Characterizing dynamic crack-tip stress distribution and evolution under blast gases and reflected stress waves by caustics method

Blast gases and reflected stress waves are crucial in the blast fracturing process. Characterizing dynamic crack-tip stress distribution and evolution under these two loadings is difficult due to challenges in visualizing blast-induced cracks, gases, and stress waves simultaneously and the subsequent interpretation. This paper presents an optical caustics method with high-speed photography to visualize gases and reflected stress waves and capture the caustics pattern in the crack tip, using a PMMA plate (400 mm x 300 mm x 5 mm) subjected to the explosion of lead azide (120 mg). And then a modified analytical model was proposed to reproduce experimental results and extract stress intensity factors (SIFs) more precisely than the classical caustics interpretation. Finally, fracture surface was examined by a light microscope. Crack-tip stress distribution and evolution are indicated by the shape and size of the caustics pattern, respectively. The classical (circle) caustics pattern under gases indicates K-dominated stress field in the crack tip, while the distorted (ellipse) caustics pattern under reflected stress waves implies K-dominated stress field is violated. By the modified SIFs measurement, it is found that SIFs under gases decrease and produce bigger and sparser micro cracks on the fracture surface, while SIFs increase under reflected stress waves with a higher loading rate and cause smaller and denser micro cracks. Based on the change of micro cracks, the curved crack front is identified in the transition of gas and reflected stress wave loadings. This paper shows that caustics method can visualize and precisely characterize crack-tip stress field under quasi-static loading of blast gases and transient loading of reflected stress waves.