Variation of fracture toughness with constraint

‘Curving’ could occur in Mode I brittle fracture and it can be attributed to the constraint. The variation of the apparent fracture toughness with constraint with or without crack curving is investigated. Using a critical opening or hoop stress as the fracture criterion, it is demonstrated that a fracture event can be quantified by two mechanics parameters, i.e. K and A3 for fracture without curving and K and T for fracture with curving. K, T and A3 are the first, second and third coefficient in the Williams’ asymptotic series solution, respectively, and are used here to quantify the constraint level. Test data are used to validate the theory. It is shown that while the apparent fracture toughness increases with increasing T it decreases with T after curving occurs.