Q. Define elastic and plastic deformation and differentiate between them.

Ans. Elastic deformation : Elastic deformation is that part of the deformed material, when the applied load is removed, would spring back to its normal space. The elastic deformation takes place of low stresses and having following characteristics.

          (i) Reversible                    (ii) Usually very small

       (iii) Obeys the hook's low i.e. stress is professional to strain.

       An elastic effect is due to : ? Grain boundary effect  ? Motion of dislocations ? Thermo elastic effects  ? Atomic diffusion 

? Magneto elastic effect

       The damping capacity of materials is also another aspects of an elastic behaviour.

       Plastic Deformation : In the plastic deformation, the material will not obeys the hook's low. This deformation is set generally 40% melting temperature of that material and depends on applied load, rate of strain, temperature etc. During the plastic deformation the metal follows the following relation i.e.

                              P = ken

             Where      P = stresses; K = strength coefficient

                              e = strain; n = power factor

      Mechanism of plastic deformation : When the crystal is subjected to sufficiently large force, plastic deformation takes place i.e. atoms moves permanently. This movement of atoms is not haphazard but the movement occurs in definite direction in the lattice. Plastic deformation takes place due to mainly : (i) Slip         (iv) Twining

Difference between elastic and plastic deformation :

     Elastic deformation                          plastic deformation

1. It is first stage of deformation.          It is second stage of deformation

                                                             I.e. after the elastic deformation.

2.  It disappears after the                      it does not disappear even the

     Removal of load.                              Load is removed.

3.  It is temporary.                                It is permanent.

4.  It is reversible.                                 It is irreversible.

5.  Stress-strain relationship is            In plastic deformation there is

     Given by a constant.                       No such well defined constant.

6.  After this next stage is plastic         After this next stage is fracture.

     Deformation.

7.  Elastic deformation is practically     Plastic deformation is high and

     Small and produced strain in          the strain produced is also quite

     The order of 0.5% only.                  high.