Experimental study on mode-I and mixed-mode crack propagation under tangentially incident P waves, S waves and reflected waves in blasts

Abstract

A dynamic crack subjected to tangentially incident P and S waves, as well as reflected waves, was studied by caustics method in a PMMA plate under two explosions of 20 mg and 60 mg lead azide. Periodic P and S waves are visualized as fringes with different densities, which is verified by calculations of arriving time and different caustics patterns in the crack tip. Distorted and classical caustics patterns occur under P and S waves, respectively, indicating that crack-tip K-dominated stress field is destroyed by P waves, while it is less disrupted by S waves. Hence, modified caustics interpretation previously proposed by authors and classical one are employed to reproduce experimental caustics patterns under P and S waves, respectively, to obtain crack-tip positions, crack velocity, KI and KII. Results show that P-wave compression loading plays a dominant role to arrest the crack, despite temporary crack re-initiation by P-wave tension loading, and that S waves temporarily accelerate the crack with alternate positive and negative KII, indicating periodic shear stress is not a sustainable driving force for crack propagation. Reflected waves cause mixed-mode crack propagation with periodic crack deflections, in which peaks of KI correspond to peaks of crack velocity and flat crack path, however peaks of KII correspond to crack slowdowns and crack deflections. Finally, this paper emphasizes a comprehensive analysis including crack path, cack-tip positions, crack velocity, caustics shape, KI and KII is required for evaluating mixed-mode crack propagation in blasts.