detection of Surface Cracks and Flaws 7-48

7.1.1 Penetrant Methods 7-48 7.1.2 Magnetic Dust Methods 7-49 7.1.3 Acid Pickling Methods 7-50

CONTENTS

7.2 Test For The Detection Of Internal Defects 7-51 7.2.1 X-Ray Methods 7-51 7.2.2 Gamma-Ray Methods 7-52

7.3 Ultrasonic testing 7-52

Chapter 8 Deformation and Recrystallization

8.0 Slip and Work Hardening 8-54 8.1 Step-Step Movement of Dislocations 8-55 8.2 Deformation by Twinning 8-55 8.3 Annealing 8-56 8.4 The Relief of Stress 8-56 8.5 Recrystallization 8-56 8.6 Grain Growth 8-57 8.7 Normalizing 8-57 8.8 Cold-Working Processes 8-57 8.9 Hot-Working Processes 8-59

Chapter 9 Mechanical Shaping of Metals

9.0 Hot-Working Processes 9-60 9.1 Forging 9-60

9.1.1 Drop-Forging 9-60

9.2 Hot-Pressing 9-61

9.2.1 Hot-Rolling 9-62 9.3 Extrusion 9-62 9.4 Cold-Working Processes 9-63 9.5 Cold-rolling 9-63 9.6 Drawing 9-64 9.7 Stretch-forming 9-65 9.8 Coining and Embossing 9-66 9.9 Powder Metallurgy 9-67

Chapter 10 Metals

10.0 Pure Metals 10-68 10.1 Alloys 10-68 10.2 Ferrous Metals 10-68 10.3 Non-Ferrous Metals 10-

CONTENTS

Chapter 11 Soldering and Brazing

11.0 Introduction 11-70 11.1 Soldering 11-70 11.2 Brazing 11-72

Chapter 12 Welding

12.0 Welding 12-74 12.1 Fusion Welding Processes 12-75 12.2 Pressure Welding Processes 12-78 12.3 Consequences of Welding 12-79

Chapter 13 Concrete

13.0 Concrete 13-81 13.1 Cements 13-81

13.1.1 Gypsum 13-81 13.1.2 Lime 13-81 13.1.3 Natural Cements 13-82 13.1.4 Portland Cement 13-82

13.2 Concrete Aggregates 13-83 13.3 Admixtures 13-83 13.4 Proportioning of Concrete 13-84 13.5 Reinforced Concrete 13-85 13.6 Uses and Properties of Concrete 13-86 13.7 Pre-stressed Concrete 13-87 13.8 Failure of Concrete 13-88 13.9 Special Applications of Concrete in a Nuclear

Station 13-89 Chapter 14 Lubricants

14.0 Lubricants 14-92 14.1 Oils 14-92 14.2 Greases 14-97 14.3 Solid Lubricants 14-98

Chapter 15 Plastic, Rubber and Protective Coatings

15.0 Plastics 15-99

CONTENTS

15.1 Rubber composition 15-101 15.2 Vulcanization 15-101 15.3 Bits and Pieces 15-103 15.4 Protective Coatings 15-104

Chapter 16 Adhesives

16.0 Adhesives 16-109 16.1 Principles of Adhesion 16-109 16.2 Functions 16-110 16.3 Sealing and insulating 16-111 16.4 Other uses 16-111 16.5 Chemical forms 16-112 16.6 Types of Adhesives 16-112 16.7 Hot melts 16-113 16.8 Others 16-114 16.9 Tapes 16-114 16.10 End Use Requirements 16-116 16.11 Conclusion 16-116

Chapter 17 Radiation Damage

17.0 Radiation damage 17-117 17.1 Types of Radiation 17-117 17.2 Effects of Radiation on the Bond Types 17-119 17.3 Effects of Radiation on Materials 17-119

Classification of Materials

Materials Version (1.0)

1-1

CHAPTER 1

Classification of Materials 1.0 Introduction:

Materials are substances of which something is composed or made. For convenience, most engineering materials are divided into three main classes: metallic, polymeric (plastic), and ceramic materials. In addition to these main classes, two more types, composite materials and electronic materials also have great engineering importance. All important properties of solid materials may be grouped into six different categories: mechanical, electrical, thermal, magnetic, optical, and deteriorative. Mechanical properties relate deformation to an applied load or force; examples include elastic modulus and strength. Electrical properties demonstrate the behavior of material in electric field; examples include electrical conductivity and dielectric constant. The thermal behavior of solids can be represented in terms of heat capacity and thermal conductivity. Magnetic properties demonstrate the response of a material to the application of a magnetic field. Example of optical properties are index of refraction and reflectivity. Finally, deterriorative characteristics indicate the chemical reactivity of materials. In addition to structure and properties, two other important components are involved in the science and engineering of materials, i.e. processing and performance.

1.1 Metallic Materials:

These materials are inorganic substances which are composed of one or more metallic elements and may also contain some nonmetallic elements. They have large number of unlocalized electrons; that is, these electrons are not bound to particular atoms. Many properties of metals are due to the presence of this sea of free electrons. Examples of metallic elements are iron, copper, aluminum, nickel, and tungsten. Nonmetallic elements such as carbon, nitrogen, and oxygen may also be contained in metallic materials. Metals have crystalline structure in which the atoms are arranged in an orderly manner. Metals, in general, are good thermal and electrical conductors. They are non-transparent to visible light. Polished metal surface has a lustrous appearance. Many metals are relatively strong and ductile at room temperature, and many maintain good strength even at high temperatures. Metals and alloys are commonly divided into two classes: ferrous metals and alloys that contain a large percentage of iron such as steels and cast iron and nonferrous metals and alloys that do not contain iron or only a relatively small



1-2

amount of iron. Examples of nonferrous metals are aluminum, copper, zinc, titanium, and nickel.

1.2 Polymeric (Plastic) Materials:

Most polymeric materials consist of organic (carbon containing) long molecular chains or networks. Polymers include the familiar plastic and rubber materials. Structurally, most polymeric materials are noncrystalline but some consist of mixtures of crystalline and noncrystalline regions. The strength and ductility of polymeric materials vary greatly. Because of their nature, most polymeric materials are poor conductors of electricity. Some of these materials are good insulators and are used for electrical innovative applications. In general, polymeric materials have low densities and may be extremely flexible. They have relatively low softening or decomposition temperatures.

1.3 Ceramic Materials: Ceramic materials are inorganic materials which consist of metallic and nonmetallic elements chemically bonded together. They are most frequently oxides, nitrides, and carbides. The wide range that falls within this classification includes ceramics that are composed of clay minerals, cement, and glass. They can be crystalline, noncrystalline, or mixtures of both. Most ceramic materials have high hardness and high temperature strength but tend to have mechanical brittleness. Lately, ceramic materials have been developed for engine applications. Advantages of ceramic materials for engine applications are light weight, high strength and hardness, good heat and wear resistance, reduced friction, and insulative properties. Ceramics are low in cost, but their processing into finished products is usually slow and costly. Also, most ceramic materials are easily damaged by impact because of their low or nil ductility.

1.4 Composite Materials:

Composite materials are misture of two or more materials. Fiberglass is a familiar example, in which glass fibbers are embedded within a polymeric material. A composite is designed to display a combination of the best characteristics of each of the component materials. Fiberglass acquires strength from the glass and flexibility from the polymer. Most composite materials consist of a selected filler or reinforcing material and a compatible resin binder to obtain the specific characteristics and properties desired. Usually, the components do not dissolve into each other and can be physically identified by an interface between the components. Composites can be of many types. Some of predominant types are fibrous (composed of fibbers in a matrix) and particulate (composed of particles in



1-3

a matrix). There are many different combinations of reinforcements and matrices used to produce composite materials. Two outstanding types of modern composite materials used for engineering applications are fibre-glass reinforcing material in a polyester or epoxy matrix and carbon fibbers in a an epoxy matrix.

1.5 Electronic Materials:

Electronic materials, i.e semiconductors, are not a major type of material by volume but are an extremely important type of material for advanced engineering technology. Semiconductors have electrical properties that are intermediate between the electrical conductors and insulators. Furthermore, the electrical characteristics of these materials are extremely sensitive to the presence of minute concentrations of impurity atoms. The most important electronic material is pure silicon which is modified in various ways to change its electrical characteristics. A very large number of electronic circuits can be miniaturized on a silicon chip which is about 0.25 squared inch. Microelectronic devices have made possible such new products as communication satellites, advanced computers, hand-held calculators, digital watches, and welding robots.

Mechanics of Materials


2-4

CHAPTER 2


2.0 Normal Stress and Strain:

The fundamental concepts of stress and strain can be illustrated by considering a prismatic bar that is loaded by axial forces 'P' at the ends (Figure 2-1a). In this case, the axial forces produce a uniform stretching of the bar and the bar is said to be in tension. The internal stresses produced in the bar by the axial forces are exposed if we make an imaginary cut at section mn (Figure 2-1b). The intensity of force (i.e. the force per unit area) is called the Stress 'σ'.

AP

=σ -------------------(2-1)

Figure 2-1a Figure 2-1b When the bar is stretched by the forces 'P', as shown in the figure, the resulting stresses are tensile stresses, if the forces are reversed in direction, causing the bar to be compressed, we obtain compressive stresses. In as much as the stress c axis in a direction perpendicular to the cut surface, it is referred to as a normal stress. Thus, normal stresses may be either tensile or compressive stresses. Later, we will encounter another type of stress, called a shear stress, that acts parallel to the surface.



2-5

When a sign convention for normal stresses is required, it is customary to define tensile stresses as positive and compressive stresses as negative. The stress distribution at the ends of the bar depends upon the details of how the axial load P is actually applied. If the load itself is distributed uniformly over the end, then the stress pattern at the end will be the same as elsewhere. However. the load usually is concentrated over a small area, resulting in high localized stresses (called stress concentrations) and nonuniform stress distributions over cross sections in the vicinity of the load. As we move away from the ends, the stress distribution gradually approaches the uniform distribution shown in Figure 2-1c.

Figure 2-1c

An axially loaded bar undergoes a change in length, becoming longer when in tension and shorter when in compression. The change in length is denoted by 'δ '. The elongation per unit length is called strain 'ε'.

Lδε = ----------------------------(2-2)

If the bar is in tension, the strain is called a tensile strain, representing an elongation or stretching of the material. If the bar is in compression, the strain is a compressive strain and the bar shortens. Tensile strain is usually taken as positive and compressive strain as negative. The strain 'ε' is called a normal strain because it is associated with normal stresses.



2-6

Because normal strain is the ratio of two lengths, it is a dimensionless quantity, that is, it has no units.

Example 2-1 A prismatic bar with rectangular cross section (20 x 40 mm) and length L = 2.8 m is subjected to an axial tensile force of 70 KN (Figure 2-2). The measured elongation of the bar is δ = 1.2 mm. Calculate the tensile stress and strain in Ihe bar.

Figure 2-2

Solution: Using equation 2-1 to calculate normal stress:

MPa87.5mm)(40mm)(20

kN70APσ ===

The normal strain from equation 2-2 is:

610429m2.8

mm1.2Lδε −×===

The quantities σ and ε are the tensile stress and strain, respectively, in the longitudinal direction of the bar.



2-7

Example 2-2 A deep-well pump is operated by a crank that rotates in fixed bearings and moves a piston up and down in the well (Figure 2-3). The pump rod has a diameter d = 15 mm and a length L = 97.5 m. It is made of steel having specific weight γ = 7.85 t/m3. The resistance encountered by the piston during the downstroke is 0.89 kN and during the upstroke is 8.90 kN. Determine the maximum tensile and compressive stresses in the pump rod due to the combined effects of the resistance forces and the weight of the rod.

Figure 2-3 Solution: During the downstroke, the resistance of the piston creates a compressive force C = 0.89 k.N throughout the length of the rod, and during Ihe upstroke it creates a tensile force T = 8.90 kN. The weight of the rod produces a tensile force that varies from zero at the lower end of the rod to a maximum at the upper end. The maximum force equals the weight of the entire rod, given by the expression

ALγW = 233 m)(0.015

4πm)(9.75N/t)109.81t/m(7.85W ×××××=

N1327W = The maximum tensile force in the pump rod occurs during the upstroke at the upper end and is equal to T + W or 10.227 kN. The corresponding maximum tensile stress is



2-8

MPa57.9/4mm)(15πN1010.227

APσ 2

3

t =×

×==

The maximum compressive stress occurs at the lower end during the downstroke:

MPa5/4mm)(15π

N10APσ 2

3

c 0.89.0

=×

×==

These calculations give the axial stresses in the pump rod due only to the specified loads. Other considerations, such as bending of the pump rod and dynamic effects, have not been taken into account in this example.

2.1 Stress-Strain Diagrams:

After performing a tension or compression test and determining the stress and strain at various magnitudes of the load, we can plot a diagram of stress versus strain. Such a stress-strain diagram is characteristic of the material and conveys important information about the mechanical properties and type of behavior. Stress-strain diagram for a typical structural steel in tension is shown in Figure 2-4. Strains are plotted on the horizontal axis and stresses on the vertical axis.

Figure 2-4 The diagram begins with a straight line from the origin 'O' to point 'A', which means that stress and strain are proportional. Beyond point 'A', the proportionality between stress and strain no longer exists: hence the stress at



2-9

'A' is called the Proportional Limit. The slope of the straight line from 'O' to 'A' is called the Modulus of Elasticity, which is usually represented by 'E'. Because strain is nondimensional, this slope has the same units as stress. With an increase in the load beyond the proportional limit, the strain begins to increase more rapidly for each increment in stress. The stress-strain curve then has a smaller and smaller slope, until, at point 'B', the curve becomes horizontal. Beginning at this point, considerable elongation occurs with no noticeable increase in the tensile force (from 'B' to 'C'). This phenomenon is known as yielding of the material, and point B is called the Yield Point. The corresponding stress is known as the Yield Stress. In the region from 'B' to 'C', the material becomes Perfectly Plastic, which means that it can deform without an increase in the applied load. After undergoing the large strains that occur during yielding in the region 'BC', the material begins to Strain Harden. During strain hardening, the material undergoes changes in its atomic and crystalline structure, resulting in increased resistance of the material to further deformation. Additional elongation now requires an increase in the tensile load, and the stress-strain diagram has a positive slope from 'C' to 'D'. The load eventually reaches its maximum value, and the corresponding stress (at point D} is called the Ultimate Stress. Further stretching of the bar is actually accompanied by a reduction in the load and Fracture finally occurs at point 'E'. Lateral contraction of the specimen occurs when it is stretched, resulting in a decrease in the cross-sectional area. as previously mentioned. The reduction in area is too small to have a noticeable effect on the calculated value of stress up to about point 'C', but beyond that point the reduction begins to alter the shape of the diagram. The true stress is larger than the nominal stress because it is calculated with a smaller area. In the vicinity of the ultimate stress, the reduction in area of the bar becomes clearly visible and a pronounced Necking of the bar occurs (Figure 2-5). If the actual cross-sectional area at the narrow part of the neck is used to calculate the stress, the true stress-strain curve will follow the dashed line CE' in Figure 2-4. For most practical purposes. however, the conventional stress-strain curve OABCDE. which is based upon the original cross-sectional area of the specimen and is easy to calculate, provides satisfactory information for use in design. Materials that fail in tension at relatively low values of strain are classified as Brittle Materials. Examples are concrete, stone, cast iron, glass, ceramic materials, and many common metallic alloys. These materials fail with only little elongation after the proportional limit is exceeded.



2-10

Stress-strain diagrams for compression have different shapes from those for tension. Ductile metals such as steel, aluminum, and copper have proportional limits in compression very close to those in tension; and therefore the initial regions of their compression stress-strain diagrams are very similar to the tension diagrams. However, when yielding begins the behavior is quite different. In a tension test. the specimen is stretched, necking may occur, and fracture ultimately takes place. When a small specimen of ductile material is compressed, it begins to bulge outward on the sides and become barrel shaped. With increasing load, the specimen is flattened out, thus offering increased resistance to further shortening (which means the stress-strain curve goes upward).

Figure 2-5 2.2 Elasticity and Plasticity:

The stress-strain diagrams illustrate the behavior of various materials, as they are loaded in tension or compression. Now let us consider what happens when the load is slowly removed and the material is Unloaded. Assume that we apply a load to a tensile specimen so that the stress and strain go from 'O' to 'A' on the stress-strain curve in Figure 2-6a. Suppose further that when the load is removed, the material follows exactly the same curve back to the origin 'O'. This property of a material, by which it returns to its original dimensions during unloading, is called Elasticity, and the material itself is said



2-11

to be Elastic. The stress-strain curve from 'O' to 'A' need not be linear in order for the material to be elastic.

Figure 2-6a

Now let us suppose that we load this same material to a much higher level, so that point 'B' is reached on the stress-strain diagram (Figure 2-6b). In this case, when unloading occurs, the material follows line BC on the diagram. When point 'C' is reached, the load has been entirely removed, but a Residual Strain, or Permanent Strain, 'OC' remains in the material. The corresponding residual elongation of the bar is called the Permanent Set.

Figure 2-6b

The stress at the upper limit of the elastic region is known as the Elastic Limit of the material. Usually the elastic limit is slightly above or nearly the same as



2-12

the proportional limit. In the case of mid steel, the yield stress is also very close to the proportional limit, so that for practical purposes the yield stress, the elastic limit, and the proportional limit are assumed to be equal. This situation does not hold for all materials. Rubber provides the outstanding example of a material that is elastic far beyond the proportional limit. The characteristic of a material by which it undergoes inelastic strains beyond those at the elastic limit is known as Plasticity. Thus, in Figure 2-6a, an elastic region is followed by a plastic region. When large deformations occur in a ductile material loaded into the plastic region, the material is said to undergo Plastic Flow.

2.3 Creep:

Some materials develop additional strains over long periods of time and are said to Creep. For example, suppose that a vertical bar (Figure 2-7a) is loaded slowly by a force 'P', so that the bar elongates by an amount 'δo'. Assume that this loading and the corresponding elongation take place during a time interval 'to'. After this time, The load remains constant. However, due to creep, the bar may gradually, lengthen as shown in Figure 2-7b, even though the load does not change. This behavior occurs with many materials, although sometimes the change is too small to be of concern.

Figure 2-7a As a second example of creep, consider a wire that is stretched between two immovable supports so that it has an initial tensile stress 'σo' (Figure 2-7a). After the initial loading time 'to', the stress in the wire gradually diminishes with the elapse of time. Eventually it reaches a constant value, even though the



2-13

supports at the ends of the wire do not move. This process, which is due to creep, is called Relaxation of the material.

Figure 2-7b Creep is usually more important at high temperatures than at ordinary temperatures, and therefore it should always be considered in the design of engines, furnaces, and other structures that operate at elevated temperatures for long periods of time. However, materials such as steel. Concrete and wood will creep slightly even at atmospheric temperatures.

2.4 Linear Elasticity and Hooke's Law:

When a material behaves elastically and also exhibits a linear relationship between stress and strain, it is said to be Linearly Elastic. It is a property of many solid materials, including metals, wood- concrete, plastics, and ceramics. The linear relationship between stress and strain for a bar in simple tension or compression can be expressed by Hooke's Law, given by the following equation.

εσ E= ----------------------(2-3) 'E' is a constant of proportionality known as the Modulus of Elasticity for the material. The modulus of elasticity is the slope of the stress-strain diagram in the linearly elastic region, and its value depends upon the particular material being used. It has relatively large values for materials that are very stiff, such as structural metals. Steel has a modulus of approximately 200 GPa: for



2-14

aluminum it is approximately 70 GPa. More flexible materials have a lower modulus: typical values for plastics range from 0.7 to 14 GPa. The modulus of elasticity is also called Young's Modulus.

2.5 Poisson's Ratio:

When a prismatic bar is loaded in tension, the axial elongation is accompanied by Lateral Contraction, i.e. normal to the direction of the applied load (Figure 2-9). It is readily seen in a stretched rubber band, but in metals the changes in lateral dimensions are usually too small to be visible. However, they can easily be detected with measuring devices.

