Evaluation of fracture toughness of HSLA80 steel at high loading rates using stretch zone measurements

There is increased interest in using the stretch zone width (SZW) as a parameter for evaluation of the fracture toughness of ductile metallic materials. This approach is particularly useful in the case of fracture at high loading rates where evaluation of fracture toughness criteria such as JIc from compliance measurements is not feasible. In the current study, specimens of HSLA80 were fractured at very high loading rates, corresponding to K larger than 106MPa m12s−1 using a modified Split Hopkinson Bar system. These tests were performed on compact tension specimens of 12.6 mm thickness at −30°C. Other experiments were performed using a drop weight tester on specimens of 25 mm thickness at −40, −20, 0 and 20°C. The stretch zone width was determined using scanning electron microscopy. It was found that the steel retains a high fracture toughness at −30°C, albeit lower than at 20°C. In measurement of the SZW, for the 12.6mm specimens there was no significant difference between using a nine-point approach (ASTM) and a three-point approach, while for the 25 mm specimens, the nine-point approach gave a much lower value.