A Microcrack Description of Unilateral Fatigue Damage in Crystal1ine Solids

A constitutive model is developed to describe the multiaxial fatigue damage for polycrystalline materials with two families of parallel flat microcracks. This model describes simultaneously the directional aspect of fatigue damage， the influence of positive mean stress as well as negative mean stress， unilateral fatigue damage and mlcrocrack closure effect. The equivalent stress in the fatigue damage evolution equation contains the stress intensity for the amplitudes as well as joint invariants for the mean values of the stress tensor and for the vectors associated with the directions of microcracks. It is shown how material parameters in the proposed model can be determined from a series of basic experiments. The results generated from the model are compared with those from experiments under uniaxial cyclic loading of brass specimens in a low-cyc1e fatigue range. It is shown that the proposed model describes well the influence of positive and negative mean stresses on the fatigue life of brass