Work Hardening, Strain Hardening
Work hardening (strain hardening) manifests as the increase in stress that is required to cause in increase in strain as a material is plastically deformed. On the diagram, the red curve is for a material that does not work harden - an ideal plastic material. Plastic deformation begins when the yield stress is reached and this material deforms to fracture at the same stress value. The black curve shows the true resolved shear stress/shear strain response of a material that work hardens. Yield again starts at the yield stress, but as the strain increases an increase in stress is required to maintain the same strain rate. The difference between the two curves measures the degree of work hardening.

The insert on the diagram shows a mechanism for work hardening. Dislocations on intersecting slip planes permit both elastic interactions and dislocation reactions to contribute to work hardening.

From: Ashby and Jones, 
"Engineering Materials I," Pergamon (1980)