Figure 2-9 The Lateral Strain at a point in a bar is proportional to the axial strain at that same point if the material is linearly elastic. The ratio of the strain in the lateral direction to the strain in the axial direction is known as Poisson's ratio 'ν'.

strainaxialstrainlateral

ν −= ---------------(2-4)

For a bar in tension, the lateral strain represents a decrease in width (negative strain) and the axial strain represents elongation (positive strain). For compression we have the opposite situation, with the bar becoming shorter



2-15

(negative axial strain) and wider (positive lateral strain). Therefore, Poisson's ratio has a positive value for most materials.

2.6 Shear Stress And Strain:

There is another very common type of stress, known as a Shear Stress, that acts parallel or tangential to the surface of the material. As an example, consider the bolted connection shown in Figure 2-10. This connection consists of a flat bar 'A', a clevis 'C', and a bolt 'B' that passes through holes in the bar and clevis. Under the action of the tensile loads P. the bar and clevis will press against the bolt in bearing, and contact stresses, called Bearing Stresses, will be developed against the bolt. In addition, the bar and clevis tend to shear off the bolt, and this tendency is resisted by shear stresses in the bolt.

Figure 2-10a Figure 2-10b To show more clearly the action of these stresses, let us look at the connection in a side view (Figure 2-10b). Then a free-body diagram of the bolt (Figure 2-10c) shows the bearing stresses exerted by the clevis and the bar against the bolt; the stresses on the left hand side (labeled 1 and 3) are from the clevis and those on the right hand side (labeled 2 are from the bar. The free-body diagram of shows that there is a tendency to shear the bolt along cross sections 'mn' and 'pq'. From a free-body diagram of the portion mnpq of the bolt (Figure 2-10d), we see that shear forces 'V' must act over the cut surfaces of the bolt. In this particular example there are two planes of shear, and so the



2-16

bolt is said to be in Double Shear. Each of the shear forces V is equal to P/2. The average shear stress 'τ' on the cross section of the bolt is obtained by dividing the total shear force 'V' by the area 'A' over which it acts:

AVτ = ----------------(2-5)

Figure 2-10c Figure 2-10d Under the action of shear stress, material is deformed, resulting in Shear Strain. Shear stresses have no tendency to elongate or to shorten an element. They produce a change in the shape of the element and it is deformed in such a way that its opposite faces are moved with respect to one another, measured in terms of angular deformation. This angular deformation is called Shear Strain 'γ' and it is measured in radian. For linearly elastic region of stress-strain diagram, the shear stress and shear strain are directly proportional, and we have the following equation for Hooke's law in shear:

γGτ = -------------------(2-6) 'G' is the constant of proportionality called the Shear Modulus of elasticity (also called the Modulus of Rigidity).



2-17

Example 2-3 A punch with a diameter of 19 mm is used to punch a hole in a 6 mm steel plate (Figure 2-11). A force P = 116 kN is required. What is the average shear stress in the plate and the average compressive stress in the punch?

Figure 2-11 Solution: The average shear stress is obtained by dividing the force 'P' by the area being sheared by the punch This area is equal to the circumference of the hole times the thickness of the plale:

2s mm358mm)(6mm)(19πA =××=

Therefore, the average shear stress is

MPa324mm358

10kN116APτ 2

3

s=×==

Also, the average compressive stress in the punch is

MPa409/4mm)(19π

10kN116APσ 2

3

cc =

×

×==

in which 'Ac' is the cross-sectional area of the punch.



2-18

2.7 Allowable Stresses and Allowable Loads:

If structural failure is to be avoided, the loads that a structure actually can support must be greater than the loads it will be required to sustain when in service. The ability of a structure to resist loads is called Strength. The actual strength of a structure must exceed the required strength. The ratio of the actual strength to the required strength is called the Factor of Safety 'n':

strengthrequiredstrengthactual

nsafetyofFactor == ----------------(2-7)

Of course, the factor of safety must be greater than 1.0 if failure is to be avoided. Depending upon the circumstances, factors of safety from slightly above 1.0 to as much as 10 are used. The incorporation of factors of safety into design is not a simple matter, because both strength and failure have many different meanings. Failure can mean the fracture or complete collapse of a structure, or it can mean that the deformations have exceeded some limiting value so that the structure is no longer able to perform its intended functions. The latter kind of failure may occur at loads much smaller than those that cause actual collapse. The determination of a factor of safety must also take into account such matters as the following: probability of accidental overloading of the structure by loads that exceed the design loads; types of loads (static, dynamic, or repeated) and how accurately they are known; possibility of fatigue failure; inaccuracies in construction; quality of workmanship; variations in properties of materials; deterioration due to corrosion or other environmental effects; accuracy of the methods of analysis: whether failure is gradual (ample warning) or sudden (no warning); consequences of failure (minor damage or major catastrophe): and other such considerations. If the factor of safety is too low, the likelihood of failure will be high and hence the structure will be unacceptable; if the factor is too large, the structure will be wasteful of materials and perhaps unsuitable for its function (for instance, it may be too heavy). Because of these complexities, good engineering judgment is required when establishing factors of safety. They are usually determined by groups of experienced engineers who write the codes and specifications used by other designers. In actual practice, there are several ways in which factors of safety are defined and implemented. For many structures, it is important that the material remain within the linearly elastic range in order to avoid permanent deformations when the loads are removed. Under these conditions, a common method of design is to use a factor of safety with respect to yielding of the structure. The structure begins to yield when the yield stress is reached at any point within the



2-19

structure. By applying a factor of safety with respect to the yield stress, we obtain an Allowable Stress, or Working Stress, that must not be exceeded anywhere in the structure. Thus,

safetyoffactorstressyield

stressAllowable =

ny

allowσ

σ = ----------------------------(2-8)

Another method of design is to establish the allowable stress by applying a factor of safety with respect to the Ultimate Stress instead of the yield stress. This method is suitable for brittle materials, such as concrete and some plastics, but it also is used for wood and high-strength steels.

nu

allowσ

σ = ---------------------------(2-9)

The last method involves the application of factors of safety to loads rather than to stresses. Ultimate Loads mean the loads that produce failure or collapse of the structure. The loads that the structure must support in service are called Service Loads.

loadserviceloadultimate

nsafetyofFactor ==

Example 2-4 A short, hollow, circular, cast-iron cylinder (Figure 2-12) is to support an axial compressive load P = 580 kN. The ultimate stress in compression for the material is σu = 240 MPa. It is decided to design the cylinder with a wall thickness 't' of 25 mm and a factor of safely of 3.0 with respect to the ultimate strength. Compute the minimum required outside diameter 'd' of the cylinder.

Figure2-12



2-20

Solution: The allowable compressive stress is equal to the ultimate stress divided by the factor of safety, equation 2-9.

MPa803.0

MPa240nσ

σ uallow ===

The required cross-sectional area can now be found:

23

allowmm7250

MPa8010kN580

σPA =

×==

The actual cross-sectional area is

t)(dtπ4

t)2(dπ4dπ

A22

−=−

−=

in which 'd' is the outside diameter and (d - 2 t) is the inside diameter. Solving for 'd' and then substituting t = 25 mm and A = 7250 mm2, we get

mm117.3tπ

Atd =+=

The outside diameter must be at least this large in order to have the desired factor of safety.

Atoms. Molecules and crystals


3-21

CHAPTER 3

Atoms, Molecules and Crystals 3.0 Introduction:

Atoms are very tiny particles indeed. All truly solid materials consists of atoms which are arranged in some pattern peculiar to that material. These atoms are held together by forces of attraction which are due to the electrical charges within each atom. Altogether there are ninety-two different types of atom which occur naturally though during the 'nuclear age' scientists have succeeded in producing some new ones, for example plutonium. of the naturally occurring atoms the smallest and simplest is that of hydrogen; whilst the largest, some two-hundred and thirty-eight times as massive, is that of uranium. There are ninety-two different 'natural' chemical elements of which over seventy are metals. Some of these metallic elements are extremely rare, whilst others are useless to the engineer either by virtue of poor mechanical properties or because they are chemically very reactive. Consequently less than twenty of them (Table 3-1) are in common use in engineering alloys. Of the non-metallic elements carbon is perhaps the one which forms the basis of most engineering materials since it constitutes the "backbones' of all plastics. Moreover it can be used in strong fibre form and is an essential constituent of all common heat-treatable steels. Another element silicon has become famous in the form of the 'silicon chip', but along with oxygen (as silicon dioxide or silica) it is the basis of many refractory building materials. Oxygen and silicon are by far the most common elements in the Earth's crust and account for some 75 per cent of it in the form of clays. sands, stones and rocks like granite.

3.1 Chemical Bonding of Atoms:

We all know the basic structure of atoms and the subatomic particles. The distribution of electrons in different shells or energy levels is also known to us. Since the number of protons in the nucleus governs the total number of electrons in all shells around it, it follows that there are generally insufficient electrons to complete the final outer shell. At the same time there is electrical attraction between the outer-shell electrons of one atom and the protons in the nuclei of neighboring atoms and it is these forces of attraction which cause both chemical combination and physical changes such as crystallization to take place. As a result of such reactions strong bonds are formed between atoms so that the atoms are left with completed outer electron shells. This is achieved by atoms either losing, gaining or sharing electrons.



3-22

Table 3-1 Properties of the important engineering metals.

Metal Chemical Symbol

Relative Density

Melting-point (0C)

Tensile strength (Nmm-2) (soft)

Elongation (%)

Aluminum Al 2.7 660 59 60

Antimony Sb 6.6 630 10 0

Beryllium Be 1.8 1285 310 2.3

Cadmium Cd 8.6 321 80 50

Chromium Cr 7.1 1890 220 0

Cobalt Co 8.9 1495 250 6

Copper Cu 8.9 1083 220 60

Gold Au 19.3 1063 120 30

Iron Fe 7.9 1535 500 10

Lead Pb 11.3 327 18 64

Magnesium Mg 1.7 651 180 5

Manganese Mn 7.2 1260 500 20 Mercury Hg 13.6 -39 Molten at

ordinary temperatures

Molten at ordinary

temperatures Molybdenum Mo 10.2 2620 420 50

Nickel Ni 8.9 1458 310 28

Niobium Nb 8.6 1950 270 49

Platinum Pt 21.4 1773 130 35

Silver Ag 10.5 960 140 50

Tin Sn 7.3 232 11 60

Titanium Ti 4.5 1667 230 55

Tungsten W 19.3 3410 420 16

Uranium U 18.7 1150 390 4

Vanadium V 5.7 1710 200 38

Zinc Zn 7.1 420 110 25

Zirconium Zr 6.4 1800 220 25



3-23

3.2 Electrovalent Bond:

The Electrovalent Bond is the attractive force produced by reactions which take place between metals and non-metals. For example, the extremely reactive metal sodium combines with the equally reactive non-metallic gas chlorine, to form crystals of sodium chloride (common table salt). To have an understanding of the bond, consider the combination which occurs between the metal lithium and the extremely reactive non-metallic gas fluorine. An atom of lithium contains only three protons in its nucleus and, therefore, three electrons in orbit around that nucleus. Two of these electrons complete the first shell, leaving a lone electron in the second shell (Figure 3-1).

Figure 3-1

The atom of fluorine on the other hand contains nine protons in its nucleus and, consequently, nine electrons in orbit around it. Two of these fill the first shell so that there are no less than seven electrons in the second shell (Figure 3-1). If a lithium atom and a fluorine atom approach each other, the lone electron of the lithium atom is snatched away so that it joins the outer electron shell of the fluorine atom. This leaves the lithium particle with a complete first shell (2 electrons) and at the same time the electron which it has lost goes to complete the second shell (8 electrons) of the fluorine particle (Figure 3-2). Because the lithium particle has lost a negatively charged electron it now has a resultant positive charge; whilst the fluorine particle having gained an electron now has a resultant negative charge. Charged particles of this type are called ions. Metals always form positively charged ions whilst non-metals form negatively charged ions.

Figure 3-2



3-24

As these lithium and fluorine ions carry opposite charges they will attract each other. However, whilst unlike charges attract, like charges repel so that lithium and fluorine ions arrange themselves in a geometrical pattern in which each fluorine ion is surrounded by six lithium ions as its nearest neighbours whilst each lithium ion is surrounded by six fluorine ions (Figure 3-3). This type of strucutre is called Crystal Structure. For lithium fluoride, it is a relatively simple cubic type of crystal structure. Other such suits may form more complex crystal patterns depending upon the relative sizes of the ions involved and the ratio of the electrical charges carried by each type of ion.

Figure 3-3

3.3 Metallic Bond:

Most metals have one, two or at most three electrons in the outermost shell of the atom. These outer-shell electrons are loosely held to the atomic nucleus and as a metallic vapour condenses and subsequently solidifies these outer-shell electrons get free to form a sort of common pool and are virtually shared between all atoms in the solid metal. Since the resultant metallic ions are all positively charged they will repel each other and so arrange themselves in some form of regular (crystal) pattern in which they are firmly held in position by the attractive force between them and this permeating 'cloud' of negatively charged electrons (Figure 3-4). This type of bonding is Metallic Bond.

Figure 3-4



3-25

It is due to this metallic bonding, all the metals posses following main characteristics: All metals are good conductors of electricity. Since electrons constituting the 'electron cloud' are free to move within the body of the metal. Metals are good conductors of heat. The application of heat to a piece of metal causes electrons to vibrate more actively and these vibrations can be passed on quickly from one electron to another within the electron cloud. Most metals are ductile because layers of ions can be made to slide over each other by the application of a shearing force. At the same time metals are strong because the attractive force provided by the electron cloud opposes the slipping apart of those layers of ions. Metals are lustrous in appearance since the free, vibrating surface electrons reflect back rays of light as these fall upon the surface of the metal.

3.4 Covalent Bond:

The covalent bond is formed between atoms of those non-metallic elements in which there is a strong attractive force between the nucleus and the outer-shell electrons. Hence instead of a transfer of electrons from one atom to the other there is a sharing of electrons between two atoms thus binding them together. For example. hydrogen atom consists of a single electron in orbit around a nucleus consisting of a single proton. Two atoms of hydrogen will therefore combine to form a molecule of hydrogen and in this way the two electrons are shared between the two atoms thus completing the electron shell for each atom by this process of sharing the two electrons. The element carbon has the ability to form long chain-like molecules in which carbon atoms are covalently bonded to each other and to hydrogen atoms: An important feature of covalent bonded materials is that, unlike metals in which the outer-shell electrons can travel freely within the electron cloud so making metals conductors of electricity, the outer-shell electrons in these covalent substances are securely held to the atoms to which they belong. These electrons are not free to move away and so the materials are excellent insulators since they cannot conduct electricity.

3.5 Polymorphism:

Many solid elements can exist in more than one different crystalline form and are said to be Polymorphic (the term allotropy is also used to describe this phenomenon). Generally these different crystalline forms are stable over different temperature ranges so that transition from one form to another takes place as the transition temperature is passed. For example, polymorphism of iron which



3-26

enables us to harden suitable steels. Another example is tin, which is also polymorphic existing as 'grey tin', ordinary 'white tin' and as 'brittle tin'. White tin, the form with which we are generally familiar, is stable above 13°C whilst 'grey tin' is stable below 13°C.

Crystal Structure of Metals


4-27

CHAPTER 4


4.0 Introduction:

All metals - and other elements, for that matter - can exist as either gases, liquids, or solids. The 'state' in which a metal exists depends upon the conditions of temperature and pressure which prevail at the time. In a metallic gas, the particles consist of single atoms, which are in a state of continuous motion. As the temperature falls condensation occurs and liquid metal is formed in which atoms are held together only by weak forces of attraction. During solidification, the atoms arrange themselves according to some regular pattern, 'Lattice Structure' or 'Crystal Structure'. Each atom becomes firmly bonded to its neighbours by stronger forces of attraction: so the solid metal acquires strength. Most of the important metals crystallize into one of three different patterns as solidification takes place (Figure 4-1) i.e. Body Centred Cubic (BCC), Face Centred Cubic (FCC), and Close Packed Hexagonal (CPH).

Figure 4-1 4.1 Body Centred Cubic:

It consists of atom cores (i.e. atoms without the outer electrons) at the corners of a cube and one atom core in the centre, all being held in position by the Electron Gas. Iron and Tungsten are BCC at normal temperatures.

4.2 Face Centred Cubic:

It consists of atom cores at the corners of the cube and one atom core at the middle of each cubic face, all bounded together by the Electron Gas.



4-28

Aluminium, Copper, Lead, Gold and Platinum are FCC at ordinary temperatures.

4.3 Close Packed Hexagonal:

It consists of one atom core at each corner of a solid hexagon, one atom core on the top and bottom faces and three atom cores on the corners of a triangle in the middle of the solid hexagon. Although these atoms in the centre of the hexagon look out of place, they are real part of another close packed plane displaced in space, overlapping the original one. Zinc, Zirconium, Beryllium and Cobalt are CPH at ordinary temperatures.

4.4 Dendritic Solidification:

When the temperature of a molten pure metal falls below its freezing-point. crystallization will begin. The nucleus of each crystal will be a single unit of the appropriate crystal lattice. For example, in the case of a metal with a body-centred cubic lattice, nine atoms will come together to form a single unit, and this will grow as further atoms join the lattice structure (Figure 4-1). These atoms will join the 'seed crystal' so that it grows most quickly in those directions in which heat is flowing away most rapidly. Soon the tiny crystal will reach visible size. and form what is called a 'dendrite' (Figure 4-2). Secondary and tertiary arms develop from the main 'backbone' of the dendrite - rather like the branches and twigs which develop from the trunk of a tree, except that the branches in a dendrite follow a regular geometrical pattern. The term 'dendrite' is derived from the Greek word 'dendron', meaning a tree.

Figure 4-2



4-29

The arms of the dendrite continue to grow until they make contact with the outer arms of other dendrites growing in a similar manner nearby. When the outward growth is thus restricted, the existing arms thicken until the spaces between them are filled, or, alternatively, until all the remaining liquid is used up. Liquid metal will be drawn in from elsewhere to fill the space formed as a dendrite grows. If this is not possible. then small shrinkage cavities are likely to form between the dendrite arms. Since each dendrite forms independently, it follows that outer arms of neighbouring dendrites are likely to make contact with each other at irregular angles, and this leads to the irregular overall shape of crystals (Figure 4-3).

Figure 4-3 4.5 Effect of Impurities:

In pure metals, there will be no hint of the dendritic process of crystallization once solidification is complete, because all atoms in a pure metal are identical. If impurities were dissolved in the molten metal, however, these would tend to remain in solution until solidification was almost complete. In this way, they



4-30

would reveal the dendritic pattern when a suitably prepared specimen was viewed under the microscope. It is due to this concentration of impurities at crystal boundaries that a small amount of impurity can have such a devastating effect on mechanical properties, making the cast metal brittle and likely to fail along the crystal boundaries.

4.6 Influence Of Cooling Rates on Crystal Size:

The rate at which a molten metal is cooling as it reaches the freezing-temperature affects the size of the crystals which form. A gradual fall in temperature results in the formation of few nuclei and so the resultant crystals grow unimpeded to a large size. A rapid fall in temperature, however, will lead to some degree of undercooling of the molten metal to a temperature below its actual freezing-point (Figure 4-4). Due to this undercooling and the instability associated with it, a very large number of nuclei is produced. Due to it, the ultimate crystals will be tiny. This is an advantage, since fine-grained castings are generally tougher and stronger than those with a coarse grain size.

Casting Processes


5-31

CHAPTER 5

Casting Processes 5.0 Introduction:

Most metallic materials pass through a molten state at some stage during the shaping process. Much of this molten metal is cast into ingot form prior to being shaped by some mechanical working process. Some metals and alloys - and a number of non-metallic products such as concrete - can be shaped only by casting since they lack the necessary properties of either malleability or ductility. A few metallic substances are produced in powder form. The powder is then compressed and sintered to provide the required shape. This branch of technology is termed Powder Metallurgy. Some of the common casting processes are briefly discussed in the following.

5.1 Ingot Casting:

Many alloys, both ferrous and non-ferrous, are cast in the form of ingots which are then rolled, forged, or extruded into strip, sheet, rod, tube, or other sections.

