Loading rate effect and failure mechanisms of ultra-high-strength steel under mode II fracture

40Cr and 30CrMnSiNi2A are both ultra-high-strength steel (UHSS) that is usually used in engineering structures such as aircraft landing gear, wing girder, and fastening bolts. Under dynamic loading, fracture properties of such materials are very important for structural design. A novel mode II dynamic fracture testing technique is adopted to study the mode II dynamic fracture characteristics of these two materials under high loading rates. The mode II dynamic fracture specimen is designed for the SHPB equipment, and the stress intensity factor curve at the crack tip is determined by an experimental-numerical method. The crack initiation time of the specimen is determined by the strain gage method. In the end, the mode II dynamic fracture toughness (K_IId) of the two materials is obtained, and the loading rate effects are compared and analyzed in detail. The results show that within the loading rate range of this research (1.08 ∼ 7.73 TPa·m^1/2/s), the K_IId of the two materials has a positive correlation to the loading rate. Both of the materials exhibit a gradual transition process from tensile failure to adiabatic shearing failure mode. According to the fracture morphologies of the two materials, the failure mechanism of the two materials is analyzed for different failure modes. Failure mode transition (FMT) is specially investigated with the increase of the loading rate for both of the materials.