Dynamic Crack Deflection and Propagation in PMMA under Blast Stress Waves by Caustics Method

The dynamic interaction between blast-induced stress waves and propagating cracks influences the fracture behavior and performance of materials under dynamic loading. This paper experimentally investigates how stress wave characteristics affect the propagation path, velocity, and dynamic stress intensity factors of moving cracks in polymethyl methacrylate (PMMA). Two typical stress wave loading scenarios are designed: reflected tensile waves from the upper boundary in single-borehole blasting and obliquely incident compressive waves from delayed double-borehole blasting. A dynamic caustics optical system combined with high-speed photography is employed to capture the entire crack initiation, propagation, and deflection process in real time. The results indicate that stress wave characteristics govern crack behavior through a dual-parameter coupling mechanism of energy and direction. Tensile waves increase the mode I dynamic stress intensity factor K_Id, promoting crack propagation, whereas compressive waves decrease K_Id, inhibiting crack propagation. The incident direction and wave type jointly determine the sign and magnitude of the mode II dynamic stress intensity factor K_IId, thereby controlling the crack deflection direction. The findings establish a quantitative correlation between stress wave parameters and crack tip stress field evolution, providing experimental insights into dynamic fracture characterization of materials under complex transient loading conditions.