Plastic zone formation and stable crack growth in high-strength alloy sheets

Abstract

Plastic zones generated at notches have been studied in detail for six different high-strength sheet alloys (0·03–0·14 in. thick) using a replication technique. The size and shape of the zone is related to the material, the sheet thickness, and, in particular, the progress of stable crack growth. In these sheets there are two components of stable growth: (a) a triangular flat normal to the tensile axis and coplanar with the original notch, (b) shear cracks on oblique planes joining the shank of the flat with the sheet surface. The length of the flat depends only on sheet thickness and not on the material. Since the plastic zone represents shear deformation extending from the flank of the crack to the surface, the length of the internal flat fracture can be deduced from the shape of the plastic zone. The shear fracture begins at a higher stress level than the flat, but grows more rapidly. Accordingly, unstable fracture occurs on shear planes. Because of these complexities, it is difficult to predict values of stable crack growth in terms of available models.