Critical plane approach with two families of microcracks for modelling of unilateral fatigue damage

New multiaxial fatigue damage model based on the critical plane approach is proposed. Two different physical mechanisms of the fatigue damage developm ent on each potential failure plane (critical plane) are considered. In gen eral, each critical plane contains two families of a parallel microcracks. The proposed model reproduces simultaneously fatigue damage induced anisotr opy, the influence of positive and negative mean stresses, unilateral fatig ue damage, microcrack closure effect and fatigue behaviour under variable a mplitude loading. The expression for the equivalent stress in the damage ev olution equation includes the stress intensity for the amplitudes as well a s joint invariants for the mean values of the stress tensor and for the vec tors associated with the directions of microcracks. The theoretical predict ions are compared with experimental data under uniaxial cyclic loading of b rass specimens. The influence of positive and negative mean stresses on the fatigue life of brass is investigated.