Figure 5-1

Casting Processes


5-32

When produced as single ingots, steel is generally cast into large iron moulds holding several tonnes of metal (Figure 5-1). There is a reservoir of molten metal which shall be preserved until solidification of the body of the ingot is complete. This reservoir feeds metal into the 'pipe' which forms as the main body of the ingot solidifies and contracts.

5.2 Continuous Casting:

This process is used to produce ingots of both ferrous and non-ferrous alloys (Figure 5-2). Here the molten metal is cast into a short water-cooled mould. As solidification begins, the base is withdrawn downwards at a rate which will keep place with that of pouring.

Figure 5-2

In continuous casting, there is little process scrap, since very long ingots are produced and consequently there is proportionally less rejected pipe. In modern steelmaking, pipe is eliminated since the continuous-cast ingot is conveyed by a system of guide rolls direct to the rolling mill from which the emerging product is cut to the required lengths by a flying saw.

5.3 Sand Casting:

In sand-casting process, sand mould is made by using a suitable pattern. The pattern is withdrawn to leave a cavity of the correct shape in the sand. Liquid metal is poured into the mould which is solidified in the mould shape. Sand-casting is a very useful process, since very intricate shapes can be produced in a large range of metals and alloys. Moreover, relatively small numbers of castings can be made economically, since the necessary outlay on the simple

Casting Processes


5-33

equipment required is low. Wooden patterns are also cheap to produce, as compared with the metal die which is necessary in die-casting processes.

5.4 Die Casting:

In die-casting, a permanent metal mould is used, and the charge of molten metal is either allowed to run in under the action of gravity (gravity die-casting), or is forced in under pressure (pressure die-casting). The product of die-casting is metallurgically superior to that of sand-casting because the internal structure is more uniform, and the grain much finer, because of the rapid cooling rates. Moreover, output rates are much higher when using a permanent metal mould than when using sand moulds. Greater dimensional accuracy and a better surface finish are also obtained by die-casting. However, some alloys which can be sand-cast cannot be die-cast, because of their high shrinkage coefficients. Such alloys would inevitably crack due to their contraction during solidification within the rigid, non-yielding metal mould. Die-casting is confined mainly to zinc or aluminium-base alloys.

5.5 Centrifugal Casting:

This process is most commonly used in the manufacture of cast-iron pipes for water, sewage and gas mains. A permanent cylindrical metal mould, without any central core, is spun at high speed, and molten metal is poured into it (Figure 5-3). Centrifugal force flings the metal to the surface of the mould, thus producing a hoilow cylinder of uniform wall thickness. The product has a uniformly fine-grained outer surface, and is considered superior to a similar shape which has been sand-cast.

Figure 5-3

Mechanical Testing


6-34

CHAPTER 6

Mechanical Testing 6.0 Properties of Materials:

Various structure materials react differently to the same loads; some fracture, some stretch greatly, some react very little. The behaviour of a metal under a load depends upon its certain properties. We have been referring to many of them throughout but now they are formally discussed under:

6.1 Strength:

For a material to be suitable for use in a structure, it must be able to withstand the expected forces or loads without rupture, collapse, or undue distortion. That is, it must have sufficient Strength. Thus, strength can be defined as the maximum stress which a material can develop in resistance of being lengthened, shortened, or distorted to the point of failure.

6.2 Stiffness:

If a material changes form only slightly, i.e. small strain, under stress then it is said to have a high degree of Stiffness. Stiffness of a material is, therefore, its resistance to strain as measured by its modulus of elasticity.

6.3 Elasticity:

The property of a material which causes it to return to its original shape upon release of an applied stress is called Elasticity. All materials are elastic below their elastic limit, however they lose their elasticity when stressed beyond this point and only partially regain their original shape on release of the load.

6.4 Plasticity:

When a stress beyond the elastic limit is placed on a material, it may become permanently deformed or plastic. Thus, a plastic material is one which easily becomes permanently deformed or has a comparatively low modulus of elasticity.

6.5 Toughness:

Toughness is the property of a material which allows it to withstand a great deal of deformation together with high stress without rupture. Toughness

Mechanical Testing


6-35

actually is a measure of the energy which a material will absorb without failure.

6.6 Brittleness:

A brittle material is one which can be permanently deformed only to a very limited extent following which it will beak suddenly. Brittleness is opposite to plasticity.

6.7 Ductility and Malleability:

Ductility is the property of a material which allows it to be drawn out by tension resulting in a permanent set. A ductile material is one which may be stretched considerably before rupture takes place. A malleable material is one which can be hammered or rolled into thin sheets. Thus, ductility is the ability of a material to undergo large permanent deformation in tension while malleability is the ability to undergo large permanent deformation in compression.

6.8 Hardness:

Hardness is the ability to resist very small indentations, abrasion and plastic deformation. It is a combination of many properties. While designing a structure, we are primarily concerned with the balancing of forces We must therefore know the effects which the application of forces may have on a material, before we can use it. Various mechanical tests have therefore been devised over the years, with the object of comparing the amount of deformation produced in a metal with the force which was employed to produce it. Thus. in a tensile test we measure the force required to stretch a specimen of the metal until it breaks; whilst in various hardness tests we produce a small dent in the surface of a test piece by using a compressive force. The hardness number is then calculated as the force used divided by the surface area of the impression produced by it. In the type of test mentioned above, the test-piece is destroyed during the testing process. Such tests are therefore known as Destructive Tests, and can only be applied to individual test pieces. These are taken from a batch of material and they are therefore assumed to be representative of the batch. Tests of a different nature and purpose are used to examine manufactured components for internal flaws and faults; for example X-rays are used to seek internal cavities in castings. These tests are generally referred to as

Mechanical Testing


6-36

Non-Destructive Tests (NDT). In such tests the component is not destroyed during the test.

6.8.1 Tensile Test:

The tensile strength of a material is the stress required to cause fracture of a test piece in tension. A test piece of known cross-sectional area is gripped in the jaws of a testing-machine, and is subjected to a tensile force which is increased by suitable increments. For each increment of force, the amount by which the length of a known 'gauge length' on the test piece increases is measured. This process continues until the test piece fractures. Force-Extension diagram is plotted with the data to analyse the behaviour of the material.

Fig. 6-1

Mechanical Testing


6-37

Figure 6-1 shows a typical force-extension diagram for an annealed carbon steel. It shows that at first the amount of extension is very small, compared with the increase in force. Such extension as there is directly proportional to the force; that is, 'OA' is a straight line. If the force is released at any point before 'A' is reached, the test piece will return to its original length. Thus, the extension between 'O' and 'A' is elastic, and the material obeys Hooke's law. If the test piece is stressed past the point 'A' (known as the elastic limit or limit of proportionality), the material suddenly suffers a sudden extension for very little increase in force. This is called the Yield Point (Y), and, if the force is now removed, a small permanent extension will remain in the material. Any extension which occurs past the point 'A' is of a plastic nature. As the force is increased further, the material stretches rapidly - first uniformly along its entire length, and then form a 'neck'. This 'necking' occurs just after the maximum force has been reached, at 'M', and since the cross-section decreases rapidly at the neck, the force at 'B' required to break the specimen is much less than the maximum load at 'M'.

6.8.2 Hardness Tests:

A true definition of surface hardness is the capacity of that surface to resist abrasion. Earlier, Moh's scale was used to assess the relative hardness of minerals. This consists of a list of materials arranged in order of hardness, with diamond, the hardest of all (with a hardness index of 10), at the head of the list and talc (with an index of 1) at the foot (Table 6-1). Any mineral in the list will scratch any one below it and in this way the hardness of any 'unknown' substance can be related to the scale by finding which substance on the scale will just scratch it and a hardness index thus assigned to it.

Table-1

Moh’s scale of hardness Material Hardness index Diamond 10 Sapphire 9 Topaz 8 Quartz 7 Feldspar 6 Apatite 5 Fluorspar 4 Calcite 3 Gypsum 2 talc 1

Mechanical Testing


6-38

Modern methods of hardness testing really measure the materials resistance to penetration rather than to abrasion.

6.8.3 Brinell Test:

It is probably the best known of the hardness tests. A hardened steel ball is forced into the surface of a test-piece by means of a suitable standard load (Figure 6-2). The diameter of the impression is then measured, using some form of calibrated microscope, and the Brinell hardness number (H} is found from:

Figure 6-2

impressionofsurfacecurvedofarea(P)load

H =

If 'D' is the diameter of the ball. and d that of the impression, it can be shown that:

( )22 dDDD2πimpressionofsurfacecurvedofarea −−=

It follows that

( )22 dDDD2π

(P)loadH−−

=

'H' is generally found by reference to tables which relate 'H' to 'd', a different set of tables being used for each possible combination of 'P' and 'D'.

Mechanical Testing


6-39

6.8.4 Vickers Pyramid Test:

The Vickers pyramid hardness test uses a square-based diamond pyramid (Figure 6-3) as the indentor. One great advantage of this is that all impressions will be geometrically similar, and, within limits, the accuracy of the result will not vary with depth of the impression. Consequently, the operator does not have to choose a P/D2 ratio as he does in the Brinell test, though he must still observe the relationship between depth of impression and thickness of specimen, for reasons similar to those indicated in the case of the Brinell test. A further advantage of the Vickers hardness test is that hardness values for very hard materials (above an index of 500) are likely to be more accurate than the corresponding Brinell numbers since diamond does not deform under high pressure to the same extent as does a steel ball, and so the result will be less uncertain.

Figure 6-3 6.8.5 Rockwell Test:

The Rockwell test is particularly useful for rapid routine testing of finished material, since the hardness number is indicated directly on a dial, and no subsequent measurement of the diameter of the impression is involved. Although the depth (h) (Fig-6-4) of the impression is measured by the instrument, this is converted (on the dial) to hardness values in which the surface area of the impression is related to the load in the usual way. The test piece is placed on the table of the instrument, and the indentor is brought into contact with the surface under 'light load'. This takes up the 'slack' in the system, and the scale is then adjusted to zero. Full load is then applied, and when it is subsequently released (timing being automatic), the test-piece

Mechanical Testing


6-40

remains under 'light load' whilst the hardness index is read direct from the scale.

Figure 6-4

There are also other methods for hardness testing like the Shore scleroscope, wear resistance tests, etc.

6.8.6 Impact Tests:

These tests are used to indicate the toughness of a material, and particularly its capacity for resisting mechanical shock. Brittleness of a material is often not revealed during a tensile test. In an impact test, however, the unsatisfactory material would prove to be extremely brittle as compared with the correctly treated one, which would be tough.

6.8.7 Izod Test:

The Izod impact test employs a standard notched test piece (Figure 6-5), which is clamped firmly in a vice. A heavy pendulum, mounted on ball-bearings. is allowed to strike the test-piece after being released from a fixed height. The striking energy of approximately 163 J is partially absorbed in breaking the test-piece, and as the pendulum swings past, it carries with it a drag pointer which it leaves at its highest point of swing. This indicates the amount of mechanical energy used in fracturing the test piece. To set up stress concentrations which will ensure that fracture does occur, the test piece is notched. It is essential, however, that this notch always be standard, for which reason a standard gauge is supplied to test the dimensional accuracy of the notch, both in this and the other impact tests.

Mechanical Testing


6-41

Figure 6-5 There is also another very common impact test known as the Charpy impact test.

6.8.8 Creep Test:

When stressed over a long period of time, some metals extend very gradually, and may ultimately fail at a stress well below the tensile strength of the material. This phenomenon of slow but continuous extension under a steady force is known as Creep. Such slow extension is more prevalent at high temperatures, and for this reason the effects of creep must be taken into account in the design of steam and chemical plant, gas and steam turbines, and furnace equipment.

Mechanical Testing


6-42

Creep occurs generally in three stages (Figure 6-6). At low stress and/or low temperature (Curve I) some Primary Creep may occur but this falls to a negligible amount in the Secondary Creep (the creep curve becomes almost horizontal). With increased stress and/or temperature (Curves II and III) the rate of secondary creep increases leading into Tertiary Creep and inevitable catastrophic failure. The Limiting Creep Stress of a material at any given temperature is the maximum stress it can withstand without showing any measurable extension.

Figure 6-6

Creep tests are carried out on test pieces which are similar in form to ordinary tensile test pieces. A test piece is enclosed in a thermostatically controlled electric tube furnace which can be maintained accurately at a fixed temperature over the long period of time occupied by the test. The test piece is statically stressed, and some form of sensitive extensometer is used to measure the extremely small extension at suitable time intervals. A set of creep curves, obtained for different static forces at the same temperature, is finally produced, and from these the limiting creep stress is derived.

6.8.9 Fatigue Test:

Experiments show that a girder which would support a static load of 12 tonf for an indefinite period, would fail if a load of only 3 tonf were raised and lowered on it some three million times. Fatigue Failure is associated with the effects which a fluctuating or an alternating force may have on a member, orwhen subjected to the action of a 'live load'.

Mechanical Testing


6-43

To find the fatigue limit, a number of similar specimens of the material are tested by a fatigue testing machine in which alternations of stress can be produced in a test piece very rapidly (Figure 6-7). Each test piece is tested at a different value of W, until failure occurs, or, alternatively, until about 20 million reversals have been endured. From these results, an S-N curve is plotted; that is, stress (S) against the number of reversals (N) endured. The curve becomes horizontal at a stress which will be endured for an infinite number of reversals. This stress is the fatigue limit or endurance limit.

Figure 6-7 Fatigue failure will ultimately occur in any member which is stressed above its fatigue limit in such a way that the operating stress fluctuates or alternates. Such failure can be due simply to bad design and lack of understanding of fatigue, but is much more likely to be due to the presence of unforeseen high-frequency vibrations in a member which is stressed above the fatigue limit. This is possible since the fatigue limit is well below the tensile strength for all materials. Any feature which increases stress concentrations may precipitate fatigue failure. Thus a fatigue crack may start from a keyway, a sharp fillet, a microstructural defect, or even a bad tool mark on the surface of a component which has otherwise been correctly designed with regard to the fatigue limit of the material from which it was made.

Mechanical Testing


6-44

6.9 Some Other Mechanical Tests:

Most of these tests are designed to evaluate some particular property of a material which is not revealed adequately during any of the preceding tests.

6.9.1 Erichsen Cupping Test:

Materials of very high ductility are used for deep-drawing. However, a simple measurement of ductility in terms of percentage elongation (obtained during the tensile test) does not always give a complete assessment of deep-drawing properties. Erichsen cupping test is used in such cases. In this test, a hardened steel ball is forced into the test piece (Figure 6-8), which is clamped between a die-face and a blank-holder. When the test piece splits, the height of the cup which has been formed is measured, and this height (in mm) is taken as the Erichsen value. The most useful aspect of the test is that it gives some idea of the grain size of the material, and hence its suitability for deep-drawing. Coarse grain, always associated with poor ductility in a drawing operation, will show up as a rough granular surface in the dome of the test piece.

Figure 6-8 6.9.2 Bend Test:

Bend tests are often used as a means of judging the suitability of a metal for bending treatment during a production process. It requires little equipment other than a vice. The wire is bent through 90° over a cylinder of specified radius 'R', then back through 90° in the opposite direction (Figure 6-9). This is continued until the test piece breaks, the number of bending cycles being counted. The surface affected by the bending process is examined for cracks.

Mechanical Testing


6-45

Figure 6-9

6.9.3 Compression Test:

Compression tests are used mainly in connection with cast iron and concrete since these are materials more likely to be used under the action of compressive forces than in tension. A cylindrical block, the length of which is twice its diameter, is used as a test piece (Figure 6-10(i)). This is compressed (using a tensile-testing machine running in 'reverse') until it fails.

Figure 6-10

Mechanical Testing


6-46

Malleable metals do not show a well-defined point of failure (Figure 6-10(ii)) but with brittle materials (Figure 6-10(iii)), the ultimate compressive stress can be measured accurately, since the material fails suddenly, usually by multiple shear at angles of 45° to the direction of compression.

6.9.4 Torsion Test:

Torsion tests are often applied to wire, steel wire in particular. The test consists of twisting a piece of wire in the same direction round its own axis until it breaks or until a specified number of twists has been endured. The simple machine used (Figure 6-11) consists essentially of two grips which remain in the same axis and do not apply any bending moment to the test piece. One grip remains stationary whilst the other is rotated. The test piece is held in the machine so that its longitudinal axis coincides with the axis of the grips and so that it remains straight during the test. This is achieved by applying a constant tensile force just sufficient to straighten it but not exceeding 2% of the nominal tensile strength of the specimen.

Figure 6-11

Mechanical Testing


6-47

6.10. Significance Of Mechanical Properties:

Hardness is of most interest where wear resistance is needed. Other than this, its main use is to estimate the tensile strength or as a quick check that a material meets a specification. Ultimate tensile strength is of little use to designers since most components will be hopelessly deformed and usable long before they break. Yield strength is of great importance, since it is the stress which mustn't be exceeded if the component is to stay the same size and shape for its whole working life. Generally, a designer will work to 1/2 or 1/3 of the yield strength to allow a margin of safety. Elongation is a measure of difficulty (the ability to be plastically deformed in tension without failure) and so may be an important factor in the fabrication cost of many components. In addition, ductility can significantly affect the load carrying ability of the material. A small surface mark will reduce the fatigue strength of a brittle material enormously; sharp corners will do the same thing. A ductile material, on the other hand, will yield locally under a scratch or a corner and increase its own strength at that location. Elongation is a safety device given to designers and fabricators by metallurgists to save them from their own poor work. Impact resistance is of value in components subject to high rates of loading. It is also a test for poor heat treatment and can give an indication of a material's fatigue resistance in high load rate applications. Fatigue resistance is primarily of importance to designers. It is important to remember, however, that overloading can cause premature failure and is not always obvious.

Non-Destructive Testing


7-48

CHAPTER 7

Non-Destructive Testing 7.0 Introduction:

Those tests described in the previous chapter are destructive tests carried out on samples which, it is hoped, are rep resentative of a batch of manufactured material. In many cases such tests will be adequate, since properties are generally uniform throughout a large batch of material derived from a single cast ingot. Components which are produced individually, however, such as castings and welded joints, may vary in quality. Even with fairly rigorous on-line inspection, faulty components can arise due to the influences of many variable factors such as working temperature, surrounding atmosphere, and the operator skill. If the quality of such components is very important, as for example castings used in nuclear power plant, it may be necessary to test each component individually using some type of nondestructive examination, Non-Destructive Testing (NDT). During such testings, faults and flaws are detected either at the surface or below it. and a number of suitable methods are available in each case. These tests give an overall assessment of the quality of the product, therefore, the term nondestructive testing is often replaced by Non-Destructive Evaluation (NDE).

7.1 Tests for the Detection of Surface Cracks and Flaws:

Surface cracks may arise in a material in a number of ways. Some cracks show up during inspection using a simple hand magnifier, others may be far harder to detect. For example, steel tools may develop hair-line cracks which are not apparent during ordinary visual inspection, Castings may crack due to contraction during the period of solidification and cooling, etc.

7.1.1 Penetrant Methods:

In these methods, the surface to be examined is first cleaned adequately to remove grease, and then dried (Figure 7-1). The penetrant liquid is then sprayed on to the surface, which should be warmed to about 90°C. Small components may be immersed in the heated penetrant. After sufficient time has elapsed for the penetrant to fill any cracks, the excess is carefully washed from the surface with warm water (the surface tension of water is too high to allow it to enter the fine cracks). Other suitable solvents may also be used to remove excess penetrant. The test surface is then carefully dried and coated with a 'developer' such as powdered chalk. This developer can be blown on as a



7-49

powder or sprayed on as a solution or suspension. In some cases, the component is dipped into a suspension or solution of the developer which is allowed to dry on the surface. The component is then set aside for some time. As the coated surface cools it contracts and penetrant tends to be squeezed out of any cracks, so the chalk layer will become stained, thus revealing the presence of the cracks. Most penetrants of this type contain a coloured dye which makes the stain immediately noticeable.

Figure 7-1 In some cases, the penetrant contains a compound which becomes fluorescent under ultraviolet light and thus, the use of chalk becomes unnecessary. When the prepared surface is illuminated by ultraviolet light, the cracks containing the penetrant are observed as bright lines on a dark background. Penetrant methods in general are particularly useful for the examination of non-ferrous metals and austenitic (non-magnetic) steels. Aluminium-alloy castings are frequently examined in this way.

