In accidents some structures should be fully plastic, even during extensive crack growth, to help share the load. K and J fracture mechanics apply to initial but not extensive growth. Plane strain, rigid-plastic, non-hardening plasticity often gives just two symmetrical slip lines at the crack tip for Mode I; one for mixed mode. These lines are the basis for the new slip line fracture mechanics, SLFM2,1. In SLFM2, the crack tip driving parameters CTDP are the angle of, and the normal stress and slip displacement across, the two slip lines. The response functions of the CTDPs can be taken as the crack tip opening displacement for initiation, CTODⁱ, and the angle CTOA for growth. SLFM1 has different response functions of only the stress and slip as driving parameters.

Slip line analyses for exact and approximate stress and deformation fields are given for typical specimens that provide data for structures. For structural steels the CTOA is typically 8 to 25°. The relations to K, J, and FEA are discussed, as are instability and cleavage.