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Mechanical and Physical Properties of Materials
When a product is designed the designers must choose the materials the product is made of
A number of factors influence this selection
The different properties of materials is the biggest consideration
Engineering Stress vs. Load Stress = load/area Two different pieces of the same
material can take different amounts of load before they break
The amount of load a material can take doesn’t tell us anything
The amount of stress is the amount we want to know
Strain
The percent that the length of the material changes
Strain = (length – length original)/
length
True Stress and Strain
Engineering stress is a number that we look at to determine how strong a material is if it is in its original state
True stress and strain are instantaneous measurements and apply to a material as its cross section is changing
Ductility
The extent of the permanent (plastic) deformation that the material undergoes before failing
The ductility of gum or a balloon is high
The ductility of chalk is basically zero because it does not stretch
Stress during Manufacturing
Temperature Effects Rate of deformation Effects Hydrostatic Pressure Effects
Compression
A force that applies squeeze pressure
For ductile materials the true stress-strain curves coincide
For brittle materials the disk test is used
Disk Test
Pressure is applied to both sides of a disk
Eventually a fracture will develop in the direction of one point of force to the other
Stress= 2(load) / (diameter * thickness *
)
Torsion
A twisting force Sheer stress Punching a hole in sheet metal
produces sheer strain Tested by twisting a thin tube of
material
Bending
This is actually a bending test to used on brittle materials such as carbides or ceramics
The stresses can be calculated by a simple beam equation in a mechanics text.
Hardness Hardness is
generally strength and resistance to wear and scratches
Diamonds are the hardest material known
They are used is several hardness tests
Hardness tests
Various tests have been developed to determine the hardness of materials such as:
Brinell test- This test uses a 10 mm ball and is
pressed into the material, the hardness is determined by the diameter of the indention.
Rockwell test- Very similar to rockwell test. Uses the depth of penetration to determine
hardness
Vickers test- Uses pyramid shape with diamond at the tip This test can determine the hardness of most
materials
Hardness tests
Knoop test- This test uses a elongated pyramid with a
diamond tip. This test is considered a micro test because
of the light tools applied This test can do thin or brittle materials Scleroscope- This uses a diamond tipped hammer in a
glass tube Hardness is determined by rebound of
hammer
Hardness tests
Mohs hardness- This uses no tools Hardness is determined by
scratching two materials together Hot Hardness- This is bacially any test at elevated
temperature
Hardness tests
Fatigue Fatigue is considered when making tools
like dies, cams, gears, shafts, and springs that are subjected to rapid fluctuating.
Fatigue failure is when a crack is formed and continues to grow with every stress that is applied to it.
This failure is responsible for the majority of failures in mechanical components.
Creep
The permanent elongation of a component under static load maintained for a long period of time in high temp.
Such as turbine blades in a jet engine and components in a rocket motor.
Generally a great resistance to creep is using a material with very high melting point
Impact or dynamic loading Test
This test is used usually for bolts or drop forged materials
The test consists of notching material on one side and using a pendulum
From the amount of the swing of the pendulum, the energy dissipated is the impact toughness
The high strength materials also have a high impact toughness
Failure and Fracture of Materials in Manufacturing and Service Types of Failure 1.- Fracture: process of breaking either
internal or external of a material. 2.-Buckling: some products are designed in
such as way that failure is essential for their function.
Ductile fracture: is characterized by plastic deformation. Ductile fracture generally takes place along planes on which the shear stress is a maximum.
Brittle Fracture: occurs with little or no gross plastic deformation.
Residual Stresses
When workpieces are subjected to plastic deformation that is not uniform through out the part, they develop residual stresses.
Reduction and elimination of residual stresses: Residual stresses can be reduced or eliminated by deformation of the part, such as stretching it.
Work, Heat, and TemperatureAlmost all of the mechanical deformation is
converted into heat.Temperature rise:
△Τ=ˍuˍ ρc
T = Temperature U = specific energy (work of deformation per
unit volume ρ= density c = specific heat of the material.
Physical Properties of Materials. Density: The density of a
material is its mass per unit volume.
Density=Mass/Volume
DT301Smart Concentration/Density Transmitter
Melting Point Melting point: The temperature at which
a solid substance changes into a liquid state
Depending on the composition of an alloy the melting point has a wide range of temperatures
When designing a component it is important to consider the temperature range that it will be functioning when choosing materials
The melting point has indirect effects on manufacturing such as, with the process of annealing,heat treating, hotworking, and with making castings
Specific Heat
Definition: The energy needed to raise the temperature of a unit mass by one degree
Alloying elements have a relative minor effect on specific heat of materials
If the temperature rises excessively in a workpeice it can be disastrous
Thermal Conductivity Definition: the rate at which heat
flows within and through a material In a product, when heat is generated
it needs to be conducted away at a high enough rate to prevent a severe rise in temperature
Low thermal conductivity can result in deformation of products
Thermal Expansion of Materials
The relative expansion and contraction of deferent materials in an assembly
Parts that utilize thermal expansion and contraction are known as shrink fit assembly
Thermal expansion along with conductivity together produce stresses on components and tools which are undesirable
The undesirable effects that occur during a product service life is known as thermal shock
To reduce the problems caused by thermal expansion, metals were replaced with iron-nickel alloys
Thermal Expansion of Materials
Electrical, and magnetic properties of materials
Electrical conductivity: used to specify the electrical characteristics of a material.
This is measured in mho not to be confused with the reverse; ohm, which is to measure electrical resistance
Different things that are include with electrical properties
Conductors: materials with high electrical conductivity
Insulators: the materials that have high electrical resistivity
Superconductors: materials that have resistivity at very low temperatures, that plunge from a finite value to one that is virtually zero
Superconductivity: electrical phenomenon where electrical resistivity occurs in some metals and alloys at temperatures around absolute zero (0K)
Simi-conductors: devise that is used in extremely miniaturized electronic circuitry
Electrical properties
Magnetic properties
Ferromagnetism: phenomenon characterized by high permanent magnetization due to the alignment of iron, nickel, and cobalt atoms
Ferrimagnetism: permanent and large magnetization exhibited by ceramic materials
Corrosion resistance
All materials; metal, ceramics, and plastics are all subject to forms of corrosion
Corrosion leads to deterioration of components
Resistance depends on the composition of the materials and the environment their in
Nonferrous metals have very high corrosion resistance
Cold worked metals are more susceptible to corrosion than hot worked metals
Tool and die materials are susceptible to chemical erosion from lubes and coolants
Corrosion
Conclusion
Mechanical and Physical properties of materials must be considered when choosing a material for your design
Your design will be better with the right material
References
Lindbeck, John R. . Product Design and Manufacture. Prentice Hall. 1995.
Kalpakjin and Shmid. Manufacturing Engineering and Technology. Prentice hall, 5th edition.
Callister, William. Materials Science and Engineering. John Wiley & Sons. 4th edition. 1997
www.dictionary.com Pictures: Railroad picture:
www.physics.brocku.ca/courses/1p23/Heat/rail.html
Corroding plate: ht
tp://www.sculptures.freeserve.co.uk/images/crack%20corrosion%202.jpg Corroded bolts: www.yachtsurvey.com/ corrosion.htm
www.pesprings.com http://www.madisongroup.com/Services/Failure/failureanalysis.html