7.1.2 Magnetic Dust Methods:

These methods can be applied only to magnetic materials. They provide a very quick and efficient method of detecting cracks. A further advantage over the penetrant method is that flaws which are immediately below the surface are also detected. Thus, the magnetic dust method is particularly suitable for examining machined or polished surfaces, where the mouth of a crack may have closed. The magnetic method involves making the component part of a 'magnetic circuit' (Figure 7-2(i)). This magnetic field can be induced in the component either by permanent magnets or by electromagnetic means. It can also be produced by passing a heavy current at low e.m.f. through the component. Magnetic lines of force pass easily through a magnetic material than through air. In air, they repel each other and spread (Figure 7-2(iii)). Therefore, when there is a gap or other discontinuity at or just below the surface, the lines of



7-50

force spread outwards. If some iron dust is now sprinkled on the surface of the component, it will stick to the surface where the lines of force break out and therefore, the fault is observed.

Figure 7-2 The magnetized component can also be observed by placing it in paraffin containing a suspension of tiny iron particles. The particles will be attracted to the surface of the component at any points where, due to the presence of a fault, lines of force cut through. The sensitivity of this method can be improved by coating the surface to be treated with a suitable liquid, containing magnetic particles which have been treated with a fluorescent compound. The surface is then examined under 'black light' in a darkened cubicle.

7.1.3 Acid Pickling Methods:

Surface defects in steel castings can generally be revealed by pickling the casting in a 10% solution of sulphuric acid at about 50°C for up to two hours. Another way is to use a 20 to 30% solution of hydrochloric acid at ambient temperature for up to twelve hours. For stainless steels, mixtures of concentrated nitric and hydrofluoric acids are used. This mixture is extremely corrosive and thus, the tests require great care. After removal from the acid bath, the casting is washed thoroughly and, if necessary, any residual acid is neutralized by using a hot suspension of slaked lime. Finally, the casting is washed again and dried quickly. This simple procedure provides a cheap but effective way of dissolving oxide layers and revealing most surface defects.

7.2 Test For The Detection Of Internal Defects:



7-51

Castings may contain unwanted internal cavities, in the form of gas blow-holes and shrinkage porosity. Similarly, wrought materials and welded joints may also contain such flaws for several reasons. These internal flaws are hidden from us because metals are opaque to light. But high frequency electromagnetic radiation, like x-rays and gamma rays, may penetrate through metals. So, these radiations are most widely used in the detection of internal faults.

7.2.1 X-Ray Methods:

X-rays travel in straight lines like light rays and can penetrate through metal provided they posses enough energy and the metal is not so thick in cross-section that the X-rays are completely absorbed. In this way, an x-ray photograph of the interior of the metal is obtained, with the help of which faults can be detected (Figure 7-3). A fluorescent screen may be substituted for the photographic film, so that the resultant radiograph may be viewed instantaneously. This type of fluoroscopy is much cheaper and quicker, but is less sensitive than photography, and its use is limited to the less-dense metals.

Figure 7-3

7.2.2 Gamma-Ray Methods:



7-52

γ-rays can also be used in the radiography of metals. They are more energetic i.e. 'harder' than X-rays. They can be used for more dense metals and also when the material is thick. They are particularly useful in the radiography of steel, which absorbs radiation more effectively than do the light alloys. Exposure to γ radiation can be extremely dangerous. Originally, radium was used as a source of γ-rays. Since it is very rare and expensive, now artificially activated isotopes are used. One of the most useful activated isotopes is cobalt-60. γ-ray radiography is in many respects simpler than using x-rays but security arrangements need to be even more stringent. They are more energetic than x-rays and unlike X-rays, cannot be 'switched off' because a radioactive isotope may emit continuously for a period varying from a few seconds to thousands of years. A lead safe is therefore used to protect against them. γ-rays radiograph has the advantage over x-rays that it is more mobile and less cumbersome to transport.

7.3 Ultrasonic testing:

In ultrasonic testing very high-frequency vibrations are used, which are beyond the acoustic range which can be received by our ears. Frequencies between 500 000 and 10 000 000 Hz are commonly used whereas our ears can detect sounds at frequencies only between 30 and 16 000 Hz. A probe containing a quartz crystal, which can both transmit and receive high-frequency vibrations, is passed over the surface of the material to be tested (Figure 7-4). The probe is connected to an amplifier, which converts and amplifies the signal, before it is recorded on the cathode-ray tube.



7-53

Figure 7-4

Under normal conditions, the vibrations will pass from the probe. unrestricted through the metal, and be reflected from the bottom inside surface at 'B' back to the probe, which also acts as a receiver. Both the transmitted pulse and its echo are recorded on the cathode-ray tube, and the distance 'T1', between the peaks of the pulse is proportional to the thickness 'T' of the test material. If any discontinuity is encountered, such as the blowhole 'D', then the pulse is interrupted, and reflected back as indicated. Since this echo returns to the receiver in a shorter time. an intermediate peak appears on the cathode-ray tube. Its position relative to the other peaks indicates the distance of the fault below the surface. This method is particularly suitable for detecting faults in sheet, plate and strip materials more than 6 mm thick. Its modified equipment is used for detecting faults in welds.

Deformation and Recrystallization


8-54

CHAPTER 8

Deformation and Recrystallization 8.0 Slip and Work Hardening:

When the shape of a piece of metal is changed by the application of forces, deformation takes place. At first crystals within the metal are distorted in an elastic manner and it increases proportionally with the increase in stress. If the stress is removed during this stage, the metal returns to its original shape. If the stress is increased further, a point is reached (the yield point) and the layers or planes of atoms begin to slide over each other. This process of 'slip' is irreversible, and so, if the stress is now removed, permanent deformation remains in the metal, called the plastic deformation (Figure 8-1).

Figure 8-1

Slip of this type can occur along a suitable plane, until it is prevented by some fault or obstacle within the crystal A further increase in the stress will then produce slip on another -plane or planes, and this process goes on until all available slip planes in the piece of metal are used up. The metal is then said to be 'work-hardened', and any further increase in stress will lead to fracture. Microscopic examination will show that individual crystals have become elongated and distorted in the direction in which the metal was deformed. By this phenomenon, the metal becomes hard and strong; but it loses its ductility.

Table 8-1 Recrystallization temperature of some metals. Metal Recrystallization temperature (0C)

Tungsten 1200 Nickel 600

Pure iron 450 Copper 190

Aluminium 150 Zinc 20 Lead tin

Below room temperature; hence they Cannot be ‘cold-worked’



8-55

8.1 Step-Step Movement of Dislocations:

Slip does not occur simultaneously by one block of atoms sliding over another block across a complete crystal plane. It occurs step-by-step by the movement of faults or discontinuities within the cystallographic planes. These faults where atoms are missing within a crystal (Figure 8-2) are known as 'dislocations'. When the crystal is stressed to its yield point these dislocations will move stopped by some obstruction such as the atom of some alloy metal which is larger (or smaller) than those of the parent metal. Movement of dislocation is stopped also by a crystal boundary or another dislocation moving across its path.

Figure 8-2 The step-by-step movement of a dislocation through the crystal structure of a metal requires a much smaller force than would the movement of a large block of ions by simultaneous slip. Therefore, real strength of a metal is much less than that calculated on the baisi of a perfect crystal. A metal becomes work-hardened when all dislocations are jammed against each other or against various other obstacles within the crystals or against the crystal boundaries.

8.2 Deformation by Twinning:

Some metals deform plastically by a process other than slip, known as 'twinning'. During slip, all ions in a block move through the same distance. But in the case of twinning (Figure 8-3), ions within each successive plane in a block move through different distances. When twinning is complete the lattice direction wilt have altered so that one half of the twinned crystal is a mirror image of the other half. The stress required to cause deformation by twinning is generally higher than that needed to produce deformation by slip.



8-56

Figure 8-3 8.3 Annealing:

When a metal is deformed or worked at normal temperature i.e. without the application of heat, it is said to be cold-worked. A cold-worked metal has considerable internal stress due to the presence of elastic strains internally balanced within the distorted crystal structure. Annealing is the process by which these residual stresses developed as a result of are relieved and the metal regains its original ductility. It occurs in three stages i.e. the relief of stress, recrystallization, and grain growth.

8.4 The Relief of Stress:

As the temperature of the cold-worked metal is gradually raised, some of the internal stresses disappear, as atoms and dislocations move through small distances into positions nearer equilibrium. At this stage, there is no alteration in the generally distorted appearance of the structure, and the strength and hardness produced by cold-working remain high.

8.5 Recrystallization:

When the temperature is further increased, a point is reached where new crystals begin to grow from nuclei which form within the structure of the existing distorted crystals. As the new crystals grow, they take up atoms from the old distorted crystals which they gradually replace. Unlike the old crystals, which had become elongated in one direction by the cold-working process, these new crystals are small and regular in shape. This phenomenon, known as Recrysiallizanon. The minimum temperature at which it will occur is called the 'Recrystallization Temperature' for that metal. The resrystallization temperature for a metal cannot be determined precisely because it varies with the amount of cold-work to which the metal had been subjected before the annealing process. The more heavily the metal is cold-worked, the greater the internal stress, and the lower the temperature at which recrystallization will begin. Alloying, or the presence of impurities. raises the recrystailization temperature of a metal.



8-57

8.6 Grain Growth:

If the annealing temperature is well above that for recrysiallization of the metal, the new crystals will increase in size by absorbing each other, until the resultant structure becomes relatively coarse-grained. This is undesirable, since a coarse-grained material is generally less ductile than a fine-grained material of similar composition. Both the time and temperature of annealing must be controlled, in order to limit grain growth. Temperature has a much greater influence on grain growth than does time. The amount of cold-work the material receives prior to being annealed also affects the grain size produced. In heavily cold-worked metal, the amount of locked-up stress is great and so, when heat is supplied during annealing, a large number of new crystal nuclei will form instantaneously as the recrystallization temperature is reached. So, the resultant crystals will be small since there will be less space in which individual crystals can grow. In very light cold-work metal, very few nuclei are formed, because the metal is not highly stressed. Consequently the grain size will be large, and the ductility poor.

8.7 Normalizing:

The term Normalizing is used in the engineering industry to denote a similar treatment to Annealing. The difference is that an annealed metal is usually allowed to cool in the heat treatment furnace after being heated, while a normalized metal is taken out of the furnace and allowed to cool in air.

8.8 Cold-Working Processes:

Most metals and alloys are produced in wrought form by hot-working processes, because they are generally softer and more malleable when hot and consequently require much less energy to shape them. Hot-working processes make use of compressive forces to shape the work-piece. The reason for this is that metals become weak in tension at high temperatures, so their ductility decreases. Any attempt to pull a metal through a die at high temperature would fail, because the metal would be so weak as to tear. Consequently, those shaping processes in which tension is employed are cold-working processes. Main disadvantage of cold-working is that most metals work-harden quickly during cold-working operations. Therefore, frequent inter-stage annealing is required during cold-working which increases the cost of the process. For example, figure 8-4 shows typical sequence of operations for the manufacture of a cartridge-case by deep-drawing brass sheet. As far as possible, operations involving the use of hot-working by compression are used. rather than cold-working processes.



8-58

Figure 8-4

Some of the reasons for using cold-working processes are: • To obtain the necessary combination of strength, hardness and toughness for

service. Mild steel and most non-ferrous materials can be hardened , only by cold-work.

• To produce a smooth, clean surface finish in the final operation. Hot-working generally leaves an oxidized or scaly surface, which necessitates 'pickling' the product in an acid solution.

• To attain greater dimensional accuracy than is possible in hot-working processes.

• To improve the machinability of the material by making the surface harder and more brittle.

Cold-working processes include: • The drawing of wire and tubes through dies. • The cold-rolling of metal plate, sheet, and strip. • Spinning and flow-turning, as in the manufacture of aluminium kitchenware. • Stretch forming, particularly in the aircraft industry. • Cold-heading, as in the production of nails and bolts. • Coining and embossing.



8-59

8.9 Hot-Working Processes:

A hot-working process is one which is carried out at a temperature well above the recrystallization temperature of the metal or alloy. At such a temperature, recrystallization will take place simultaneously with deformation (Figure 8-5). For this reason, the metal will not work-harden, and can be quickly and continuously reduced to the required shape. The expended energy is minimum because metals are more malleable at high temperatures. Metals remain soft, because they are recrystallizing continuously during the working process. Thus, hot-working saves both energy used and production time. It also results in the formation of a uniformly fine grain in the recrystallized material, replacing the original coarse cast structure. For this reason, the product is stronger, tougher, and more ductile than was the original cast material.

Figure 8-5

The main disadvantage of hot-working is that the surface condition is generally poor, due to oxidation and scaling at the high working temperature. Moreover, accuracy of dimensions is generally more difficult to maintain. Consequently, hot-working processes are usually followed by some surface-cleaning process, such as water-quenching, shot-blasting and/or acid pickling prior to at least one cold-working operation which will improve the surface quality and accuracy of dimensions. The principal hot-working processes are: • Hot-rolling, for the manufacture of plate, sheet, strip, and shaped sections. • Forging and drop-forging, for the production of relatively simple shapes, but

with mechanical properties superior to those of castings: • Extrusion, for the production of many solid and hollow sections (tubes) in

both ferrous and non-ferrous materials.

Mechanical Shaping of Metals


9-60

CHAPTER 9

Mechanical Shaping of Metals 9.0 Hot-Working Processes:

Now we know that a hot-working process is one which is carried out at a temperature above the recrystallization temperature of the material. Deformation and recryslallization take place at the same time. so the material remains malleable during the working process. Intermediate annealing processes are therefore not required, so working takes place very rapidly. Some of these processes are as under:

9.1 Forging:

The simplest metal-working process is that of hand-forging. With the aid of simple tools called 'swages', the smith can produce relatively complex shapes, using either a hand- or a power-assisted hammer.

9.1.1 Drop-Forging:

If large numbers of identically shaped components are required, then it is convenient to mass-produce them by drop-forging. A shaped two-part die is used, one half being attached to the hammer, whilst the other half is carried by a massive anvil. For complicated shapes, a series of dies may be required. The hammer, working between two vertical guides, is lifted either mechanically or by steam pressure, and is then allowed to fall or is driven down (Figure 9-1) on to the metal to be forged. This consists of a hot bar of metal, held on the anvil by means of tongs. As the hammer comes into contact with the metal, it forges it between the two halves of the die.



9-61

Figure 9-1

9.2 Hot-Pressing:

This is a development of drop-forging and is generally used in the manufacture of simpler shapes. The drop hammer is replaced by a hydraulically driven ram, so that. instead of receiving a rapid succession of blows, the metal is gradually squeezed by the static pressure of the ram. The downwards thrust is sometimes as great as 500 MN. The main advantage of hot-pressing over drop-forging is that mechanical deformation takes place more uniformly throughout the work-piece, and is not confined to the surface layers, as it is in drop-forging, This is important when forging large components, which may otherwise have a non-uniform internal structure.



9-62

9.2.1 Hot-Rolling:

Traditionally a steel-rolling shop consists of a powerful ' reversing mill (Figure 9-2) to reduce the section of the incoming white-hot ingots, followed by trains of rolls which are either plain or grooved according t0 the product being manufactured. Now that the bulk of steel is produced from continuous-cast ingots, which leaves the casting unit via a series of guide rolls which convey it to a train of reduction rolls followed by a set of finishing rolls. The finished strip passes into. a coiling machine and is cut by a flying saw as required.

Figure 9-2

Hot-rolling is also applied to most non-ferrous alloys in the initial shaping stages but finishing is more likely to be a cold-working operation.

9.3 Extrusion:

The extrusion process is used for shaping a variety of both ferrous and non-ferrous alloys. The most important feature of the process is that, in a single operation from a cast billet, quite complex sections of reasonably accurate dimensions can be obtained. The billet is heated to the required temperature and placed in the container of the extrusion press (Figure 9-3). The ram is then driven forward hydraulically. with sufficient force to extrude the metal through a hard alloy-steel die. The solid metal section issues from the die in a manner similar to the flow of toothpaste from its tube.



9-63

Figure 9-3 9.4 Cold-Working Processes:

The surface of a hot-worked component tends to be scaled, or at least heavily oxidized, so it needs to be sand-blasted or 'pickled' in an acid solution if its surface condition is to be acceptable. A much better surface quality is obtained if the workpiece is cold-worked after being pickled. Therefore, some degree of cold-work is applied to most wrought metallic materials as a final stage in manufacture. Cold-working is also a means of obtaining the required mechanical properties in a material. By varying the amount of cold-work in the final operation, the degree of hardness and strength can be adjusted. Some operations which involve drawing or pulling the metal can be carried out only on cold metals and alloys, since ductility is usually less at high temperatures. Some cold-working processes are as under:

9.5 Cold-rolling:

Cold-rolling is used during the finishing stages in the production of both strip and section, and also in the manufacture of foils (Figure 9-4). To roll very thin material, small-diameter rolls are necessary; and. if the material is of great width, this means that the working rolls must be supported by backing rolls, otherwise they will bend to such an extent that reduction in thickness of very thin material becomes impossible. For rolling Thicker material, ordinary two-high mills are generally used. The production of mirror-finished metal foil necessitates the use of rolls with a highly polished surface.



9-64

Figure 9-4

9.6 Drawing:

Drawing is exclusively a cold-working process, because it relies on high ductility of the material being drawn. Rod, wire and hollow sections (tubes) are produced by drawing them through dies. For example, in the manufacture of wire (Figure 9-5), the material is pulled through the die by winding it on to a rotating drum. In all the cases, the material is lubricated with oil or soap before it enters the die aperture.



9-65

Figure 9-5

9.7 Stretch-forming:

In any forming process, permanent deformation can only be achieved in the workpiece if it is stressed beyond the elastic limit. In stretch-forming, this is accomplished by applying a tensile load to the work-piece such that the elastic limit is exceeded, and plastic deformation takes place. The operation is carried out over a form-tool or stretch-block, so that the component assumes the required shape. In the 'rising-table' machine (Figure 9-6), the workpiece is gripped between jaws. and the stretch-block is mounted on a rising table which is actuated by a hydraulic ram. Stretch-blocks are generally of wood or compressed resin-bonded plywoods, though other materials, such as cast synthetic resins, zinc-base alloys, or reinforced concrete, are also used. Proper lubrication of the stretch-block is necessary.

Figure 9-6



9-66

9.8 Coining and Embossing:

Coining is a cold-forging process in which deformation takes place entirely by compression. It is confined mainly to the manufacture of coins, medals, keys, etc. High pressures are necessary to produce sharp impressions. and this limits the size of work which is possible. The coining operation is carried out in a closed die (Figure 9-7). Since no provision is made for the extrusion of excess metal, the size of the blanks must be accurately controlled to prevent possible damage to the dies, due to the development of excessive pressures. Embossing differs from coining in that virtually no change in thickness of the workpiece takes place during pressing. Consequently, the force necessary to emboss metal is much less than in coining. The material used for embossing is

Figure 9-7

generally thinner than that used for coining, and the process is affected by using male and female dies (Figure 9-8).

Figure 9-8



9-67

9.9 Powder Metallurgy:

Powder-metallurgy processes were originally developed to replace melting and casting for those metals, called 'refractory' metals, which have very high melting-points. For example, tungsten melts at 3410 °C. and this is beyond the softening temperature of all ordinary furnace-lining materials. Hence tungsten is produced from its ore as a fine powder. This powder is then 'compacted' in a die of suitable shape at a high pressure Under the high pressure, the particles of tungsten become joined together by 'cold-welding' at the points of contact between particles. The compacts are then heated to a temperature above the recrysiallization temperature. This treatment causes recrystallization to occur and in this way the crystal structure becomes regular and continuous as grain-growth takes place across the original boundaries where cold welds have formed between particles. This heating process is known as Sintering. Although powder-metallurgy was originally used to deal with metals of very high melting-point, its use has been extended for other reasons, such as:

• to produce metals and alloys of controlled porosity, e.g. stainless steel

filters to deal with corrosive liquids, and also oil-less bronze bearings.

• to produce alloys of metals which do not mix in the molten state, e.g. copper and iron for use as a cheap bearing material.

