If you deform a solid plastically, then unload it, and then try to re-load it so as to induce further plastic flow, its yield point/elastic limit increases i.e. the material resistance to plastic deformation will have increased. There are, two simple hardening rules for modelling strain hardening in a finite element material: Isotropic and Kinematic. Isotropic hardening rule states that the yield surface expands proportionally in all directions when yield stress is exceeded. Kinematic hardening rule states that the yield surface does not exceed, but translates in the direction of the stress rising and stays in the same area and shape.