Unilateral Effect in Plastic Fatigue with Micromechanical Approach

Based on the slip theory, a micromechanical model of damaged-inelastic behavior, which was recently proposed for metallic polycrystalline structures (FCC), is extended in order to describe the unilateral effect in low-cycle fatigue with a small strain assumption. In this micromechanical model, the micro-damage variables initiate and then evolve on the activated crystallographic slip systems. These variables are coupled with micro-macro constitutive equations. The formulation of the unilateral character of the damage (activation and deactivation) is limited only to the macroscopic level using the isotropic damaged concept. At this point, the unilateral effect is introduced with the help of a simple phenomenological constant. It allows to appropriately describe the macro-damage parameter evolution during the closure of defects (inactive phase) with the continuity of the stress-strain response.