A number of the important mechanical properties of materials, predominantly metals, have been discussed in this unit.
REVIEW AND SUMMARY
✔ A number of the important mechanical properties of materials, predominantly metals, have been discussed in this unit.
✔ Mechanical properties are those characteristics of material that describe its behaviour under the action of external forces.
✔ Some of the important mechanical properties are elasticity, plasticity, ductility, malleability, brittleness, hardness, tough- ness, stiffness, resilience, creep, endurance, strength, impact strength and fatigue.
✔ Technological properties are those which have a bearing on the processing and/or application of materials.
✔ Some of the important technological properties are machina- bility, castability, weldability, and formability/workability.
✔ Factors that highly influence the mechanical properties are 1. Grain size; 2. Heat treatment; 3. Atmospheric exposure; and 4. Low and high temperatures.
✔ When force is applied on a metal piece, then the size and/or shape will be altered. Any changes in the size and/or shape of the metal is called a deformation of the metal.
✔ Deformation of metals can be classified into: 1. Elastic deformation, and 2. Plastic deformation.
✔ Elastic deformation is the deformation of a body which completely disappears as soon as the external load is removed from the body.
✔ Plastic deformation is the deformation of a body which remains even after removing the external load from the body.
✔ Mechanisms/modes of plastic deformation: 1. Slip, and 2. Twinning.
✔ These mechanisms of plastic deformation are discussed in detail in this unit.
✔ Slip may be defined as the sliding of blocks of the crystal over another along definite crystallographic planes called slip planes.
✔ Slip begins when the shearing stress on the slip plane in the slip direction reaches a threshold value called the critical resolved shear stress (τcr).
τcr = σ cos φ cos λ
where the terms τcr, σ, φ and λ have usual meanings.
✔ In the above equation, the term 'cos φ cos λ' is known as the Schmid's factor.
✔ Schmid's law states that the stress required at a given temperature to initiate slip in a pure and perfect single crystal for a material is constant.
✔ Twinning is the process in which the atoms in a part of a crystal subjected to stress, rearrange themselves so that one part of the crystal becomes a mirror image of the other part.
✔ Two types/causes of twins are: 1. Mechanical twins, and 2. Annealing twins.
✔ Fracture is the mechanical failure of the material which will produce the separation or fragmentation of a solid into two or more parts under the action of stresses.
✔ Types of fractures: 1. Brittle fracture, 2. Ductile fracture, 3. Fatigue fracture, and 4. Creep fracture.
✔ The mechanisms of these fractures are also presented in this unit.
✔ Brittle fracture may be defined as the fracture which takes place by the rapid propagation of crack with a negligible deformation.
✔ Griffith's fracture equation: 
where notations have usual meanings.
✔ Ductile fracture may be defined as the fracture which takes place by a slow propagation of crack with appreciable plastic deformation.
✔ Fatigue fracture may be defined as the fracture which takes place under repeatedly applied stresses.
✔ The S-N diagram can be obtained by plotting the number of cycles of stress reversals (N) required to cause fracture against the applied stress levels (S).
✔ Creep may be defined as the property of a material by virtue of which it deforms continuously under a steady load.
✔ A creep curve shows the variation of the extension of a metal with time under different stresses.
✔ There are many different types of test are used for determining the various mechanical properties.
✔ Various mechanical tests are: 1. Tensile test; 2. Compression test; 3. Shear test; 4. Hardness test; 5. Impact test; 6. Fati- gue test and 7. Creep test.
✔ The tensile test describes the resistance of a material to a slowly applied stress. Important tensile properties include yield strength, tensile strength, modulus of elasticity, % elongation, and % reduction in area.
✔ The compression test is used to determine the tensile properties of brittle materials.
✔ The shear test describes the resistance of a material subjected to parallel and opposite tangential forces.
✔ The hardness test measures the resistance of a material to penetration and provides a measure of the wear and abrasion resistance of the material.
✔ The three most commonly used hardness tests are : 1. Brinell hardness test; 2. Vickers hardness test; and 3. Rockwell hardness test.
✔ The impact test describes the response of a material to a rapidly applied load. The energy required to fracture the specimen is measured and can be used as the basis for compa- rison of various materials tested under the same conditions.
✔ Types of impact tests: 1. Izod tests, and 2. Charpy tests.
✔ The fatigue test determine the resistance of a material to repeated pulsating or fluctuating loads. Important fatigue properties include fatigue or endurance limit, fatigue strength, and fatigue life.
✔ The creep test provides information on the load-carrying ability of a material at high temperatures. Creep rate and rupture time are important properties obtained from these tests.
KEY TERMS YOU SHOULD REMEMBER
Mechanical Properties
Elasticity
Plasticity
Ductility
Malleability
Brittleness
Hardness
Toug'iness
Stiffness
Resilience
Creep
Endurance
Strength
Impact strength
Fatigue
Technological
Properties
Castability
Weldability
Formability/Workability
Deformation
Elastic deformation
Plastic deformation
Slip
Critical resolved shear
stress
Schmid's law
Twinning Fracture
Brittle Fracture
Cleavage planes
Griffith crack theory
Elastic strain energy
Surface energy
Fracture strength
Ductile fracture
Fatigue fracture
Stress cycles
S-N diagram
Endurance limit
Fatigue-limit
Tensile test
Shear test
Fatigue life
Creep fracture
Creep curve
Creep strength
Creep limit
Creep life
Creep resistance
Dislocation climb
Compression ¡est
Hardness test
Brinell hardness test
Vickers hardness test
Rockwell hardness test Impact test
Izod test
Vacancy diffusion
Charpy test
Grain boundary
sliding
Fatigue test
Mechanical testing
Creep test
Fatigue strength
Destructive tests
Non-destructive tests
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