Cyclic plasticity and variable amplitude fatigue

Load history effects are observed both in cyclic plasticity and during fatigue crack growth. History effects in cyclic plasticity of bulk material are inherited from material strain hardening (isotropic and kinematic hardening). Kinematic hardening is usually attributed to the development of plastic strain gradients inside the material. During fatigue crack growth, history effects, such as crack retardation after an overload for example, are also observed. An overload causes the development of a plastic strain gradient at the crack tip. This plastic strain gradient leads to the development of residual stresses during unloading, which cause a reduction of the efficiency of subsequent fatigue cycles and fatigue crack retardation. Consequently, this phenomenon is analogous to a kinematic hardening effect, but related to crack tip mechanical fields. Load-load interaction effects are therefore expected to be predictable using a phenomenological for cyclic plasticity at the crack tip, analogous to those employed for the description of the cyclic plasticity of bulk material.