Dynamic propagation behaviors of pure mode I crack under stress wave loading by caustics

It is always a difficult task to study the dynamic fracture of prefabricated cracks under stress wave loading. To investigate loadings of stress waves on dynamic cracks, a crack propagation testing configuration consisting of a one-point bend specimen loaded in split Hopkinson pressure bar (SHPB) was used, which loaded an unconstrained polymethyl methacrylate (PMMA) plate (120 mm x 60 mm x 5 mm) on the edge opposite to the cracked edge. A modified simple pre-cracked specimen geometry under impact stress wave loading to generate pure mode I crack at crack initiation was demonstrated, which can avoid the superposition and interference of various waves to facilitate the research. The numerical simulation was performed firstly by ABAQUS to prove the existence of the mode I field at the crack tip leading to crack propagation and indicate the stress distribution and evolution in the specimen caused by the propagation of the impact stress wave, analyze the propagation characteristics of the wave. Then dynamic caustics method in conjunction with high-speed photography was utilized in SHPB impact experiment. The propagation of shock stress wave in the specimen and its interaction with the prefabricated crack and the stress concentration at the tip of the prefabricated crack were analyzed. The corresponding stress intensity factor history is precisely determined. Finally, it is concluded that the observed distortion phenomenon at the impact point belongs to a caustic behavior under compression load, which reflects the stress concentration at the impact point. And the mode I failure occurs along the pre-crack direction. Specifically, the pre-crack shows obvious pure mode I crack propagation characteristics under symmetrical reflected tensile wave, the stress at the crack tip changes from compressive stress to tensile stress. And crack propagates under tensile stress wave reflected from its two free boundaries without crack, while the compressive stress wave can not make crack initiate and has little influence on crack propagation. Which agree with the numerical prediction.