Fatigue Crack Propagation Under Complex Loading

Abstract

The paper describes the recent progress of the understanding of fatigue crack propagation under complex loading. The behavior of fatigue crack propagation is dependent on the macroscopic mode of crack growth : tensile mode and shear mode growth. Tensile mode growth takes place on the plane perpendicular to the maximum tangential stress range, satisfying the local symmetry. Shear mode growth takes place on the plane of the maximum shear stress when a precrack is subjected to high cyclic shear stress. When the shear stress is low, a bent or branched crack is formed at the precrack tip, showing tensile crack growth. Under an intermediate stress, a crack first shows shear mode growth and turns to tensile mode after some extension. This transition is caused by the decrease in the mode II crack-tip stress intensity factor due to increasing crack surface contact. The criterion of the maximum crack growth rate may control the mode of fatigue crack growth. In notched components under complex loading, the life of crack initiation at the notch root is calculated from the critical plane model using the stress and strain at a critical distance from the notch root. The propagation of a fatigue crack from a notch is dependent on the geometry of notched component. The resistance-curve method is applicable for the prediction of fatigue thresholds of thin walled tubular specimens subjected to in-phase and out-of-phase combination of cyclic torsion and tension-compression.