Work hardening is when a metal is strained beyond the yield point. An increasing stress is required to produce additional plastic deformation and the metal apparently becomes stronger and more difficult to deform.
If true stress is plotted against true strain, the rate of strain hardening tends to become almost uniform, that is, the curve becomes almost a straight line, as shown below. The gradient of the straight part of the line is known as the strain hardening coefficient or work hardening coefficient, and is closely related to the shear modulus (about proportional). Therefore, a metal with a high shear modulus will have a high strain or work hardening coefficient (for example, molybdenum). Grain size will also influence strain hardening. A material with small grain size will strain harden more rapidly than the same material with a larger grain size. However, the effect only applies in the early stages of plastic deformation, and the influence disappears as the structure deforms and grain structure breaks down.
Work hardening is closely related to fatigue. Bending the thin steel rod becomes more difficult the farther the rod is bent. This is the result of work or strain hardening. Work hardening reduces ductility, which increases the chances of brittle failure.
Work hardening can also be used to treat material. Prior work hardening (cold working) causes the treated material to have an apparently higher yield stress - the metal is strengthened.