Strain hardening is a common behaviour in most materials. It consists of the yield stress increment during the different loading and unloading cycles applied on the material.
When a first load cycle is applied, a point A is reached above the elastic limit. Unloading from A develops along a straight line AB which is parallel to the linear elastic region (so the slope is the elastic module E of the material). At the end of this first cycle ( loading - unloading of the material ) an amount of permanent deformation OB is present in the material.
If a second loading cycle is applied, the origin would be the finishing point for the precedent unloading (B), and load increase would develop along BA. Until point A is reached, the deformation of the material is linear elastic. From this point on, plastic deformation increases and it is stored in the material. The main consequence is the increase of the yield stress magnitude between the first and the second load cycles. With the second load cycle, point C is reached, where unloading starts along CD line. Once the unloading process is finished, the permanent deformation in the material is OD.
An hypothetical load cycle would start at D. It would be linear elastic up point C, where plastic deformations would increase when the load increased.