Effect of loading rate on mode I dynamic fracture toughness of high strength steels 40Cr and 30CrMnSiNi2A

Mode I dynamic fracture toughnesses K_Id for high strength steels, 40Cr and 30CrMnSiNi2A, subjected to impact loading with different loading rates were measured using an experimental-numerical hybrid method. The sensitivity of K_Id to loading rate for the steels were investigated. The tests were performed on three point bending specimens with Hopkinson pressure bar and the time of crack initiation was determined by strain gauge. With 3-D transient finite element analysis, the dynamic stress intensity factor histories under different loading rates were obtained and the dynamic fracture toughnesses were determined by fracture initiation time. The results show that within the loading rate range (10⁶ MPa·m^1/2/s), 40Cr steel specimens failed by cleavage fracture, and the K_Id tendency is not clear with increasing loading rate. 30CrMnSiNi2A steel specimens failed to a large extent by ductile fracture and the K_Id value increases obviously with loading rate. Mechanisms of the results were discussed macroscopically and microcosmically.