Strain Hardening

It was stated earlier that on reloading from g the new curve joins the original curve at h, where point h is higher than point a. In other words it can be said that due to the initial loading of the specimen upto point f its yield strength has increased to h for if it were not so, the reloading curve must follow a path similar to original curve showing a yield strength equal to the height of point a. This increase in yield strength because of loading in plastic region is termed as "strain hardening". If the material does not strain harden throughout plastic deformation it will extend at constant stress level. The materials that extend at constant stress level are called ideally plastic. Very soft materials behave in this manner.

The explanation to strain hardening is found in defects in crystalline materials at atomic level. Dislocations are such defects. Strain hardening is a practical method of increasing elastic strength. Wires which make helical springs are strain hardened.