Mode II dynamic fracture toughness of two high strength steels under high loading rate

Mode II dynamic fracture toughnesses K_IId for high strength steels, 40Cr and 30CrMnSiNi2A, subjected to impact loading were measured using an experiment-number combined method. The tests were performed on shear specimens with Hopkinson pressure bar and the time of crack initiation was determined by strain gauge. With 3-D transient finite element analysis, the time evolution of dynamic stress intensity factor under different loading rates was obtained and the dynamic fracture toughness was determined by fracture initiation time. The results show that within the loading rate range (2 × 10⁶-7 × 10⁶ MPa·m^1/2/s), almost all the specimens failed by adiabatic shear bands and the K_IId values of the two steels increase with increasing loading rate. The K_IId value of 30CrMnSiNi2A is larger than that of 40Cr at the same loading rate. The effect of ligament size on fracture toughness was also discussed.