Blunt Cracks and Two Parameter Fracture Criteria

Abstract

There is a concern in the fracture testing community about the influence of crack sharpness on measured toughness values. The notion of sufficiently sharp is intrinsic in the standards but does not define how sharp is sharp enough. This requires a definition of a crack tip radius below which the toughness will be Gc. A proposal is made here to modify the stress at a distance criteria in line with cohesive zones such that G GC and a critical stress σC is achieved. This provides a critical radius, ρC, below which, G = Gc. A comparison with FEA cohesive zone results supports the idea and comparison with data on uPVC conforms well to the suggested scheme to determine ρC. Some comments on self-blunting are also included.