Three-dimensional crack initiation mechanisms in bcc-Fe under loading modes I, II and III

We present quasicontinuum simulations of three-dimensional (3D) edge cracks in single crystal bcc-Fe. Crystals with different thicknesses and crystallographic orientations have been investigated and loaded in modes I, II and III. We study incipient plasticity at the crack tip and the onset of crack propagation, and calculate the critical stress intensity factors for crack propagation and dislocation emission. The active slip systems and the character of attendant dislocations have been identified and compared with the standard 2D behavior. We find that the observed type of propagation depends on the loading mode and crystallographic orientation. Mechanisms comprise twinning, bcc → fcc phase transformation, void formation and dislocation emission.