Work (Strain) Hardening:

Work hardening is while a metal is strained beyond the yield point. An increasing stress is needed to generate further plastic deformation and the metal apparently becomes stronger and harder to deform.

The rate of strain hardening tends to become almost uniform if true stress is plotted against true strain, which is, the curve becomes almost a straight line, as display in Figure. The gradient of the straight categories of the line is known as the strain hardening coefficient or work hardening coefficient, and is closely associated to the shear modulus (about proportional). Thus, a metal along with a high shear modulus will have a high strain or work hardening coefficient (for instance, molybdenum). The Grain size will also influence strain hardening. A material along with small grain size will strain harden more rapidly than the similar material with a larger grain size. Therefore, the effect only applies in the early stages of plastic deformation, and the influence disappears as the structure grain and deforms structure breaks down.

Work hardening is nearly associated to fatigue. In the instance on fatigue given above, bending the thin steel rod becomes harder the farther the rod is bent.  This is the result of work or strain hardening. It decreases ductility that increases the chances of brittle failure.

Figure: Nominal Stress-Strain Curve vs True Stress-Strain Curve

Work hardening could also be used to treat material. The Prior work hardening (cold working) causes the treated material to have an apparently higher yield stress. Thus, the metal is strengthened.