• for the production of small components such as the G-frame of a micrometer screw gauge where the negligible amount of process scrap makes the method competitive.

Metals


10-68

CHAPTER 10

Metals 10.0 Pure Metals:

Pure metals are chemical elements that are usually solid at normal temperatures and become liquids when heated to very high temperatures. When a very hot liquid metal is allowed to cool, it begins to crystallize. These crystals grow as the liquid metal cools until a solid metal mass results. This solid mass is made up tiny crystals. These crystals may form in different sizes and shapes, and from this crystal formation the metal gains its properties of strength, hardness, ductility, etc.

10.1 Alloys:

To some extent different hot molten metals will dissolve in wach other. When a liquid containing two or more metals dissolved in each other cools, it forms crystals containing all these elements and this is termed as an Alloy. Thus, if we control the crystal structure by carefully choosing such items as alloying elements, rate of cooling and mechanical treatment, we can closely predict the final properties an alloy will have.

10.2 Ferrous Metals:

All metals may be divided into two general classes, the ferrous metals covering the many varieties of iron and steel and the non-ferrous metals covering all other types. In the extraction of iron from iron ore, the ore is burned in a fuel, usually coke. The carbon in this coke will dissolve in the molten iron upto a maximum of 5% and the liquid cools to form a carbon-iron alloy, the basic ingredient of all ferrous metals. This carbon-iron alloy may be produced with an extensive variety of crystal structure but basically they all fall into two general categories; the steels, where the carbon content is less than 1.7% and the cast irons, where the carbon content is over 1.7%. Generally, the more carbon contained in the alloy, the harder, more brittle and less ductile it will be. Cast iron is strong in compression and brittle. It can be produced in a variety of strength and hardness by varying the cooling rate. Generally, this material is

Metals


10-69

used where weight and toughness are less important than cost, hardness and strength in compression. As an example, pump casings in conventional areas of an NPP are made of cast iron. Steels are produced in varying amount of carbon content. in addition to it, they are often produced with other alloying elements e.g. with chromium and nickel which results in stainless steel, a very corrosion resistant metal. Stainless steels are used for moderator components, such as valves and pumps, and turbine blades where the properties of hardness and strength together with good corrosion resistance are essential. Low carbon steels are generally used for fabricating steel plates, medium carbon steels are used for such items as shafts, whereas high carbon steel is used for springs, tools, etc.

10.3 Non-Ferrous Metals:

Some of the more important non-ferrous metals are copper, nickel, zinc, tin, lead, zirconium and aluminium. These are produced both as pure metals and as alloys of different non-ferrous elements. The non-ferrous alloys contain neither iron or carbon. Since they are generally more expensive than ferrous metals, they are commonly used where their distinctive properties such as light weight, corrosion resistance, electrical conductivity etc. are important. Aluminum has high corrosion resistance and can be produced as an alloy with such elements as copper, silicon, manganese so that it has strength approaching that of mild or low carbon steel. Of considerable importance are the properties of aluminium being light in weight and having a comparatively small tendency to capture neutrons. Zirconium is an expensive metal. It shows greater strength than aluminum under conditions of high temperature and pressure and also has good corrosion resistance and low neutron capture probability.

Soldering and Brazing


11-70

CHAPTER 11

Soldering and Brazing 11.0 Introduction:

Apart from purely mechanical methods, such as riveting, the chief methods available for the joining of metals are soldering, brazing, and welding. Industrial applications of these processes are many and varied. In soldering, complete or incipient fusion takes place at the surfaces of the two pieces of metal being joined, so that a more or less common crystal structure exists as we pass across the region of the joint.

11.1 Soldering:

A solder must be capable of "wetting", that is, alloying with the metals to be joined, and at the same time have a freezing range appreciably lower, so that the work itself is in no danger of melting. The mechanical strength of the solder must also be adequate. Alloys based on tin or lead fulfil most of these requirements for a wide range of metallurgical materials which need to be joined, since tin will alloy readily with iron, copper, and lead. At the same time tin-lead alloys possess mechanical toughness, and melt at temperatures between 183°C and 250°C, which is comfortably below the point at which deterioration in properties of the metals to be joined will take place. Plain tin-lead solders are of two main types, depending upon whether they are to be used by the tinsmith or the plumber. Best-quality tinman's solder contains 62% tin and possesses distinct advantages in that it will pass quickly from complete liquid to complete solid without any intermediate pasty stage, during which a joint might, if disturbed, be broken. Nevertheless, since tin is an expensive metal compared with lead, the tin content may be reduced to 50% ("coarse" tinman's solder) or even less. Plumber's solder contains about 67% lead, and will consequently be pasty between 183°C and about 265°C. This extended range over which the alloy will be in a pasty state is of advantage to the plumber, since it enables him to "wipe" joints in lead piping, a feat which would be almost impossible to accomplish with an alloy melting or freezing over a small range of temperatures.



11-71

Solders are sometimes strengthened by the addition of small amounts of antimony which, within the limits in which it is added, remains in solid solution. When copper alloys are soldered it is always possible that at some point in the joint the concentrations of copper and tin will be such that a hard brittle copper-tin intermetallic compound such as Cu31Sn8, or Cu6Sn5, will be formed and cause brittleness in the joint. This difficulty can be overcome by soldering copper with an alloy consisting of 97.5% lead and 2.5% silver, for, whilst lead and copper are insoluble in each other, silver alloys with each, and thus acts as a metallic bond between the two. A list of representative solders is given in Table 11-1.

Table 11-1 Tin-Lead –Base Solders

Composition (%) Solidification Range (0C)

Types and uses

Sn Pb Sb Ag 62 38 - - Solidification

Begins and Ends at 183 0C

Tinman’s solder

50 50 - - 220-183 Coarse tinman’s solder 33 67 - - 260 – 183 Plumber’s solder 31 67 2 - 235 – 188 Plumber’s solder for wiping joints

43.5 55 1.5 - 220 – 188 General-purpose solder 30 69.35 - 0.65 250 – 180 Substitute solder for general purposes 18 80.8 0.8 0.4 270 – 180 Substitute plumber’s solder 12 80 8 - 250 – 243 For soldering iron and steel - 97.5 - 2.5 Solidifies at

205 0C For soldering copper and its alloys

In order that the solder shall "wet" the surfaces of the metals to be joined, the latter must be clean and free from oxide. (This makes it very difficult to solder alloys containing aluminium.) To clean the metal surface a flux is used which will dissolve the thin oxide layers which might form before the surface can be wetted by the solder. Possibly the best known flux is hydrochloric acid ("spirits of salts"), or the acid-zinc chloride solution which is obtained when a piece of zinc is dissolved in hydrochloric acid. Whilst being most effective in action, these fluxes have the disadvantage that the residue they leave behind is likely to lead to corrosion of the metal near to the joint. If it is inconvenient, therefore, to wash the finished joint, an organic type of flux is safer to use. Such fluxes usually have a resin base and are almost completely non-corrosive,



11-72

though only really effective on copper and tin-plate. Fig. 1 illustrates the action of a suitable flux during a soldering operation.

At (A) the oxidised metal surface is covered by a layer of flux solution, which begins to boil as the soldering bit moves towards it. The boiling flux rapidly becomes more concentrated and dissolves the oxide film (B), leaving a clean metal surface covered by a layer of protective flux (C) . Immediately the molten solder wets this clean surface and begins to alloy with it (D). As the soldering bit-moves away- the film of solder cools and solidifies (E) .

Figure 11-1

11.2 Brazing:

Although the technique of this process may vary to some extent, metallurgically it is similar to soldering. Brazing is used when a tougher, stronger joint is required, particularly in alloys of higher melting point than those usually joined by soldering. Most ferrous materials and non-ferrous alloys of sufficiently high melting point can be joined by brazing. A borax-type flux is generally used, though for lower temperatures involved in high-grade silver soldering, a fluoride type of flux may be used. Ordinary brazing solder contains about 50% copper and 50% zinc. Higher-grade brazing compounds, or silver solders, contain over 50% silver. Cheaper grades of silver solder contain between 10 and 20% silver and about 50% copper. Typical brazing alloys are shown in Table 11-2 .



11-73

Table 11-2 Brazing Solders and Silver Solders

Composition (%) Freezing Range (0C)

Type

Cu Zn Ag Cd Sn 50 50 - - - 870 – 880 Ordinary brazing alloys for 50 45 - - 5 750 Ferrous materials 16 4 80 - - 740 – 795 High-grade silver solders for use on 20 15 65 - - 695 – 720 brass and light-guage copper, monel

15.5 16.5 50 18 - 625 – 635 and stainless steel 45 30 20 5 - 775 – 815 Lower – grade silver soldering 52 38 10 - - 820 - 870

Shows example of four common types of brazed joint.

Figure 11-2

Welding


12-74

CHAPTER 12

Welding 12.0 Welding:

Some welding processes resemble both soldering and brazing n that molten metal is applied to produce a joint between the two pieces. However, in welding the added metal is, more often than not, of similar composition to the metals being joined, and .he joined pieces are melted locally so differences between the weld metal and the pieces being joined are structural rather than compositional.

Welding calls for rather different techniques to those employed in brazing and soldering. The speed of working is particularly important if complete melting of the metal near the joint is to be avoided. Some welding processes rely on pressure to effect joining of the two halves and in such cases no metal is added to form a joint, the weld metal being provided by the two parts being joined. Thus we have both fusion and pressure-welding processes, as indicated in Figure 12-1.

Figure 12-1

Welding


12-75

12.1 Fusion Welding Processes:

In all fusion welding high temperatures are employed in or to melt the parts quickly. In this way damage to the unmelted metal near the weld is minimized since the time at high tempera is reduced. Also heat must be supplied fast because the pieces being joined will conduct heat away from the weld area at quite high rate. Fusion welding can be classified according to the source o! heat. The most common sources are gas flame and electric arc,1 chemical reaction, laser beams, and electron beams are also used in special applications.

Gas flames usually use acetylene and oxygen, since both gases are readily available and very high temperatures are easily attained Hydrogen and oxygen can be used to get even higher temperatures the atomic hydrogen welding torch, which uses an electric arc to supplement the heat of combustion.

Electric arcs can be struck between two non-consumed elect a non-consumed electrode and the work, or between a consumed electrode and the work. The third is, perhaps, the most common; the electrode is continually melted to supply the weld metal nee to fill the weld. When non-consumable electrodes are used (carbon and tungsten are common),a separate filler rod is melted in the arc to fill the weld.

In all welding the molten metal is shielded by non-reactive gas to prevent oxidation. In gas flame welding the combustion products form this shield, but in electric arc methods it must h provided either by a continuous supply of gas (such as argon or helium) from a cylinder or by including a combustible material ii the flux.

Flux is used in most welding processes to slag off oxides and impurities in the molten metal as well as to burn and provide an unreactive gas shield around the molten metal. While flux is usually supplied to the joint by coating it on filter rods, it sometimes applied as a loose powder which is piled on the area be welded. Common welding methods are illustrated for your information in Figures 12-2 to 12-6.

Welding


12-76

Fig. 12.3

Fig. 12.2

Welding


12-77

Figure 12-4

Figure 12-5

Welding


12-78

Figure 12-6 12.2 Pressure Welding Processes:

In pressure welding no extra metal is added to the joint. These processes can be divided into two classes according to whether r not the joint is ever molten. The blacksmith's forge is the most common example of pressure welding without melting. The pieces to be joined are heated above their recrystallisation temperature and forced together by hammering. At the joint any oxide on the surface gets broken up, and direct metal to metal contact is established. A metallic bond is formed wherever there is direct contact, and the metal recrystallises across the joint forming a continuous crystal structure. A more modern version of this is ultrasonic welding, where the two surfaces to be joined are rubbed together by high frequency sound waves. This rubbing breaks up the oxide film and friction supplies the heat. Fine wires are welded to microscopic electronic circuits in his way.

Other common pressure welding processes depend on electric current to heat the weld area until it is soft or even molten. The parts to be joined are then pushed together and the hot met cooled. All such methods rely on the electrical resistance of weld area (caused by oxide on the surfaces and the irregularity the surfaces) to localize the heating. Such processes include spot welding, seam welding and butt welding (which is used to end caps onto CANDU fuel elements). These resistance welding methods are illustrated in Figures 12-7 to 12-10.

Welding


12-79

Figure 12-7 Figure 12-8

Figure 12-9 Figure 12-10

12.3 Consequences of Welding:

A welder may boast that his weld is stronger than the parent metal. He backs up this statement by the fact that welds seldom fail before metal near a weld. The fallacy is that the metal at ' point of failure may be much weaker or more brittle than the bulk of the material as a direct result of his welding. You will recall from previous lessons that heat can have quite severe effects the grain structure; uncontrolled heating is almost always detrimental. In steel the metal very close to the weld has been heated high in the austenite region and so the austenite may have suffered grain growth before transforming, on cooling, to pearlite or martensite. Further from the weld the steel would have been transformed to austenite but the temperature would not have been high enough to cause grain growth. Heat conduction through the steel to the cold surrounding metal might have cooled the austenite fast enough to form martensite. Further out yet, the steel would not re been transformed but will have been held in the pearlite grain growth region and considerably

Welding


12-80

softened. Thus the metal surrounding a weld could have a wide variety of structures. Since most steel plate is supplied hot rolled (a fine pearlite structure which is very tough, fairly strong and quite ductile), most of the welding induced structures will have worse properties in one way or another i.e. the region around a weld will have soft patches an brittle patches. These structures might lead to tensile failure creep (lower U.T.S. of the softened material) or fatigue or brit failure (low ductility of martensite). The degredation described above applies to steel; other problems arise when welding other materials. Aluminum alloys are very susceptible to high temperatures which can leave them very soft. Stainless steel can suffer a great loss in corrosion resistance near a weld (this is called weld decay). Any metal is liable to be softer and less ductile owing to grain growth.

These are all failures due to the alteration of the metal' s crystal structure. Other failures happen for different reasons. Welds may fail because of the existance of slag or blow holes in the weld material itself. The weld may not have penetrated right through the area to be joined or there may be incomplete melting of the parent metal leaving the weld unjoined to the component in some places. These and other faults such as cracking of the well or adjacent metal can be detected in several ways, such as X-ray; gamma radiography, ultrasonic testing, and crack detection and so will usually be detected before the weld is put into nuclear service. Structural changes can not be easily detected without destroying the component and so are far more dangerous.

Concrete


13-81

CHAPTER 13

Concrete 13.0 Concrete:

Concrete consists of a mixture of cement, water and an inert matrix of sand, and gravel or crushed stone. Although all the components of concrete are essential, the cement is the most important because it is the "weakest link in the chain". The cement binds the sand and stone together and fills up the voids between the sand and stone particles.

13.1 Cements: Cements or cementing materials are substances which, when mixed with water to form a paste, possess the ability to harden into a solid. It is important to realize that this hardening is the result of a chemical reaction between the cement and the water, and not the result of an evaporation process. Most cementing materials in fact will harden either in air or under water. Such materials are termed hydraulic cements, and concretes made up with hydraulic cements are said to possess the property of hydraulicity, or the ability to harden under water. In common use are the non-hydraulic cements, gypsum and hydrated lime, and their hydraulic counterparts, natural cement and Portland cement.

13.1.1 Gypsum:

Gypsum is a non-metallic rock found in many section of the world, and widely distributed throughout the United States. Chemically it is CaSO4.2H20. Commercially it is used as "plaster of par is", with the chemical formula, CaSO4.1/2H2O. On the addition of water to plaster of paris, a mixture results which has the property of setting by the action of recombination of some of the mixing water with the plaster of paris (CaSO4.1/2H2O) to bring it back to the original chemical composition of gypsum, CaSO4.2H2O. This action causes the mixture to harden, and subsequent removal of excess water by evaporation leaves a hard, rock-like mass.

13.1.2 Lime: Common lime, known commercially as "quicklime", is composed largely of calcium oxide (Ca0), a small amount of magnesium oxide (Mg0) , and a trace of clay. It is made by calcining* natural limestone (largely calcium carbonate,

Concrete


13-82

CaC03.). Upon the addition of a limited amount of water, the calcium oxide is converted to calcium hydroxide, Ca (OH)2 with consider able evolution of heat and change in volume. This is then referred to as slaked or hydrated lime. Commercially, all quicklime is slaked in a kiln. Hydrated lime hardens upon the addition of water in the presence of air, by absorbing carbon oxide from the air to become calcium carbonate. It is used as the cement component in mason's plaster, mortar and stucco.

13.1.3 Natural Cements: Natural cement is the finely pulverized product of the calcination of a clay-bearing limestone, and is of variable quality. It is not normally recommended for placement under water. In addition, natural cements normally set so rapidly that it becomes necessary to add a considerable amount of gypsum in order to control the set within desired limits. Thus, since rigid product control is difficult, natural cement has nowadays been almost entirely displaced by portland cement.

13.1.4 Portland Cement: Portland cement is an artificial mixture of lime-bearing and clay-bearing materials which is kiln-dried and subsequently ground to a fine power. This power is then fed into the elevated end of a long, slightly-inclined, slowly-rotating heated kiln, from which it emerges as a hard lumpy clinker, which is then coarse-ground. A calculated amount (2-3%) of raw gypsum is added to retard its rate of setting, and the whole mixture is subjected to further grinding to a fine powder. Unlike the cements discussed previously, there are two distinct stages in the setting of a portland cement: the initial set, which usually requires anything from about 45 minutes to 10 hours, depending upon the composition of the cement and the ambient conditions; and the final set, which may take days and even weeks to become complete. It is interesting to note that in a huge structure such as a dam, it is usually necessary to incorporate pipes for cooling water inside to remove the heat of reaction with the water of the concrete. All portland cements are hydraulic, have high strength and durability, and are used in making concrete for all aspects of modern constructional work. * Calcining is roasting a more or less finely-divided substance in a kiln or drum; the product is normally a metal oxide.

Concrete


13-83

13.2 Concrete Aggregates:

The inert materials such as sand, gravel, and crushed stone which are mixed with cement and water in making concrete are referred to as aggregates. Aggregates are classified into two groups, according to their size. Those retained on a ^ inch or No. 4 sieve are coarse. All particles which pass through this sieve are referred to as fine. The coarse aggregate is used primarily for the purpose of providing bulk to the concrete, whereas the fine aggregate assists the cement paste in holding the concrete together by filling in the voids between the coarse aggregate particles. Although the quality of a concrete mix is determined principally by the quality of the cement paste, the aggregates in general must possess physical properties at least equal to the properties desired in the concrete. For instance, one could not reasonably expect to make a high density concrete from low density aggregates, cement and water. Properties of concrete which are partly attributable to aggregates include density, strength, modulus of elasticity, creep, fire resistance and resistance to wear and abrasion.

13.3 Admixtures: Any material in concrete other than cement, aggregates and water that is added in the mixer, or to one of ingredients before mixing, is termed an admixture. For instance, calcium chloride can be used in amounts up to 2% of the weight of the cement to accelerate its setting. It is extremely deliquescent (attracts water from its surroundings), and its reaction with water (hydration) is quite exothermic (gives off heat). By using this material in cold weather, the length of time required for protection with covers and artificial heat can be reduced. It is not safe, however, to expose freshly poured concrete to temperatures below freezing even though calcium chloride has been added, since in the small percentages used, the freezing point is not appreciably lowered. Prevention of freezing of concrete during setting is possible by the addition of sufficient amounts of certain compounds. The amount of compound necessary to lower the freezing point of water, however, also causes serious loss of strength and durability. Such anti-freeze compounds are therefore not normally used unless their addition in the required amounts can be shown by tests not to adversely affect the properties desired in the concrete. When frequent freeze-thaw cycles are likely, entrained air can be incorporated to provide space for expansion. This was done at NPD.

Concrete


13-84

Similarly, finely-ground gypsum (not exceeding 3% by weight of the cement) is commonly used as a "retarder" to overcome the acceleration in set caused by hot weather, or to delay stiffening in difficult placing jobs. Admixtures used as waterproofing agents in concrete include soap, oils and hydrocarbons. In concrete, soap functions as a water repellent and is of use in situations where exposure to moisture is at worst only moderate. Under more adverse conditions, certain oils and hydrocarbons are more effective.

