UNIT – III MECHANICAL PROPERTIES &

TESTING

Properties of a material:Strength:• It is the ability of the material to withstand

various forces to which it is subjected.Elasticity:• It is the ability of a material to recover its

original shape when load is removed.Plasticity:• It is the property that enables the formation of

permanent deformation in a material.• It is reverse of elasticity.

Stress- strain curve indicating elastic and plastic deformation

Ductility:• It is a measure of degree of plastic deformation

that has been sustained during fracture.• Due to this property, wires are made by

drawing operation.Brittleness:• Lack of ductility is brittleness• A material that experiences very little or no

plastic deformation upon fracture is called brittle material.

Stress- strain curve for ductile and brittle material

Malleability:• This is the property by virtue of which a

material may be hammered or rolled into thin sheets without rupture.

Toughness:• It is the strength with which the material

opposes rupture.Hardness:• It is defined as the resistance of material to

penetration.• It is a measure of material’s resistance to

localized plastic deformation.

Fatigue:• When subjected to fluctuating or repeating

loads, materials tend to develop a characteristic behavior which is different from the behavior under steady loads.

• Fatigue is the phenomenon that leads to fracture under such conditions

Creep:• It is the slow plastic deformation of metals

under constant or prolonged loading usually at high temperature.

DEFORMATION OF METALS• The change in dimensions of forms of matter

under the action of applied force is called deformation.

• There are two types of deformation. They are, (i) Elastic deformation (ii) Plastic deformation(i)Elastic deformation :• The deformation which disappears when load

is removed is called elastic deformation.

• During elastic deformation, strain is proportional to stress.

• For some materials (grey cast iron, concrete and polymers), the deformation is non-linear.

Anelasticity:• In some materials, elastic deformation occurs

when load is applied and upon release some finite time is required for complete recovery. This time-dependent elastic behavior is called anelasticity.

(ii) Plastic deformation :• It is the deformation which persists even after

the is removed.• It is observed at stresses exceeding the elastic

limit.

Atomic dislocations :• Atomic dislocation occurs by two phenomena (i) Slipping, (ii) Twinning(i)Slip:• It is defined as a shear deformation that moves

atoms by many interatomic distances in one crystal plane over atoms of other crystal plane.

• It is the process of sliding of blocks of crystal over one another along definite crystallographic planes called slip plane.

Atomic dislocation by slip

(ii) Twinning:• It is the process by which a portion of the crystal

takes up an orientation which makes that portion a mirror image of parent crystal.

Fracture:• The breaking of material to yield an irregular

surface is called fracture.• There are two types of fractures found in

metals.They are, (i) Ductile fracture, (ii) Brittle fracture

(i) Ductile Fracture:• During ductile fracture, an appreciable plastic

deformation prior to failure occurs and the fractured surfaces form cup and cone appearance.

(ii) Brittle Fracture:• It occurs when crack propagates through the cross

section without an appreciable plastic deformation.

Testing of materials• Tension test• Compression test• Shear test• Hardness test• Impact test• Fatigue and creep test• Fracture toughness test

Tension test:• The standard specimen with diameter

approximately 12.8 mm and length (at least four times this diameter) 60 mm is used.

• The specimen is mounted by its ends into the holding grips of the testing apparatus.• The tensile testing machine is designed to

elongate the specimen at a constant rate, and to continuously and simultaneously measure the instantaneous applied load (with a load cell) and the resulting elongations (using an extensometer)