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Tensile test

Objective

To perform the tensile test on the given samples and to determine the associated properties of

specimens using universal testing machine.

Apparatus

Universal testing machine Given specimens (cast iron and aluminium) Vanier calliper Extensometer

Introduction

Mechanical testing plays an important role in evaluating fundamental properties of engineering

materials as well as in developing new materials and in controlling the quality of materials for

use in design and construction. If a material is to be used as part of an engineering structure

that will be subjected to a load, it is important to know that the material is strong enough and

rigid enough to withstand the loads that it will experience in service. As a result engineers have

developed a number of experimental techniques for mechanical testing of engineering materials

subjected to tension, compression, bending or torsion loading.

Tensile properties indicate how the material will react to forces being applied in tension. A

tensile test is a fundamental mechanical test where a carefully prepared specimen is loaded in

a very controlled manner while measuring the applied load and the elongation of the specimen

over some distance. Tensile tests are used to determine the modulus of elasticity, elastic limit,

elongation, proportional limit, reduction in area, tensile strength, yield point, yield strength and

other tensile properties.

The most common type of test used to measure the mechanical properties of a material is the

Tension Test. The main product of a tensile test is a load versus elongation curve which is then

converted into a stress versus strain curve. Since both the engineering stress and the engineering

strain are obtained by dividing the load and elongation by constant values (specimen geometry

information), the load-elongation curve will have the same shape as the engineering stress-

strain curve. The stress-strain curve relates the applied stress to the resulting strain and each

material has its own unique stress-strain curve. A typical engineering stress-strain curve is


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shown below. If the true stress, based on the actual cross-sectional area of the specimen, is

used, it is found that the stress-strain curve increases continuously up to fracture.

Elastic Region

Stress = s = P/A(Load/Initial cross-sectional area)Strain =e = L/L(Elongation/Initial gage length)

Engineering stress and strain are independent of the geometry of the specimen. In start stress

and strain are in linear relationship.

This is the linear-elastic portion of the curve and it indicates that no plastic deformation has

occurred. In this region of the curve, when the stress is reduced, the material will return to its

original shape.

In this linear region, the line obeys the relationship defined as Hooke's Lawwhere the ratio of

stress to strain is a constant.

= Ee

Where

= engineering stress

e = engineering strain

E = elastic modulus or youngs modulus

The slope of the line in this region where stress is proportional to strain and is called the

modulus of elasticity orYoung's modulus. The modulus of elasticity (E) defines the

properties of a material as it undergoes stress, deforms, and then returns to its original shape

after the stress is removed. It is a measure of the stiffness of a given material

Plastic region

The part of the stress-strain diagram after the yielding point. At the yielding point, the plastic

deformation starts. Plastic deformation is permanent. At the maximum point of the stress-strain

diagram (UTS), necking starts.

Yield Point

In ductile materials, at some point, the stress-strain curve deviates from the straight-line

relationship and Law no longer applies as the strain increases faster than the stress. From this

point on in the tensile test, some permanent deformation occurs in the specimen and the

material is said to react plastically to any further increase in load or stress. The material will

not return to its original, unstressed condition when the load is removed. In brittle materials,


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little or no plastic deformation occurs and the material fractures near the end of the linear-

elastic portion of the curve.

For most engineering design and specification applications, the yield strength is used. The yield

strength is defined as the stress required to produce a small, amount of plastic deformation.

The offset yield strength is the stress corresponding to the intersection of the stress-strain curve

and a line parallel to the elastic part of the curve offset by a specified strain (in the US the offset

is typically 0.2% for metals and 2% for plastics while in UK offset method is 0.1% or 0.5%.

Stress corresponding to 0.1% strain is known as proof strength).

In some materials there is upper yield point and lower yield point. In these materials load at

yield point suddenly drops this is known as yield point. After decreasing load, strain increases

while load remain almost constant. This phenomena is known as yield-point elongation. After

yielding stress increases. The deformation occurring throughout the yield-point elongation is

heterogeneous. At the upper yield point, a discrete band of deformed metal, often readily

visible, appears at a stress concentration, such as a fillet. Coincident with the formation of the

band, the load drops to the lower yield point. The band then propagates along the length of the

specimen, causing the yield-point elongation.

Figure 1: yield elongation phenomena in material by Figure 2: Upper and lower yield point in material by ASM hand book

volume 8 - Mechanical Testing and Evaluation ASM hand book volume8 - Mechanical Testing And Evaluation

A similar behaviour occurs with some polymers and superplastic metal alloys, where a neck

forms but grows in a stable manner, with material being fed into the necked region from the

thicker adjacent regions. This type of deformation in polymers is called drawing.


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Ultimate Tensile Strength

The ultimate tensile strength (UTS) or, more simply, the tensile strength, is the maximum

engineering stress level reached in a tension test. The strength of a material is its ability to

withstand external forces without breaking. In brittle materials, the UTS will at the end of the

linear-elastic portion of the stress-strain curve or close to the elastic limit. In ductile materials,

the UTS will be well outside of the elastic portion into the plastic portion of the stress-strain

curve.

Measures of Ductility

The ductility of a material is a measure of the extent to which a material will deform before

fracture. The amount of ductility is an important factor when considering forming operations

such as rolling and extrusion. It also provides an indication of how visible overload damage to

a component might become before the component fractures.

In general, measurements of ductility are of interest in three ways:

1. To indicate the extent to which a metal can be deformed without fracture inmetalworking operations such as rolling and extrusion.

