Experimental study on the effects of joint span and shape on crack initiation and propagation in PMMA plates by the caustics method

• Filling material type significantly affects PMMA joint crack propagation, with air-filled joints producing multi-cracks and silicone/epoxy-filled joints generating single cracks at different positions. • Quasi-static loading leads to more pre-crack energy accumulation and a higher initial stress intensity factor than dynamic loading, and silicone/epoxy-filled specimens show longer energy recovery time than air-filled ones. • Under dynamic loading, PMMA joint length positively correlates with crack initiation and propagation time, and material hardening parameter and fractal dimension also exert notable impacts on PMMA fracture. This study employed a dynamic caustics system integrated with a Hopkinson pressure bar, Schlieren optics, and a high-speed camera to investigate how joint span and shape affect crack initiation and propagation. First, crack penetration into joints with different spans (10 mm, 30 mm, 50 mm) and different shapes (“u” and “n”) was visualized. Then, crack-tip stress intensity factors and propagation velocity were measured by high-speed caustics patterns. Finally, fractal dimensions of crack trajectories were obtained to quantitatively evaluate the complexity of the crack layout. Based on loading time, the crack behavior is divided into 4 phases: first precrack initiation, propagation toward the joint, secondary initiation from the joint and final propagation toward the boundary. Since the phase 1 duration increases with span, crack initiation from precracks clearly depends on span length. In phases 2 and 3, reflected waves occur from the joint interface; furthermore, they are confirmed to be Rayleigh waves through wave velocity. Meanwhile, the reflected Rayleigh waves from the “n”-shaped joint have a significant effect on crack propagation in phase 2. In phase 4, crack trajectories initiating from joint ends are heavily influenced by joint span, which is associated with crack interaction. Furthermore, different opening orientations (“u” and “n”) of arc-shaped joints have different effects on crack behavior. The “u”-shaped joint exhibits crack behavior similar to that of same-span line-shaped joints. The “n”-shaped joint demonstrates a strong fracture resistance. This work advances the understanding of fracture resistance as influenced by joint span and shape variations.