Modified mixed-mode caustics interpretation to study a running crack subjected to obliquely incident blast stress waves

Abstract

Crack-wave interaction is common in blast, producing mixed-mode cracks when stress waves are oblique to cracks. How to measure mixed-mode crack-tip stress under blast stress waves is difficult. To this end, optical caustics method was employed to study a running crack in a PMMA plate subjected to obliquely incident blast stress waves, by which stress problem was transformed into optical problem. Incident P and S waves, as well as reflected waves, were visualized as fringe patterns, and mixed-mode crack-tip stress was represented by a caustics pattern (shadow spot). However, mixed-mode caustics patterns under P waves were not consistent with the classical interpretation, hence a modified mixed-mode caustics interpretation was proposed and then verified by P-wave compression and tension loading cases. In modified interpretation, an iteration procedure was proposed to obtain modified stress intensity factors, i.e. KI and KII, and crack-tip positions which were more precise than those by the classical interpretation. P-wave compression phase decreased KI and crack velocity sharply but produced the largest KII, while the following P-wave tension phase had an opposite effect. S waves and reflected waves produced higher KI and crack velocity with lower KII. KII and crack velocity histories explained curved crack path and microscopic fracture surface. Finally, the reason for the success of the modified interpretation was discussed, and it is indicated that transient stress superposition and delayed redistribution in the crack tip are the mechanism underlying the modified interpretation.