13.4 Proportioning of Concrete: The proportions of the component materials (cement, water and aggregates) in a concrete mix directly affect the properties of both the mix and the hardened concrete. The cement-water paste is the major active ingredient in the mix and the quality of the concrete, in so far as proportioning is concerned, is determined by the quality of the paste. As water is added to cement, the paste so formed becomes increasingly more dilute, with a corresponding decrease in the ultimate strength of the hardened concrete. This is illustrated in Figure 13-1, which shows the relationship between compressive strength and the water-cement ratio.

Figure 13-1

Concrete


13-85

Of all the water added to the cement to form the paste, only a small percentage is necessary for the hydration of the cement; the remainder, though it decreases strength, is necessary to provide the proper workability in the mix. As a general rule, therefore, the amount of mixing water used should be kept to a minimum. From a purely economic standpoint it is desirable to use as little cement as possible, since this is the most expensive constituent of concrete. To achieve this objective it is important to choose a suitable ratio of coarse to fine aggregates, as the greater the percentage of fines in a mix, the larger will be the surface area to be covered by the cement paste. In addition, economy can be further improved by using larger sizes of coarse aggregate since such a mix would have fewer voids to be filled, thus required a reduced amount of cement paste. This is illustrated in Fig. 13-2.

Figure 13-2

A fairly typical mix for a normal type concrete would be 1: 3: 5: by volume (ie, 1 part Portland Cement; 3 parts fine aggregate; 5 parts coarse aggregate; all by volume). Any voids or air pockets existing between pieces of aggregate in the mix can be eliminated by careful mixing and the use of a vibrator-compacting process as the concrete is poured.

13.5 Reinforced Concrete: Concrete used alone is generally referred to- as plain concrete. Such concrete is suitable only for use in situations where the loads or stresses to which it may

Concrete


13-86

be subjected are of a purely compressive nature. Reinforcement is necessary whenever the concrete is to be subjected to tensile stresses*; this is achieved by embedding in the concrete steel bars which have high tensile strength. Typical strengths for concrete (2400 kg/m3 are 20MN/m2 in compression and 2.4MN/m 2 in tension, while for steel the corresponding values are 185MN/m for both tension and compression. From this we can see how reinforcement works (Figure 13-3)

Figure 13-3

*This includes bending stresses (beams) and buckling (columns). F is the load and R1 and R2 are supports. Here the top fibres are in compression and the bottom ones in tension. This is why reinforcing rods are placed as close to the bottom as possible, ie, to resist the tensile stresses. In addition to enabling the concrete to resist the application of tensile stresses, reinforcement also serves to limit the size of any cracks caused by shrinkage during setting or changes in length by temperature variations.

13.6 Uses and Properties of Concrete: Plain concrete, as used in construction work, will support considerable compressive loads, and if reinforced with steel bars will in addition resist the application of moderate tensile stresses. As a structural material its uses are further enhanced by its good wear resistance, durability and resistance to corrosion, which, as you might have expected, are dependent upon the choice of suitable aggregates. Similarly, the fire resistance of a concrete is directly related to the properties of its associated aggregates. Fortunately, the majority of concretes exhibit excellent resistance to fire - the main exception being those concretes which employ flint aggregates, as these become badly disrupted on exposure to fire.

Concrete


13-87

All in all, it is true to say that reinforced concrete is, for most applications, the least expensive fully fireproofed construction material available. It is particularly suited for use in the construction of multi-storey buildings, although its relatively high density, (around 2400kg/m3 ), will usually limit the number of storeys to seven, since above this, the size of the columns required and their associated dead weight becomes unduly excessive. Apart from its bulk, however, the only significant limitation of concrete as a construction material is the difficulty in altering a concrete column, as compared with, say, a steel girder or a wooden beam.

13.7 Pre-stressed Concrete: By passing steel cables through ducts in concrete, and putting these under considerable tensile stress in such a manner that the stresses will oppose any subsequent loads, "pre-stressed"concrete can be made. There are two methods of pre-stressing: 1. "pre-fab" at the factory (pre-tensioning) 2. gradual on-site (post-tensioning)

The first is more economical, and is more widely used in conventional construction projects. It consists of stretching tendons (cable) near the base of the form and then pouring the concrete on top. When the concrete has set sufficiently (normally 28 days), the clamps holding the tendons are released, transferring the load to the concrete (Figure 13-4):

Figure 13-4

Tendon

Concrete


13-88

The solid lines represent the initial state, and the dotted lines the final pre-tensioned state. The bending shown here is greatly exaggerated but the bending in actual beams can be observed I with the eye. From this it can be seen that when such a beam is put into use, the forces already present have to be overcome (reversed) before any further stresses are exerted. In other words, when the beam is loaded the compressive forces in the bottom concrete fibres have to be overcome before they can go into tension. On-site pre-stressing consists of applying the design load gradually to already-cast concrete while increasing the stress on the tendons. This prevents possible failure from tension (buckling) which might occur if complete pre-stressing was done at the beginning. This type of pre-stressing has the feature that the structure can be designed for the removal of the tendons (if they are likely to corrode, for instance), one at a time, and replacement with fresh ones. This has been done in the giant pre-stressed concrete pressure vessels of Britain's advanced gas-cooled reactors.

13.8 Failure of Concrete:

Like any other structural material, concrete is liable to fail if it is overloaded; as previously pointed out, it is more vulnerable to tensile (and shear) stresses than compression. For brief discussion here, however, are the effects of heat and radiation on the one hand, and some chemicals on the other. Gamma rays which do not penetrate concrete interact with the atoms there; the energy of the rays is converted finally to heat. The chemical identity of the atoms is unchanged. Neutrons, on being slowed down by either elastic or inelastic collisions, give up their lost energy directly or indirectly as heat. We will ignore the effect of the relatively few neutrons which are captured (and thereby produce new atoms). Heat drives water out of the concrete, which can result in cracking and spall ing (pieces of concrete separate from the surface) . There are two types of water in concrete: chemically bound (water of hydration) Heat drives water out of the concrete, which can result in cracking and spalling (pieces of concrete separate from the surface) . and physically trapped (the excess over that required chemically which had to be added for workability) . Heat tends to drive away both particularly, as regards the interior, the latter. The internal stresses resulting from this inner water trying to get out cause cracking. In the case of concrete as a neutron shield, there is another ramification of this water loss: the shielding efficiency decreases. An important contributor to the slowing down of neutrons is elastic scatter, which is done by light nuclei. In concrete this is mainly done by the hydrogen atoms in the contained water.

Concrete


13-89

On the chemical side, the worst offenders are chlorides and strong acids. The acids (particularly nitric and hydrochloric) dissolve the cement, with the result that the remaining aggregate just crumbles away. Chloride and nitrate salts act in a similar albeit much less vigorous way: they render the cement component somewhat water soluble. Acid spills on concrete should be treated with an alkali along with the flushing with water; sodium carbonate. (washing soda) or bicarbonate (baking soda) are safer to work with than aqueous ammonia and especially sodium hydroxide (caustic soda, lye). Concrete sidewalks, etc. can be de-iced just as cheaply using either ammonium phosphate or urea fertilizers as with the highly deleterious common salt (sodium chloride).

13.9 Special Applications of Concrete in a Nuclear Station:

In a nuclear station, concrete serves not only as a structural material but also as a shielding material for gamma-neutron radiation fields. In this respect the dense aggregate materials in the concrete are responsible firstly for reducing the intensity of any gamma radiation fields, and secondly for slowing down any fast neutrons by the process of "inelastic scatter". Subsequently, the hydrogen content of the water of hydration in the cement is responsible for further slowing down these neutrons to thermal energies by the process of "elastic scatter". These thermal neutrons are then readily captured by nearby nuclei, unfortunately with the release of "capture gamma radiation", which of course presents an additional gamma field for aggregate shielding. The effectiveness of concrete as a shielding material depends principally, therefore, upon the densities of its aggregates, since the most effective gamma-absorbing materials consist of high atomic weight elements having high densities. In other words, enhancing the shielding ability of a concrete by using high density aggregates will permit the use of reduced concrete thicknesses without any reduction in shield effectiveness. This fact is advantageous, firstly, in that it permits the use of concrete shields in areas where available space is limited and secondly in that it can reduce heavy water holdup at points where it is necessary for heavy water lines to pass through the shielding.

The density of ordinary concrete (2400kg/m3) can be increased by replacing the usual fine and coarse aggregates with high density ores such as ilmenite, barytes, or magnetite. A heavy concrete employing such aggregates would have a density of around 3500kg/m3. Even higher densities (4800kg/m3) can be obtained by using ferrophosphorus ore and iron shot as the aggregate materials. It should be noted, however, that the 4800kg/m3 concrete is weaker in tension, compression and shear than the 2400kg/m3 material.

Concrete


13-90

Figure 13-5 illustrates some of the locations in which the various density concretes have been utilized in the construction of NPD. In conclusion it should be borne in mind that although concrete is by no means the best shielding material available, it is, on account of its excellent structural qualities, the most economic. Lead, for instance, is a far better gamma shield than concrete, since only a quarter of the thickness is required for a given effectiveness. This is more than offset, however, by the cost, and the fact that being such a poor structural material lead shielding requires some sort of supporting frame.

Likewise steel also makes an excellent gamma shield, and ^ such is useful in shielding doors that would be difficult to fabricate from concrete. Unfortunately, steel is also relative expensive, and if neutron shielding is required, then laminations of some hydrogenous material such as masonite must be included. (See Figure 13-5).

Experience has shown that the weight of material required for given shield is generally the same, regardless of the material used. Thus the use of dense materials such as lead and steel, or for that matter the heavier concretes, can only be justified economically when space is restricted, or when pipes containing heavy water have to be as short as possible.

Concrete


13-91

Figure 13-5

Lubricants


14-92

CHAPTER 14

Lubricants 14.0 Lubricants:

There are basically two types of lubricants in common usage: oil, which is always a liquid at operating temperature, and grease which is generally a solid. Table 14-1 gives a comparision between the two types of lubricants.

Advantages of Grease Advantages of Oil

1. Maintenance may be reduced ; no oil level to maintain, and regreasing is infrequent

1. Oil is easier to drain and refill. This is important if lubricating intervals are close together

2. Proper grease quantity is easily

confined in housing which simplifies bearing design

2. Use of oil makes it easier to control the correct amount of lubricant.

3. Freedom from leakage is important

in food, textile and chemical industries

3. Same lubricant may be used on other types of bearings on the same machine

4. Improved efficiency of labyrinth

enclosures gives better bearing protection

4. If bearing must operate under high temperatures, conditions favour oil,

Table 14-1. Comparative Advantages of Grease and Oil in Bearings 14.1 Oils:

The three general types of oil are as follows: (1) Mineral oils, which are the most common type and are produced by refining petroleum crude; (2) Fixed oils are produced from plants, such as castor shrub and rape seed, and animals. These oils are generally used with mineral oils to improve the oiliness of the resulting mixture; (3) Synthetic oils are man-made chemical compounds with better properties than mineral oils. An example of a synthetic oil is phosphate ester which will be used in the power control system at Bruce,. because of its fire-resistant properties. The properties of an oil may be divided into six main groups:

Lubricants


14-93

1. Flow properties - viscosity, viscosity index, pour point 2. High temperature properties - volatility, thermal stability, evaporation residue, decomposition products and residues. 3. Oxidation characteristics " inhibitor susceptibility / resistance to oxidation. 4. Hydrolysis - resistance to bases, acids, water or steam 5. Solubility - in water, in hydrocarbon solvents 6. Miscibility - how well it mixes with petroleum products.

Now that the properties of oils have been grouped let us examine the properties in greater detail. Viscosity - is basically a measure of the ability of an oil to resist shear stress or in other words an oil of low viscosity flows more easily than an oil of high viscosity. An oil's viscosity is its most important property, as it is this property that ' most affects its load bearing capacity and the amount of internal heat generated. Hence in deciding which oil to use for a particular application, the viscosity must be high enough to maintain the required lubrication film, but not so high that undue friction losses might occur. Fig 14-1 indicates how viscosity is affected by various factors. Viscosity Index - indicates how viscosity depends on temperature; thusan oil of a high viscosity index will show little change in viscosity as temperature varies. The V-I can be improved by the addition of a colloid.

Temperature Stability - is the ability of a lube oil to maintain its load bearing capacity over the range of temperature variations under which it is likely to be used. As the temperature increases an oil may begin to decompose into carbonaceous and tarry products, especially if oxygen or metallic oxides are present in the system.

Flash Point - is the temperature to which a lubricant must be heated before its vapour, when mixed with air, will ignite but not continue to burn. At its fire point a lubricant will keep on burning. Temperature Increasing temperature lowers oil viscosity. A high-viscosity oil can support a heavy load, especially at low temperatures. High-viscosity oils also have more internal friction. Pressure

Lubricants


14-94

Increasing pressure increases oil viscosity. However, this only becomes important when pressures are in the neighborhood of many megapascals. Load Capacity Viscosity of an oil must be matched to the application. The oil must have enough viscosity to handle the load, yet increasing the viscosity causes an increase in fluid friction, which heats the oil and lowers the viscosity. Shaft Speed High speed means faster shearing of oil layers/ and more fluid friction. As temperature goes up/ viscosity goes down to decrease load capacity. However, a high speed helps to form a hydrodynamic wedge in bearings.

Lubricants


14-95

Oil Viscosity Characteristics

Figure 14-1

Increasing temperature lowers oil viscosity, A high-viscosity oil can support a heavy load, especially at low temperatures. High-viscosity oils also have more internal friction.

Increasing pressure increases oil viscosity. However, this only becomes important when pressures are in the neighborhood of many megapascals.

Viscosity of an oil must be matched to the application. The oil must have enough viscosity to handle the load, yet increasing the viscosity causes an increase in fluid friction, which heats the oil and lowers the viscosity.

High speed means faster shearing of oil layers, and more fluid friction. As temperature goes up, viscosity goes down to decrease load capacity. However, an high speed helps to form a hydrodynamic wedge in bearing.

Lubricants


14-96

Pour Point - is the temperature at which an oil just barely flows, under certain prescribed conditions. This quantity is important in refrigeration applications or in any equipment which is intended for operation at low temperatures.

Oiliness - is the ability of a lube oil to cling to the surface of a material. Usually a fixed oil is added to a mineral oil to increase its oiliness. This property is of prime importance in thin film lubrication. In addition to the above properties of an oily a number of additives are available to enhance existing or add further properties to an oil. Some of the more usual additives will be discussed below. Anti-oxidants - deter the reaction of oxygen with oil and also inhibit the catalytic action of metals upon the oil-oxygen reaction and hence extend the useful life of the oil. If oxidation is not prevented, the oil will become acidic and form sludges and may promote corrosion. The effect of heat, especially if "hot spots" are present, on entrained air is primarly responsible for oxidation.

Detergents - aid in maintaining cleanliness in internal combustion engines. In the crankcase the detergent keeps solids sus-pended in the oil so that they can be filtered out elsewhere, while I in the area swept by the rings' the detergent prevents sludge or lacquer from building up. Rust Inhibitors - enable the oil to protect steel surfaces from rusting when in contact with water or moist air within the oil circulating system. These additives act by either forming an adsorbed film which prevents moisture from making contact with the metal or by forming a surface coating by chemical action. Emulsifying Agents - are used to enable the oil to surround each particle of water to prevent metal corrosion. Demulsifiers - aid in the separation of an oil-water emulsion which is usually caused by the presence of contaminants such as rus and sludge. An emulsion can generally be broken by heating to 75°-100°C then allowing adequate settling time. If water is allowed to accumulate in the circulating system it may lead to the formation of an oil-water emulsion which may lead to oil breakdown or corrosion or both. Anti-foaming Agents - are used to prevent entrained air from being turned into foam by turbulence in the system. Foam is undesirable because it can lead to oxidation, interfere with heat transfer, cause spillage at vents as well as creating difficulties in regulatory systems.

Lubricants


14-97

14.2 Greases:

There are four types of grease in use: ie, water resistant, water soluble, multi-purpose, and synthetics. Water resistant grease is generally used in low temperature applications (below 80°c) since above this temperature the water in the grease, which acts as a binder, evaporates allowing the oil to separate and "bleed" 3Ut of the bearing leaving a sticky soap mass which can lead to failure. These greases are usually made by cooking tallow or fatty acids with lime and water thus forming the base or "soap',' then a definite amount of water and oil are added to get the required emulsion of base and oil. The amount of oil added determines the consistency of the grease.

Water soluble (soda based) greases are made from fatty acids, water and lube oil, but caustic soda (Na0H) is used instead of lime. The main difference between these greases and the calcium based (water resistant) greases lies in the fact that the former are more nearly a chemical mixture of soap and oil and don't depend entirely on water content to stay jelled. In general in soda based greases a lower water content means a better products as excessive water causes the grease to lose consistency. At low temperatures these greases become stiff and hence cause high starting torques; also as they are water soluble they cannot be used in contact with water or steam. Multipurpose greases are so named because their worked and unworked properties are similar. They are formed with a barium or lithium base depending on the temperature they are to be used at. The barium base is suitable for use at temperatures up to 200°c and beyond if regreasing is frequent enough to remove by-products that will form at these elevated temperatures, while the lithium base has a dropping point about 175 °C but its low temperature characteristics are far better. The water resistance of a lithium base grease is between a good soda-base grease and a calcium-base grease. Because of their good properties it is often possible to meet all the grease requirements of a large plant with one high quality lithium-base grease. Synthetic greases are made from standard soaps with a synthetic lubricant substituting for mineral oil or are made entirely from silicone. These greases are produced in both water soluble and insoluble types of many consistencies and most types have little or no effect on natural or synthetic rubber. Generally silicone greases are highly resistant to water and oxidation as well as chemical fumes. The synthetics are expensive but useful in applications where there is a wide temperature variation , rapid oxidation or gumming.

Lubricants


14-98

14.3 Solid Lubricants:

In certain solids the interatomic bonds are strong in two directions but weak in the third, hence the material forms layers which can easily slide upon each other. Among these solids are graphite and molybdenum disulphide.

Graphite is used in two forms: natural graphite, a mineral which is used as a lubricant and conductor in electrical brushes and also may be used in a dry form or mixed with solvents for use in such places as oil-less bearings; colloidal graphite is manufactured in an electric furnace from anthracite coal plus petroleum coke forming a soft greasy material, almost chemically pure, then mixed with a solvent. This second form of graphite is good for high temperature applications. Molybdenum disulphide closely resembles graphite in appearance but has twice the density of graphite. This substance starts to slowly oxidize at 400°C but can be used at temperatures up to 565 °C at which it rapidly oxidizes. Molybdenum disulphide is used in the machine tool industry for lubrication.

Plastic, Rubber and Protective Coatings


15-99

CHAPTER 15

Plastic, Rubber and Protective Coatings 15.0 Plastics:

The definition given by the American Society for Testing Materials is that " a plastic is a material that contains as an essential ingredient an organic* substance of large molecular weight, is solid in its finished state, and at some stage in its manufacture or in its processing into finished articles, can be shaped by flow. What this means in English is that it is an organic polymer which is or was moldable.

Plastics in fact usually consist principally of carbon, hydrogen, oxygen and nitrogen. The raw materials normally originate from petroleum and/or coal, whence the monomers are extracted. The monomer is reacted either with itself or with other monomers, usually under the influence of one or more of heat, pressure or a catalyst. Perhaps the most widely known plastic is polyethylene, which is made by reacting ethylene (a low boiling gas manufactured from natural gas) with itself in the presence of heat, catalyst and high pressure: n C2H —> (C2H4) n n typically 100-1000 ethylene polyethylene (monomer) (polymer)

The properties of the plastic depend on the structure of its molecule. Most polyethylene, for example, is flexible and quite soft and tough; this is due to the fact that the molecules are for the most part long straight chains which are not bonded to each other in very many places. Such "sideways-bonding" (known as cross-linking) imparts hardness and higher softening point, but also rigidity and brittleness. Increased cross-linking can be incorporated by gamma, beta or neutron irradiation; this is done with polyethylene, for example, to raise its softening point above the boiling point of water (eg for kitchen utensils). One place where this versatile material is found in the Plant: the high density concrete beams on either side of the hatches above the reactor at NPD contain a layer of polyethylene to aid in neutron shielding. Besides the shape of the * An organic substance is considered to be any compound containing carbon (with the exception of carbonate (CO3) and cyanide (CN) groups). Plastic



15-100

molecules, the properties depend also on the nature of the chemical groups found in the plastic.

