Mechanical testing
Transcript of Mechanical testing
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)