In this study, the effects of preloaded uniaxial compressive stress and defect (prefabricated by laser cutting) curvature on blast-induced cracks and stress wave propagation were investigated in polymethyl methacrylate (PMMA) specimens using caustics theory. Based on the mathematical relationship between stress and optics, the fracture behavior (the propagation path, dynamic stress intensity factors (DSIFs), propagation velocity, initiation angle of the main crack, and damage degree between the defect and the blasthole) was calculated and analyzed quantitatively. The results show that the preloaded stress could mainly restrain the main crack propagation in the horizontal direction and reduce the initiation angle and damage degree at the defects. Meanwhile, the crack initiation position of the positive curvature defect was not at the end of the defect. In addition, the curvature of the defect significantly affected the blast-induced fracture compared to preloaded stresses. Comparing the main crack with other curvature defects, the DSIFs, velocity, crack arrest time, crack length, and horizontal offset distance of the main crack with negative curvature defect were the largest. The variation in the full stress field in the specimen under blasting and preloaded stress was simulated by a numerical model. The results show that the curvature of the defect has a significant effect on tensile waves at the end of defects, and the tensile stress from high to low was C_-25, C₀, and C₂₅, in that order.

This study investigates the dynamic mechanical characteristics of strength-gradient composite rock masses under one-dimensional explosive plane waves by integrating digital image correlation (DIC) and Lagrangian inverse analysis. Using a one-dimensional explosive plane wave generator, high-spatiotemporal resolution displacement and strain data were acquired from specimen surfaces via an ultra-high-speed camera and DIC. The study compared the decay patterns of blast stress waves and deformation features of rock under two loading paths (forward and backward gradients) for three explosive charges, and employed Lagrangian inverse analysis to determine the strength-gradient distribution within the composite rock mass. The Lagrange inverse analysis method was employed to derive the constitutive relationship of the strength-gradient composite rock mass. The results indicate that in forward gradient rock masses, stress waves undergo stress jumps at joint surfaces, leading to increased wave amplitudes. In backward gradient rock masses, stress wave attenuation is more pronounced. In forward gradient coarse sandstone, stress attenuation rates are significantly higher than in the other two sandstone types. In backward gradient gray sandstone, attenuation rates are markedly greater than in the other two sandstones. However, under identical charge conditions, coarse sandstone exhibits a higher attenuation coefficient than gray sandstone. This indicates that stress waves decay more rapidly in the immediate vicinity of the explosion and that weaker media exhibit faster decay rates. The findings reveal the propagation patterns of explosive stress waves in layered gradient materials, providing a theoretical basis for engineering blasting in layered rock formations.