The term "plastic" covers as wide a field as does "metal" ; there are, however, two broad classifications: 1. Thermoplastic: Thermoplastic describes plastics that may be softened by heat, and which upon cooling regain their solid state, even if the process is repeated. Typical of this group are polystyrenes, acrylics and vinyls (Table 1 in the Appendix).

2. Thermosetting: Thermosetting describes plastics that solidify or set on heating and cannot be remelted. In general, thermoset materia1 cannot be reshaped once they have been fully cured. Typical of this group are the phenolics (bakelite)and epoxies. (Appendix 2) Plastics are light but for their weight are fairly strong. They can be shaped by relatively simple means (such as die casting, pre"* extrusion). Compared to other materials they have a wide range of colours, are adaptable to mass production methods, have useful physic properties and excellent chemical resistance. The overall cost is usually low.

Their primary disadvantage is susceptibility to heat. Most of the thermoplastics are limited to about 150°C, and even at this temperature the strength begins to deteriorate. Thermosetting plastics will eventually char or even burn, and again when compared to metals have low strengths. The strength problem can be overcome in some respects by fibre reinforcement.

Fibre reinforcement most commonly entails glass, because of the ease with which long, strong fibres can be obtained. In fact/ fibres can be obtained which match the tensile strength of the strongest steels. Glass fibre reinforced plastics find extensive use in aviation. An important application of plastics is as insulators. Whereas the atoms in a piece of metal are held together by a Free-Flowing electron "sea", giving rise to good electrical conductivity, a piece of plastic has no such mobile electrons. All the electrons are held within the individual molecules (most of the valence (outer shell) electrons are involved in covalent bonding)/ and cannot leave ____ unless of course the rated voltage is sufficiently exceeded. If this happens, conduction (with some decomposition) can occur either across the surface or through the bulk of the material.



15-101

Believe it or not, plastics can be springy, especially when glass fibre reinforced (Fibre glass re-wrote the pole vault record books). A most odd and valuable property of many plastics and resins, in fact, is their behaviour under strain. Unlike -metals, in which strain is induced in proportion to the stress applied, plastics show a stress-strain relationship which depends on the rate at which the stress is applied; moveover they deflect much more for a given load. Research is continually working towards the development of better and better plastics. Two excellent examples are duPont's Teflon and Kapton. The former features almost universal chemical resistance while possessing the added property that few things stick to it; the latter is good for continuous service at temperatures up to 4000C, and shows an astonishing capacity to cope briefly with far higher temperatures, red-heat and beyond.

15.1 Rubber composition:

Rubber was discovered by early Spanish explorers of America, and received its name when it was used by an inspired individual to erase pencil marks. Contrary to popular belief, its occurrence, rather than being limited to the rubber tree, is widespread in the plant kingdom. It usually occurs as a colloidal solution in a white fluid known as latex. (If "the milky" fluid from goldenrod or dandelion is rubbed between the fingers, a little ball of rubber will result). This latex is not the sap of the rubber tree. It occurs "in microscopic tubules distributed throughout the plant and is obtained from those in the cortex layer between the bark and the cambium layer," which roughly translated means that although the latex is found in little tubular cells throughout the plant, we draw it out from the layer between the bark and the layer containing the sap. It contains 35% rubber. After tapping, it is usually diluted, coagulated, rolled into sheets, and washed.

Rubber consists mainly of a hydrocarbon polymer, (C5H8)x; the monomeric compound CSHB is called isoprene. The molecular weight of rubber hydrocarbon is not definitively known; one investigation indicates a range of 50,000 to 3,000,000, with 60% of the molecules having molecular weights > 1,300,000.

15.2 Vulcanization:

Because there is little or no cross-linkage of the chains of the molecules, rubber from the tree is thermoplastic and becomes soft and sticky on heating. When cooled to low temperatures it becomes hard and brittle. These properties were undesirable even in the • early use of rubber, which was chiefly for the



15-102

waterproofing of textiles. And then one day in 1839 a New England inventor by the * name of Charles Goodyear, in the course of trying to ameliorate the rubber situation, accidently dropped one of his brews on a hot stove and discovered what he called vulcanization.* Development of the process led to the production of a material with much greater toughness and elasticity, and one which withstood relatively high temperatures without softening and which retained its elasticity and flexibility at low temperatures. Vulcanization is a chemical reaction of the rubber hydrocarbon with sulfur: the chains of rubber molecules are tied together by sulfur atoms giving a huge cross-linked macro-molecule. So what does this accomplish? These (few) cross-links that we introduce prevent the original rubber molecules from slipping pass one another ie. plastic deformation cannot occur ? the paucity of the cross-links,! however, does permit the original molecules to be aligned and elongated to a considerable extent by stretching ______ the stuff is now elastic. When the tension is removed, the molecules spring back to the original alignment. Pictorially the situation is as follows:

Figure 15-1

Commercial rubber is either low in sulfur (1-3%) for soft rubber, on high in sulfur (23-35%) for hard rubber or ebonite. Rubbers containing intermediate amounts are useless. Why? • Although he (to an extent) fluked this one, he was genuinely talented. He

ended up with over 60 patents and many bestowed honours, although he was kept in poverty by infringements of his patents.



15-103

15.3 Bits and Pieces:

(a) One of the outstanding developments in the rubber industry has been the use of anti-oxidants to prolong the life of rubber articles. The aging of rubber is due to reaction with oxygen. The net result chemically is that the macro-molecules get chopped up somewhat; the lower molecular weight is manifested in reduced elasticity and tensile strength, and eventually crumbliness and even stickiness. Other additives include carbon black and "carbon white" (finely divided silica) to increase stiffness and tear and abrasion resistance, fillers to reduce cost where strength is not important, and softeners such as pine oil.

(b) Rubber articles are usually formed in either of the following two ways:

(i) The ingredients are compounded on a rolling mill, after which

the sheeted material is used to line molds which are then subjected to heat and pressure.

(ii) The additives are simply mixed into the latex, after which the articles are shaped by gelation on a mold and then vulcanized.

(c) Did you know that those old, worn-out, non-recappable, fabric-filled,

insoluble-in-everything tires are reclaimable? It is done on a large scale. The tire is shredded and stewed in dilute caustic soda at 180-200°C; this softens the rubber and disintegrates the fabric. Pine oil is added and after straining, the mixture is pressed into sheets. This reclaimed rubber can be revulcanized, and costs only a fourth to a third as much as natural rubber. Most rubber articles contain some reclaim.

(d) Two other naturally occurring rubber-like materials are gutta-percha and balata. Their hydrocarbons are indentical to each other and have the same chemical composition of rubber. The atoms are arranged somewhat differently, however, with the result that these two are hard at room temperature. One vitally important application is in the manufacture of golf balls. Chicle is a resin containing polyisoprenes along with carbohydrates. Its chief use is in the manufacture of, you guessed it, chewing gum.

(e) There is such a thing as synthetic rubber, but it was slow in coming

because it costs more than natural rubber, and for most uses its properties have been inferior. Two products which emerged after World War I are Buna rubber (European) and Neoprene (American). Because of the loss the East Indies and Malay Peninsula during World



15-104

War 11, great impetus was given the synthetic rubber industry in the United States, with the result that the synthetic product is now firmly established. Although a synthetic true rubber is unknown, many different synthetic products are made, each of which has its own desirable characteristics that frequently are superior to those of natural rubber. The synthetics produced in largest amounts are manufacture from butadiene (C4He), which in turn is made from alcohol petroleum or acetylene. The butadiene is usually co-poly-merized with another compound, such as styrene. Neoprene is a good general purpose rubber, but its relatively high cost of manufacture has limited its use to those applications that require its unique properties, such as resistance to oil, chemicals, air, light and heat.

15.4 Protective Coatings:

Everybody knows of the word "unsaturated" being used to describe fats and vegetable oils. What it refers to can most simply be described as latent or unused reactivity*. A fully saturated fat has I no unused reactivity. In the body, this latent reactivity aids in the breaking up of the otherwise nearly inert molecules, thus reducing accumulation of body fat (and cholesterol). This same unsaturation, if present to at least a moderate degree, enables a thin layer of a vegetable oil to react with the oxygen of the air, and polymerize somewhat to become a solid, ie. a dry film. Although linseed oil is the classic example, many edible oils can "dry" in this way___if you happen to be using one of these in the kitchen and the bottle has been around for quite a while, chances are it will have a visible dried film on the outside, or at least be sticky Such an oil is called a drying oil. Mineral (e.g. lubricating) oils do not possess this property. Table 1 below lists some common vegetable oils in order of increasing unsaturation. The division into the three categories is somewhat arbitrary; the semi-drying oils leave a more or less tacky film.

Table 1 Order of Drying Ability of Some Common Vegetable Oils

Non- 1. Olive Drying 2. Castor

* Chemically described, unsaturation refers here to covalent bonds involving more than one pair of electrons per carbon-carbon bond.



15-105

Non- 3. peanut Drying 4. Rape

Semi- 5. cottonseed Drying 6. sesame

7. corn

8. sunflower Drying 9. soybean

10. linseed

"Boiled" linseed oil, which was used a lot in the olden days, is really just ordinary linseed oil to which driers have been added. Driers are metallic salts of organic acids;* they catalytically accelerate the reaction with atmospheric oxygen. Varnish is a mixture of drying oil, thinner and rosin (pine tree resin). The thinner is called mineral spirits and is what you get when you buy "paint thinner" or "solvent". It is essentially identical to kerosene, solvent naphtha and coal oil, and turpentine may be substituted for them. The drying of varnish then, entails first the evaporation of the solvent (giving the tacky stage), followed by oxidative curing of the remaining film. The dried film from drying oils will not dissolve in anything, so if you forgot to clean your brush, heave it in the garbage. The rosin imparts hardness and high gloss to the film; it may be replaced by natural or synthetic resins. There are basically two categories of varnish: indoors and outdoors. The former features better resistance to abrasion, while the latter can better withstand sunlight. One component (poly) urethane is a modern synthetic indoors varnish (requires only paint thinner for cleanup).

Paint is a mixture of drying oil, pigment, thinner (again mineral spirits), and driers• The pigment (very finely ground after blending) provides colour and covering power; it is normally either a metallic oxide (Ti02 for white) or salt, or an organic dye. The latter provide most of the bright colours, but are rarely colour-fast to sunlight. The drying oil is called the vehicle because after dry-ing it holds the pigment. Modern paint vehicles now contain, or are entirely composed of, alkyd resins, which are synthesized from medium molecular weight polyhydric alcohols and unsaturated acids. The alkyds dry much faster and may give superior films.



15-106

Oridinary paint is manufactured in three classifications of gloss: the shiniest is called "enamel", the dullest is "flat", while the one in between is "semi-gloss". The difference is achieved by loading up the paint with what is called an inert, or extender.

• usually naphthenates of Co, Mn, Pb, Ca ** more than one OH group per molecule This is a finely-divided inorganic compound, usually magnesium sil; (talc), which disrupts the otherwise smooth film of the vehicle, ir addition to its virtue of being cheap.

Specialty paints include primers, which are used as undercoats when painting new metal (to minimize corrosion); stains, which are essentially ordinary paint thinned way down to penetrate new wood well to colour it; latex, a finely-dispersed emulsion of paint in v (the droplets are so small that they can be seen only under a microscope), which permits water thinning and clean-up of the undried material; expensive two-component protective coatings such as epoxies, polyurethanes and polyesters/ which feature superb adhesion and/or resistance to abrasion, acid, caustic and/or solvents.

All of the coatings discussed so far produce dried films which will not dissolve in anything. The only way to remove these dried films is by scraping or sanding. The commercial paint removers can greatly aid scraping; they do this by loosening and wrinkling the old film. Shellac and Lacquer differ from all the proceeding coatings in one basic respect: they dry by evaporation only; there is no oxidation or polymerization of the remaining film. This means that the dried film can be removed from equipment, and the coated article itself, by a simple dissolving action. Shellac, not used much any more is a solution of a resin called lac* in alcohol (methyl hydrate at the hardware store). It is orange-brown in colour, used for coating wood. Lacquer in theory takes in any protective coating which dries by evaporation only, but by far the most widely used lacquer is that based on nitrocellulose, which is made by treating cotton with a mixture of nitric and sulfuric acids. The lacquer, which often contains pigment, consists of the nitrocellulose dissolved in a mixture of low and medium molecular weight esters (eg.butyl acetate) and/or ketones (eg. acetone), alcohol, and toluene, along with a placticizer to give flexibility to the film. The solvent mixture comprises "lacquer thinner," which must be used for thinning and clean-up. * secreted on trees by certain insects in Southern Asia.



15-107

APPENDIX

Table 1 Term Features Some Trade Name

ABS* Acetal Acetates Butyrates Cellulose propionate Cellulose nitrate Ethyl cellulose Fluorocarbon Polyamides (nylon) Polycarbonate Polyether (chlorinated) Polyethylene Polypropylene Polystyrene Vinyls

Excellent toughness Extremely riged Tough, hard, easily Coloured Tough, good weather-ability No odor, stable, bright finish Tough, hard surface, inflammable Tough, stands hard treatment Highly chemical resistant Strong and extra temperature High impact strength Excellent chemical resistance Light weight and squeezable Light weight, un-usual chemical Brilliant, rigid colorful Versatile, multipurpose, colourful

Kralastic-Naugaturc Kralastic-Naugatuck Chemical Division Delrin-Dupont Lumarith-Celances Plastacele-Depont Vuepak-Monsanto Tenite Butyrate-Eest-man Forticel-Celances Nixon C/N-Nixon Nit-ration Herculoid-Hercules Powder Hercocel E-Hercules powder Nixon E/C-Nixon Nit-ration Teflon-Dupont Zytel-Dupont Lexan-General Electric Penton-Hercules Polyeth-Spencer Alathon-Dupont Escon-Humble Oil Styron-Dow Lustrex-Monsanto Exon-Firestone Marvinol-Naugatuck

* Acrylonitrile-Butadiene-Styrene



15-108

Table 2

Term Features Some Trade Name Alkyds Good electrical properties Plaskon-Allied Allyls Low electrical loss Dapon-FMC Corp Caseins Good colour range Cascoloid-Borden Epoxies Excellent adhesion Epon-Shell Chemical Melamines Strong and light-fast Melmac-Americal Cyanamid Phenolics Hard, rigid, strong Resinox-Monsanto Polyesters Tough hard surface Mylar-Dupont Silicones Resistant to 590 F Silastic-Dow Corning Ureas Colourful, dimensionally stable Lauxite-Monsanto Urethanes Tough, tear resistant Mondeur-Mobay Chemical

Adhesives


16-109

CHAPTER 16

Adhesives 16.0 Adhesives:

An adhesive is a substance applied as a thin intermediate layer which is capable of holding materials together in a useful manner by surface attachment. Two basic but important facts about adhesive materials are:

1. An adhesive does not perform its function independently of its environment

or context of use. 2. An adhesive does not exist which will bond anything to anything with equal

utility.

In other words, adhesives influence and are influenced by the materials with which they come in contact. For this reason it is virtually impossible to compile a selection guide. These would at best remove grossly unsuitable materials from consideration or at worst they could be misleading. Table 1 (appendix) lists general comments which may be useful as a preliminary guide.

16.1 Principles of Adhesion:

Chemically adhesives are with few exceptions derived from* materials which contain carbon and other elements in combination. These are what are known as organic compounds, examples of which are gasoline, alcohols and all living organisms. Although not usually used as an adhesive, ethylene (C2H4) can be used to illustrate how the bonding works. Structurally it can be represented as H H and this is called a monomer.

1 1 C = C 1 1 H H

Since in this form it exists independently. When linked with others in a chain, these molecules are called Polymers.

H H H H 1 1 1 1

C C C C 1 1 1 1

H H H H

Adhesives


16-110

• One inorganic adhesive is silicate of soda-the solution is called waterglass.

Now if the hydrogen atoms are removed there will be bonds available for adhesion. This can be visualized as:

Figure 16-1

It is the primary bonds which link the monomer into chains, while the secondary bonds provide the adhesive force by linking the polymer chains and ultimately the adherents. This type of bonding is termed specific adhesion, that is, it is essentially chemical in nature.(molecular forces) The second type of adhesion is mechanical. This occurs where porous, penetrable surfaces are involved, such as wood, paper "and cloth, and as a result the adhesive is able to enter crevices of the material and flow around fibres to provide an effective mechanical anchorage.

16.2 Functions:

In many situations/ adhesives are more practical or essential than conventional bonding. A glued joint produces a distributed stress over the whole adhesive and thus eliminates the local stress concentration attendant upon screws, rivets, spot welds etc. For example, adhesives can join thin metal sections to thick sections so that the full strength of the thin section is utilized. Conventional mechanical fastening or spot welding produces a structure whose strength is limited to that of the areas of the thin section in contact with the fasteners or the welds. Some applications such as bonded honeycomb structures rely exclusively on adhesives the thin metal facings could not be bonded to the core without them.

Adhesives


16-111

16.3 Sealing and insulating:

Since the adhesive provides full contact with mating surfaces of the joint, it forms a barrier to fluids which do not attack or soften it. This property is used to advantage in nuclear operations At Pickering G.S. a fill (thiokol) is used as a semi-permanent seal and fill above the steel wedge in the seal plate of the boiler room hatch, (Figure 16-2).

Figure 16-2

This fill effectively prevents any movement of the wedge as well as completing the seal. Further use of thiokol is made in the other hatch covers. A clearance space is allowed between adjacent covers to permit easier handling, and when they are closed the space is filled to provide continuity.

16.4 Other uses:

Bonded joints can serve as insulation, damp out sound and vibration, and minimize contact corrosion in metals. Their smooth appearance can also be an aesthetic advantage. There are disadvantages of bonded structures which have limited their use in engineering. These include the need for long processing times for curing and the dependence of joint durability on the quality of the bonding process. As well, they have limited resistance to extreme temperatures and humidities. The need for special joint design to prevent cleavage is another. very important factor and this is outlined in the appendix.

Adhesives


16-112

16.5 Chemical forms: Adhesives are available as liquid, paste or dry, but they can be further classified as solutions or emulsions depending on their behaviour with water. In a solution, the polymeric molecules are "water-loving', that is, they are very soluble in water as are suqar and salt. In emulsions on the other hand, the molecular chains are "water-hating". The particles do not dissolve in water but disperse discretely as droplets. They are usually very sticky and will attach themselves to many surfaces or bunch together unless a buffer is placed between them and the water. The buffers are "water-loving" and are called emulsion agents.

16.6 Types of Adhesives:

Many kinds of adhesives are being used, depending largely on the type of materials being bonded and also on the service conditions of the bonded assembly. Something like 5000 products are available today, based on 200 basic polymers, which precludes any intensive study at this stage. However, the most widely used ones can be grouped under a few broad headings. Adhesives are classed as Thermoplastic or Thermosetting depending on whether they do or do not revert to a fluid state upon application of heat. Thermoplastic adhesives (also plastics) have this property while thermosetting ones are insensitive to heat once they have been subjected to heat treatment or to some agent -which- promotes the development of polymers.* For Engineering applications the majority of structural adhesives are of the thermosetting type, cured under heat and pressure. Epoxies and their modifications find the widest use for bonding metals, because they do not contain solvents and are true liquids Single component epoxy resins have become available in the last few years as liquids or films which avoid the need for critical premixing of resin and simplified processing. Epoxy adhesives with good gap filling properties are used for bonding pumps, compressors and cast-iron casings with significant saving in weight and design complexity. Some have good resistance to radiation and for this reason are used in nuclear operation. This was vital in the decision to line (waterproof) the spent fuel bays at Pickering G.S. with epoxy. Thermoplastics, which soften with heat, give considerably lower strengths than the thermosetting types, but nevertheless are used where lower joint strengths are acceptable. These tend to be sticky and are the most common in packaging, eg. resin type emulsions. • This classification is not absolute, for some adhesives are mixtures of

polymers and possess thermal characteristics of both.

