Uploaded by Pethuraj M

Unit 1

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Deformation of Metals
Introduction
• When metals or alloys are stressed, they get deformed
• Deformation is the change in dimension or form of matter under the
action of applied forces.
• Deformation is caused by
• Mechanical action of external forces
• Various physical and physico chemical process (change in volume of separate
crystallites in phase transformation)
• Deformation may be wholly temporary – ELASTIC DEFORMATION –
which disappears after the deforming load is removed.
• Permanent deformation may also occur- PLASTIC DEFORMATION –
which remains after the removal of the deforming load.
Elastic Deformation
• If a metal is loaded, a temporary deformation of the crystals takes
place through displacement of the atoms.
• As the deforming load is removed, the atoms return to their original
stable position and the crystals recovers its original shape.
Elastic Deformation
• It occurs with comparatively smaller deforming loads, which can keep
working stress within the elastic range.
• Under tensile deforming load, the metal piece becomes slightly
longer as a result of slight elongation of the unit cell in the direction
of the tensile load.
• This elongation in the metal piece disappears as soon as the
deforming load is removed.
• When elastic deformation occurs, the strain in metal piece is nearly
proportional to the stress, i.e. the condition in the elastically
deformed region are expressed by Hooke’s Law
Elastic Deformation
• Hooke’s law which says in case of the tension test that deformation e
is proportional to applied stress:
𝑃𝐿
𝑒=
𝐴𝐸
•
•
•
•
P is the applied force
L is the length of the metal specimen
A is the cross sectional area of the specimen
E is modulus of elasticity
Plastic Deformation
• Plastic deformation follows elastic deformation
• If the deforming load is such that the stresses in the metal piece cross
the elastic limit, the specimen gets plastically deformed.
• If the deforming load, at this stage is removed, the metal piece does
not regain its original shape; a permanent (plastic strain) set always
remain.
Plastic Deformation
• When the load is suddenly removed from a plastically deformed
specimen, the total strain is immediately reduced by the amount
Stress/ Young’s Modulus i.e. the elastic strain
Plastic deformation
• Next, the anelastic strain disappears gradually after the load has been
removed but the plastic deformation remains.
• Plastic deformation is the function of
• Applied stress
• Temperature
• Strain rate
• It is accompanied by change in both the internal and external state
and it is not reversible. It involves distortion of crystal and
microstructure.
• It is carried out during shaping process such as bending, stamping etc.
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