2. To indicate to the designer, in a general way, the ability of the metal to flow plasticallybefore fracture.

3. To serve as an indicator of changes in impurity level or processing conditions. Ductilitymeasurements may be specified to assess material quality even though no direct

relationship exists between the ductility measurement and performance in service.

The conventional measures of ductility are the engineering strain at fracture (usually called the

elongation) and the reduction of area at fracture. Both of these properties are obtained by fitting

the specimen back together after fracture and measuring the change in length and cross-

sectional area.

% elongation =LfLo

100

Where Lf = final length

Lo = initial length


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% Reduction in area =

100

Where Ao = initial length

Af = final length

Resilience

Resilienceis the capacity of a material to absorb energy when it is deformed elastically and

then, upon unloading, to have this energy recovered. The associated property is the modulus of

resilience, Ur which is the strain energy per unit volume required to stress a material from an

unloaded state up to the point of yielding

Ur = dx

Assuming a linear elastic region,

Ur = yy

Toughness

Toughnessis a mechanical term that may be used in several contexts. For one, toughness (or

more specifically, fracture toughness) is a property that is indicative of a materials resistance

to fracture when a crack (or other stress-concentrating defect) is present. Because it is nearly

impossible (as well as costly) to manufacture materials with zero defects (or to prevent damage

during service), fracture toughness is a major consideration for all structural materials.

Another way of defining toughness is as the ability of a material to absorb energy and

plastically deform before fracturing.

For dynamic (high strain rate) loading conditions and when a notch (or point of stress

concentration) is present, notch toughness is assessed by using an impact test.

Several mathematical approximations for the area under the stress-strain curve have been

suggested.

For ductile metals that have a stress-strain curve like that of the structural steel, the area under

the curve can be approximated by


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For brittle materials, the stress-strain curve is sometimes assumed to be a parabola, and the

area under the curve is given by

Poisson's ratio

Poissons ratio is defined as the negative of the ratio of the lateral strain to the axial

strain for a uniaxial stress state.

Only two of the elastic constants are independent so if two constants are known, the third can

be calculated using the following formula:

E = 2G (1 + v)

Where: E = modulus of elasticity (Young's modulus)

V = Poisson's ratio

G = modulus of rigidity (shear modulus)

Necking

Up to maximum stress deformation is homogeneous and

material deform plastically. But after maximum stress

delocalized deformation takes place. After UTS stresses are

concentrated at weaker portion of the specimen and a neck is

formed at that there. Load bearing capacity of material

decrease due to necking. Up to the point at which the

maximum force occurs, the strain is uniform along the gage

length; that is, the strain is independent of the gage length.

However, once necking begins, the gage length becomes very

important.

Specimen for Tension Test

In standard tensile specimen normally,

the cross section is circular, but

rectangular specimens are also used. The

dogbone specimen configiration was

chosen so that during testing,

Figure3: necking area in sample by AMS metal

handbook volume 8 - Mechanical Testing and

Evaluation

Figure4: standard specimen shape by AMS metal handbook volume 8 - Mechanical

Testing and Evaluation


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deformation is confined to the narrow center region (which has uniform cross section along its

length) and also to reduce the likelihood of fracture at the end of the specimen.

The standard diameter is approximately

12.8 mm (0.5 in.), whereas the reduced

section length should be at least four

times this diameter; 60 mm is common.

Gauge length is used in ductility

computations, as discussed in Section

6.6; the standard value is 50 mm (2.0 in.)

Machine for Tension Test

According to the loading type, there are two kinds of tensile testing machines;

1) Screw Driven Testing Machine: During the experiment, elongation rate is kept constant.2) Hydraulic Testing Machine: Keeps the loading rate constant. The loading rate can be

set depending on the desired time to fracture.

A tensile load is applied to the specimen until it fractures. During the test, the load required to

make a certain elongation on the material is recorded.

Procedure

Put gage marks on the specimen Measure the initial gage length and

diameter

Select a load scale to deform and fracturethe specimen. Note that that tensile strength

of the material type used has to be known

approximately.

Record the maximum load Conduct the test until fracture. Measure the final gage length and diameter.

The diameter should be measured from the

neck

Figure5: dimension of standard specimen Materials Science and Engineering by D.

Callister

Figure 6: specimen and machine arrangement for tension

test byAMS metal handbook volume 8 - Mechanical Testing and

Evaluation


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Calculation

For Aluminium specimen

Initial length of specimen = Lo = 100mm

Initial diameter = Do= Final length of specimen = Lf = Final area of specimen = Af = Yield strength = 85.59 kN Ultimate tensile strength = 108.8731628 Fracture strength = 137.755N Modulus of resilience = Modulus of toughness =

For Cast Iron specimen

Initial length of specimen = Lo = 100mm Initial diameter = Do= Final length of specimen = Lf = 68.5mm

Final area of specimen = Df = Yield strength = Ultimate tensile strength = 748.502994 kN Fracture strength = 646.43 kN Modulus of resilience = Modulus of toughness =

Application of Tension Test

Tensile testing is used to guarantee the quality of components, materials and finished products

within a wide range industries. Typical applications of tensile testing are highlighted in the

following sections on:

Aerospace Industry Automotive Industry Beverage Industry Construction Industry Electrical and Electronics Industry
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Medical Device Industry Packaging Industry Paper and Board Industry Pharmaceuticals Industry Plastics, Rubber and Elastomers Industry Safety, Health, Fitness and Leisure Industry Textiles Industry

References

www.azom.com Materials Science and Engineering by D. Callister 8 - Mechanical Testing and Evaluation Mechanical metallurgy by Dieter

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