For many materials, plastic deformations under cyclic loadings differ from monotonic loading. Cyclic plastic zone with considering the strain hardening effects becomes complicated during unloading step. In this research, the effects of nonlinear kinematic and isotropic hardening behaviours on the cyclic plastic reaction around the crack tip for different conditions are inves- tigated. For study of various hardening characteristics, as-received and annealed copper are tested under same symmetric cyclic loadings. The results indicate considerable isotropic hardening behaviour in annealed material. A Chaboche nonlinear hardening model was used to determine the hardening parameters. The cyclic plastic zone around the crack tip in C(T) specimen was measured by back stresses. The cyclic plastic zones are speci- fied via variations of the back stresses in a cycle. The cyclic plastic zones predicted by Chaboche model are smaller than those for Irwin model because of the hardening effects. Also, Irwin model for prediction of the cyclic plastic zone size is modified with considering the cyclic plastic effects. According to the results, the cyclic plastic zone around the crack tip is almost constant in the same load range, and load ratio (R) has a slight effect on this zone. Whereas, for constant load range, cyclic J-integral (ΔJ) is different for various R values. For studying of the fatigue crack growth, two parameters of ΔJ and cyclic plastic zone area were evaluated. These parame- ters can apply in fracture mechanics especially in elastic–plastic conditions with considering the cyclic plastic responses.
