Mixed-mode I/II fracture behaviour of a high strength steel

Mixed mode fracture in a certain high strength steel has been investigated through physical experiments and numerical calculations. The main objective has been to investigate the implications of local crack tip processes on the macroscopic mixed mode fracture behaviour. A scrutiny of the fractured specimens revealed evidence of crack branching in all cases where β_eq < 40° ( β_eq = atan ( K_I /K_II ) ). The appearance of branching was found to be accompanied to a rather abrupt increase in the macroscopic mixed mode fracture toughness. In the numerical calculations the effective plastic strain criterion suggested by Hallbäck and Nilsson (1994) was applied to the present material. Crack tip branching was from the analysis predicted to occur at β_eq = 60°. Besides the presence of branching, it was however questionable whether the analysis corroborated the experimentally observed behaviour.