Effects of vertical and horizontal reflected blast stress waves on running cracks by caustics method

•Effects of vertical and horizontal reflected blast waves on cracks were studied.•The geometry of caustics patterns was studied.•Distorted caustics patterns were precisely interpreted.•SIFs measurements by caustics method in blast-induced fracture are discussed. Wave reflection which is inevitable in the explosion plays a significant role in blast-induced fracture. In this paper, transient effects of vertical (P1P) and horizontal (P2P) reflected blast stress waves with wave fronts perpendicular and parallel to crack surfaces, respectively, on blast-induced cracks in a PMMA plate are investigated in the same explosion by the caustics method with high-speed photography. Reflected P and S waves are identified by bright and dark fringes and asymmetrical caustics patterns, which is verified by theoretical calculations. The emphasis is placed on the geometry of distorted caustics patterns which are not consistent with the classical ones. It is found that K-dominated field in the crack tip is violated with unstable crack propagation under reflected P waves, while it is available with stable crack propagation under reflected S waves. To precisely interpret distorted caustics patterns, a novel and simple model based on geometrical optics is established, agreeing well with experimental results and providing accurate dynamic stress intensity factors (SIFs) which are verified by the KId-v relationship. Through accurate SIFs, it is clear that vertical P1P waves tend to cause crack arrest with lower SIFs and crack velocity, while horizontal P2P waves assist in crack opening with higher SIFs and crack velocity. Finally, problems of SIFs measurements by the caustics method in blast-induced dynamic fracture are discussed.