Investigation on multiaxial fatigue crack path using polar stress–strain representation

Critical plane concept is widely used as the basis for formulating fatigue damage models. Understanding of fatigue crack behavior, initiation and early growth, under cyclic multiaxial loading is essential for fatigue damage analysis and fatigue life predictions. Full stress–strain fields surrounding an infinitesimal element at the gauge section were obtained by transforming cyclic hysteresis loops using plane stress–strain transformation relations. These fields were represented on polar diagrams to show the loci of maximum stresses or strains with respect to the plane orientation. Measured crack paths were superimposed onto polar diagrams to understand the crack growth behavior with respect to stress or strain fields. Cracks were found to initiate at both planes of maximum normal or shear strains. It was found that there might be equal chances for a crack to initiate and grow at different paths. Polar diagrams can be used to determine the likelihood regions where crack could possibly initiate and grow.