Elasticity, Plasticity and Strain hardening


This is the property of a material to regain its original shape after deformation when the external forces are removed. When the
material is in elastic region the strain disappears completely after removal of
the load, The stress-strain relationship in elastic region need not be linear
and can be non-linear (example rubber). The maximum stress value below which
the strain is fully recoverable is called the elastic limit. It is represented by point A in figure. All
materials are elastic to some extent but the degree varies, for example, both
mild steel and rubber are elastic materials but steel is more elastic than rubber.


When the stress in the material exceeds the elastic  limit, the
material enters into plastic phase where the strain can no longer be completely
removed. Under plastic conditions materials ideally deform without any increase
in stress. A typical stress strain diagram for an elastic-perfectly plastic
material is shown in the figure. Mises-Henky criterion gives a good starting
point for plasticity analysis.



Strain hardening

If the material is reloaded from point C, it
will follow the previous unloading
path and line CB becomes its new elastic region with elastic limit defined by
point B. Though the new elastic region CB resembles that of the initial elastic
region OA, the internal structure of the material in the new state has changed.
The change in the microstructure of the material is clear from the fact that
the ductility of the material has come down due to strain hardening. When the
material is reloaded, it follows the same path as that of a virgin material and
fails on reaching the ultimate strength which remains unaltered due to the
intermediate loading and unloading process.


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