Adhesives


16-113

With the exception of silicones and fluorocarbons (Teflon), all known thermosetting and thermoplastic resins are degraded below 400°C, the higher breakdown temperature corresponding to a more rigid chain structure. However, the useful temperature range of a Polymer adhesive is difficult to ascertain, since many changes can occur below the breakdown temperature. Thus, a Polymer can undergo chemical modification, irreversible decomposition, molecular rearrangement or reversible melting or softening. Ceramic adhesives, made from boric acid, silica and ferric oxide, appear to offer an answer to the high temperature problem. Up to 5500C, strength increases with temperature but there are serious shortcomings. Attempts to overcome their inherent brittleness have failed. Thermal shock resistance is poor even though their heat and oxidation resistance is better than that of organics. Some adhesives have unique cure mechanisms. Anaerobic adhesives are one of the most recent developments. These polymerize in the absence of oxygen, under the catalytic influence of metals. Thus the monomer is able to polymerize to form a tough resilient bond when confined between closely fitting parts such as on threads of a nut and bolt. The LOCTITE products are the best known example of this type which can be formulated to give various viscosities, setting times and shear strengths, and bond all common metals, glass, ceramics and phenolic plastics. The best have shear strengths up to 10 MN/m2 while-still others can be used in 20 minutes. Recently developed special grades will stand up to 200°C; above 250°C the materials soften, which may be useful for dismantling the joints.

16.7 Hot melts:

Since they are based on thermoplastics, hot melts are temperature dependent adhesives and thus achieve their strength by cooling. Unlike other adhesives, they do not depend on chemical action or solvent to effect the setting, which occurs within seconds or minutes of application. As a result, they are of considerable im-portance in mass-production industries such as paper and packaging, where speed, simplicity and mechanization are vital. They are also "water-hating" which means they show excellent moisture resistance, but problems can arise because of their limited toughness or low melt temperature (65oc to 1800C). Animal Glue, used since Egyptian times, is basically a derivative of hide and bones. It is similar to a jelly and extra-refined grades are in, fact edible Gelatin. It approaches an all purpose glue, and as a bench glue for general purposes it is unsurpassed. For optimum results it should be applied at 60°C, as it is a fluid at this temperature. It also has a rapid handling bond because of the fast initial set.

Adhesives


16-114

Table -1 Type

Source Form Remarks

Vegetable Starches of corn

Potatoes, tapioca dry (flour) low cost-not resistant to humidity or

moisture Casein Milk Powder water resistant plywood where exposure is

not extreme. Also, good resistance to heat. Urea Synthenic resin

based on carbon dioxide (C02) & (NH3) ammonia

Powder or liquid

Shorter curing cycle & temperature required than phenolics but reduced ultimate durability. Excellent under normal exposure.

Resorcinol

Synthetic-resin of phenolic type

Liquid Excellent for laminated lumber and joints subject to severe exposure such as marine conditions.

Blood Albumin

dried blood

Power Since it is cheap it is a good modifying & extending agent in resin adhesives to regulate spread & flow. Also to shorten hot press cycles.

Silicate of soda

sand & soda ash solid or liquid

Extender for other glues such as soybean. Corrugated fibre or paper packages.

Lacquer many liquid (organic) solvents used

moisture resistant used when water-containing adhesives are ruled out.

Rubber Natural or Synthetic compounds

liquid "Dry-Tack" adhesives and self-seal envelopes & shirt bands.

16.8 Others:

There are many other types of adhesives, some of which have very specialized uses owing to their characteristic properties or economies. Elastomers, for example, are used when high peel strengths are required. Soybean glues, on the other hand, are particulary suited to Douglas Firs and are used almost exclusively for these and associated species. These and others are summarized in table 1 and tables 1 and 2 of the Appendix

16.9 Tapes: Pressure sensitive tapes consist of materials (backings) with permanently tacky adhesives bonded to them. They adhere to a variety of surfaces, and require only light pressure. (Figure.16-3) A brief summary is given here because of their omnipresence in industry.

Adhesives


16-115

Table – 2

Tape type Remarks

1. Paper tapes masking tape etc. - varying properties 2. Cellophane Transparent, tough, strong. 3. Cellulose acetate

Clean, strong, better humidity resistance but not as tough as cellophane

4. Polyester and Polyvinyl

High tensile strength, toughness, moisture resistance, and durability expensive Electrical tape

5. Polyethylene

High tensile strength, low temp. flex-ibility, resistant to most solvents, low moisture transmission

6. TFE fluorocarbon High heat & chemical resistance 7. Cloth tapes Able to "breathe" - good toughness, tear

resistance and conformability 8. Metal foil

Aluminium, copper & lead are common Resistant to moisture & weathering Copper has good electrical conductivity Lead blocks nuclear radiation & x-rays

Adhesives


16-116

Figure 16-3

16.10 End Use Requirements:

The ultimate strength of an adhesive is as dependent upon the service conditions such as temperature, humidity and water as it is on the adhesive itself. The temperature which the bond is subjected to in use is just as important as that under which it tool place. (see appendix) Recall that the condition of thermoplastics is temperature dependent, and that permanent damage can be done to thermosets if their design temperatures are exceeded. For this reason maximum temperature should always be noted before use. Some of these are given in table 2 of the appendix. Since a large percentage of adhesives cure by losing entrained water many may be remoistened and consequently weakened under high humidity conditions. Water resistance may be a useful property, as is the case with beer bottle labels. These however may not be humidity resistant and may fail when exposed to 80 or 90 percent relative humidity. Properties such as solvent, shock and oil resistance may also be incorporated into the final product.

16.11 Conclusion: The use of adhesives requires a good deal of common sense, especially in sensitive industries such as nuclear plants. In many instances a patch up job could be done with an adhesive but this may not be desired. For example, if a nut is loose it can normally be secured with Loctite, but this would probably constitute a misuse. If a precision fit was designed for, an adhesive should not be used as a solution for poor workmanship; rather, only when it is designed as an integral part of a structure , such as in plywood, waterproofing, etc.

Radiation Damage


17-117

CHAPTER 17

Radiation Damage 17.0 Radiation damage:

When we say radiation, we mean either neutrons, or alpha, beta r gamma rays. These radiations can interact with the nuclei in material or the electrons around them; when this happens, radiation damage may result. First let's take a general look at what appends when these radiations do their interacting; this will be followed by a brief look at the separate behavior of covalent, ionic and metallic bonds on irradiation. Finally, short descriptions of the damage experienced by specific materials will be presented.

17.1 Types of Radiation:

Fast neutrons lose energy by colliding with nuclei. A nucleus thus clobbered takes off at high speed, and some of its electrons re stripped off as the atom passes by neighboring ones. The result is a high speed charged ion passing through the material. The damage resulting from fast neutron irradiation is actually caused by atoms knocked out of position by the neutrons. As might e expected, the more energy the neutrons have, the greater will e the resultant damage, both by electron ionization and atomic displacement effects. How fast are fast. neutrons? We measure their energies in units called electron volts.* Fission neutrons start out typically at a couple of million e.v., whereas room temperature molecules have energies only a fraction of an electron volt. Thermal neutrons are so called because they are in thermal equilibrium with their surroundings; i.e. they have the same kinetic energy, or temperature, as the (for example) heavy water in which they are bouncing around. This kinetic energy is about a fortieth of an electron volt in the moderator. Thermal neutrons therefore cannot cause any damage on collision with atoms of the medium. We get an effect only when the thermal neutron is captured by a nucleus. In some cases the addition of a neutron does not change the identity of the element, e.g., most neutron captures by iron atoms result in just a heavier stable iron atom. * Defined as the energy gained by an electron when accelerated through a potential difference of one volt.

Radiation Damage


17-118

On the other hand, capture by aluminum results in the eventual formation of silicon. About 0.4% of the aluminum at the centre of the NRX reactor is transformed to silicon in 10 years. When a thermal neutron is captured, the newly formed nucleus is in an excited (higher energy) state, and gives off its excess energy as a gamma photon. This photon is usually very energetic (6-7 Mev), with the result that the emitting nucleus recoils. On recoiling it is usually displaced from its position. To summarize neutrons, then: fast neutrons can cause a great number of ionizing effects and cause a large number of atoms to be displaced, whereas a thermal neutron can cause only one atom to be displaced, which may or may not be concomitant with the formation of an impurity atom. Gamma rays for the most part interact only with electrons, and thereby produce ionization. (Interactions with nuclei causing ejection of neutrons and other particles are not of importance in engineering design). The fast electrons resulting from the incident gamma rays cause damage by gamma radiation. Beta radiation (electrons) entails charged particles, which are Very light. Damage is caused only by interaction with the orbital electrons of the atoms in the material - ionization is the result. Beta radiation ranges anywhere from a few thousand e.v. to ten or more Mev. Since each ion pair* formed requires 25-35ev, a 1 Mev beta particle can cause substantial ionization. Unlike neutrons and gamma rays, beta particles have very limited power of penetration. They can at best penetrate 10-15mm. of water or body tissue, while the feeble ones (e.g. from tritium) cannot even penetrate a piece of paper. Alpha particles have so little penetration power they are not a factor in radiation damage to materials. It should be pointed out that none of the above radiations occurs alone. Thermal neutrons when captured give rise to what are called capture gamma rays (they are emitted by the target nucleus, and have energies in the neighborhood of 7 Mev.) The new nucleus often decays later on by emitting a beta particle and/or a gamma photon. Fast neutrons (a result of fission) are accompanied by gamma rays; some slow down to thermal neutrons. Incidentally, it is worth knowing that only neutrons can make a material radioactive. • When an electron is removed from an atom, it (which is negatively

charged) along with the remaining positively charged atom constitute an ion pair.

Radiation Damage


17-119

17.2 Effects of Radiation on the Bond Types:

Covalent bonds as you recall, entail atoms being held together by the sharing of electron pairs. Since this bond does depend on the sharing of electrons, ionizing radiation (which causes electrons to become separated from their atoms) destroys it. Ionizing radiation causes disintegration of the original molecules and the formation of new and different ones, i.e., and chemical change. Organic compounds contain almost exclusively covalent bonding. Ionic bonds entail cations and anions being held together in a crystal lattice by electrostatic attraction only; all electrons in the compound are held in their orbits around particular atoms. ionizing radiation is not nearly so destructive to an ionic bond as to a covalent one. In the case of NaCl (table salt), ionization can result in Na or a neutral Cl atom (loss of an electron from Na or Cl ) without causing the atoms to move out of their positions. The displaced electron may eventually find its way to another electron deficient site and there is no net damage. On the other hand, the electron may become trapped in a lattice imperfection and we get what is called an F centre.* This has little effect on the strength of a material, but can change its color. For example, glass is quite resistant insofar as its structural properties are concerned, but it soon turns-black-when exposed to ionizing radiation. In molecules with ionic bonding all the available electron sites are filled, so that there normally can be very little electron movement from site to site. They are thus good electrical insulators. However, since radiation causes electron migration, the resistively of these materials is reduced during irradiation. The majority of inorganic materials, other than metals, exhibit ionic bonding. Metallic bonding entails the atoms in a piece of metal being held together by an electron sea. The electrons in the sea are mobile; they do not belong to particular atoms. All of the possible electronic sites are not filled; the electrons are free to move from site to site. Ionizing radiation, therefore, which interacts only with electrons, will not have any effect on this type of material. However, fast neutrons can have a serious effect by knocking the metal atoms out of position, forming vacancies and interstitials. * From Farbe, the German word for color

17.3 Effects of Radiation on Materials:

Oils suffer increase in viscosity as a result of cross-linking, which takes place after bonds have been ruptured. The damage is caused by both neutrons and ionizing radiation. The amount of viscosity increase depends on the

Radiation Damage


17-120

composition of the particular oil; the more vulnerable types can actually solidify. In general, oils of high viscosity are more sensitive. In any event, the tendency is to form gummy, tar-like polymers, which can restrict the flow of lubricant. Greases of the soap-oil type can rapidly become fluid. Hydrocarbon mineral oils can be considered, however, a relatively radiation-resistant class of materials. Exposure to air during irradiation accelerates oxidation, and reduces threshold and limiting dosages by about a factor of 2. Additives can be very helpful in limiting radiation-induced oxidation and other damage. Polymers (plastics) undergo drastic changes under the influence of radiation, since the bond arrangement can be extensively changed when the bonds broken by the radiation come together. They need not join in the same structure as before breaking. Figure 17-1 shows the relative effect of radiation on various polymeric materials * Fragmentation into shorter chains may occur. This will result in a polymer that is much softer and weaker than the original material, perhaps even liquid. Some production of gas can also occur (mostly hydrogen and methane); this can cause swelling. New bonds may be made to make new polymers that are larger or have new arrangements. These will also have new properties, such as altered tensile strength.

Radiation Damage


17-121

Effect of Radiation on Polymeric Materials Figure 17-1

Radiation Damage


17-122

Cross-linking may occur, resulting in less freedom of movement for the indivi-dual chains/ and thus a more rigid and brittle polymer.

Paints contain polymers; they will tend to get brittle and flake away, "talus'" failing in their task as a protective coating. Electrical insulations are polymers; they will also get brittle and crack, leading to the hazard of electrical short-circuits. All plastic parts chosen for their mechanical properties will fail in time, thus great care must be taken when choosing the materials used in radiation fields. All polymers undergo change when exposed to radiation. Some are more resistant to radiation than others, but they all suffer radiolytic damage. This need not always be bad. Polyethylene, (as normally produced) softens below the boiling point of water. Under the influence of a small dose of radiation, a degree of cross-linking can be added that raises this softening point above the boiling point of water. The wide use of polyethylene in kitchen utensils is a result of this radiation induced "damage". Ion-exchange resins are polymers based on polystyrene. They will also be subjected to radiolytic breakdown. The resins in the active water systems remove radioactive corrosion products and fission products from the water. These radionuclides will deposit much of their energy within the resin bed, leading to its destruction. At the present time, the active resins are not regenerated, but are discarded (i.e., buried). It is felt that a combination of the radiolytic damage* and the added effort of regenerating active resins makes discarding the resins a more profitable procedure than regenerating them. • Which in addition might result in the release of traces of corrosive chloride

from the resin into the system.

Radiation Damage


17-123

The molecules in elastomers (the generic name for rubber-like substances) are long strings of atoms. Their properties depend on a fine balance between inherent freedom of motion of the chain and the degree of cross-linking between the chains. Of the rubbers, natural is more resistant to radiation damage than the synthetics. The tensile strength of all is reduced, but the effect on hardness depends on the material (natural rubber, for instance, gets harder; butyl rubber softens). The effects of radiation on graphite have been studied in detail. ionizing radiation has little effect on it. (Why not?). When its atoms are displaced by fast neutrons, however, the energy of the crystal lattice is increased. If the graphite is heated, these dislocations anneal out that is, the carbon atoms fall back into their correct positions. As this happens, the excess energy that went into displacing the atoms is released as heat. Careful control must be exercized during the annealing process to prevent overheating the graphite, as happened to the Windscale reactor in the UK. Radiation causes dimensional changes and an increase in resistively of the graphite. The effect of radiation on concrete was dealt with briefly at the end of the lesson covering this material. The results manifest themselves as heat damage, namely spalling and internal stresses. Damage to electrical properties of electronic circuit components can be serious. In many cases electrical property changes set in before other damage is apparent. Semiconductors are a good example. These materials depend on a very critical number of current carriers (available electrons) for their particular electrical properties. Damage to such items as crystal diodes, transistors and dry rectifiers consists of changes in conductivity, which for components such as these can mean game over. Degradation of components containing ceramic materials also consists of changes in electrical properties before physical deterioration. The resistively of these materials can drop by a factor of 10 to 100; in general, however, they can take 10-100 times the radiation that organic insulators will withstand. Careful selection of components will lead to longer circuit life in radiation fields. Wire-wound and metallized resistors on ceramic bases will experience less than a 10% change in resistance at doses up to ten times those which cause resin-bonded carbon resistors to fail (because of failure of the resin). Ceramic, glass or mica capacitors with a body of inorganic material will show little damage after 1000 times the dose that will put an oil-filled capacitor out of service.

Radiation Damage


17-124

Finally, metals. As stated a few pages ago, ionizing radiation has no effect (Why not?). To understand why the strength and hardness of metals increase while ductility decreases as a result of neutron irradiation, it is necessary to consider the ways in which high-energy particles damage the crystal structure. Neutron radiation creates three types of simple defects in the crystal lattice: vacancies/ interstitials, and impurity atoms. Vacancies may be created by collisions of neutrons with the atoms in a solid lattice—they are uninhabited sites in the crystal lattice. The energy transferred in these collisions is generally large enough to allow the thumped atom* to produce additional vacant lattice sites by further collisions. Thus for each initial neutron-atom encounter, a cascade of collisions resulting in vacancies can be initiated.

Figure 17-2

Radiation Damage


17-125

The atoms displaced from their equilibrium (initial) positions in the lattice will stop in non-equilibrium (incorrect) positions, unless they immediately recombine with adjacent vacant lattice sites. Such atoms are called interstitials. The energy required to knock an atom out of its equilibrium lattice site is of the order of 2 5ev. Thermal neutrons have an energy of about {oev, and it is normal in considering radiation damage to consider only fast neutrons, with an energy of 1 Mev or higher in fact, as effective in causing this type of damage. Impurity atoms come about through neutron capture followed by radioactive decay. The number introduced is generally small, and in reactor applications may normally be ignored in relation to other damage effects. How about a summarizing paragraph: The damage produced by irradiation depends markedly on the type of radiation involved. Gamma rays and charged particles (alpha, beta) lose most of their energy by removing electrons from the inner shells of the atoms in the metal, or by exciting the free electrons. Because these effects are rapidly removed by the electron sea, they are not expected to produce a permanent effect. Neutrons, on the other hand, act directly with the nuclei of the metal atoms, causing ejection from the lattice site. These initially energetic atoms behave like any other charged particle radiation. Most of the energy will be dissipated by ionization, or excitation of free electrons, until their velocities are reduced below that of these electrons. The remaining energy will be dissipated by collisions with other atoms; each encounter will result in a displacement if the energy transferred exceeds 25ev. After the energy of the moving atoms becomes less than about 50ev, they cannot displace further atoms without themselves remaining in the lattice position—- i.e., no further displacements occur. This means that the last 50ev or so of each knock-on atom is dissipated by hitting other atoms without displacing them permanently; this heats the lattice. The local hot spot formed is called a thermal spike. A schematic diagram of how neutrons can cause ionization,vacancies, interstitials, impurity atoms and thermal spikes is presented as Figure 17-3. Except at very low temperatures, vacancies and interstitials can rearrange themselves. Some close pairs will annihilate each other, while others move to surfaces or become associated with lattice imperfections. Still others will combine to form clusters of defects. It is generally considered that the defect clusters, by forming barriers to slip, are the primary causes of the radiation induced changes in the mechanical properties of metals.

Radiation Damage


17-126

Some specifics for two important metals will now be thrown at you. Neutron irradiation of plain carbon steel produces a marked increase in yield strength (with a lesser increase in tensile strength) accompanied by a decrease in the elongation to fracture. The increase in strength after irradiation may be bene-ficial; however, the concomitant decrease in ductility may be very bad indeed. The effect of irradiation temperature on the various -properties is complex. I-t appears that partial annealing of" radiation damage responsible for one mechanical property may take place more or less independently of another mechanical property at a given temperature of irradiation. The effects of composition are complicated, although increased carbon appears to decrease the amount of radiation damage. Damage is also apparently reduced by smaller grain size. Irradiation of zircaloy at temperatures up to 280°C considerably increases the yield strength, and to a lesser extent the tensile strength. The ductility is reduced, particularly the uniform elongation. In fact, the almost complete absence of uniform elongation causes zircaloy to show local necking and a drop in load carrying capacity at very low strains. This must be considered in designs where strains resulting from volume changes are expected e.g. differential thermal expansion or radiation swelling. Radiation damage has little effect on the life of zirconium fuel sheathing, but it does impose limitations on the positions in a reactor to which fuel may be shifted. Pressure tube life is reduced because of radiation, since creep is accelerated in a radiation field.

Documents

Materials