Macro-Mechanical Analysis for Symmetric and Asymmetric Fully Plastic Crack Growth

Asymmetries like welds or shoulders, may eliminate one of the two shear zones of symmetrically fully plastic cracked parts and thus give crack propagation along the remaining active slip band through pre-strained material instead of through the relatively unstrained region between two shear bands of the symmetric case. One thus expects a reduced ductility in the single shear band asymmetric case. A macro-mechanical analysis provides a physical basis for explaining the development of deformation in both geometries, as observed from tests on several alloys. The asymmetric case is approximated as Mode II shearing, with the crack extension occurring by sliding off along a single slip plane and fracture. In the symmetric Mode I case, the crack is assumed to extend by alternating shear on two symmetric slip planes and fracture.