Tests and analyses for fully plastic fracture mechanics of plane strain mode I crack growth

Under monotonic loading, structures should ideally be ductile enough to provide continued resistance during crack growth. Such fully plastic behavior is of interest in design against collisions, tank car accidents, earthquakes, and ship groundings. For fully plastic crack growth in low strength alloys, existing asymptotic solutions for elastic-plastic growing cracks are not applicable because they reach the fracture strain only in regions small compared to the inhomogeneities of the actual fracture process. For the limiting case of non-hardening fully-plastic plane strain crack growth, in a number of geometries and loadings the near-tip fields are characterized in terms of three parameters: an effective angle 2θ_s between a pair of slip planes, and the normal stress σ_s and the increment of displacement δu_s across the planes. This three-parameter characterization is in contrast to the one- or two-parameter (K or J and T or Q) characterization in linear or non-linear elastic fracture mechanics. These θ_s, σ_s, and δu_s parameters are found from the far-field geometries and loadings through slip line fields or least upper bound analyses based on circular arcs. The resulting crack growth, in terms of the crack tip opening angle (CTOA), is a function of θ_s, σ_s, and the material. The geometry of the crack growing between two moving slip planes emanating from its tip reduces this function to the critical fracture shear strain left behind the slip planes, γf, as a function of σ_s. γf(σ_s) is found theoretically from a hole initiation and growth model. It is also found from preliminary fully plastic crack growth experiments on unequally grooved specimens with fixed-grip extension or 4-point bending of a 1018 CF steel. At high triaxialities and also after strain aging, cleavage intervened abruptly, even during stable, slow deformation.