In the process of underground construction, it is common to spray concrete on the working surface, which will form a concrete-rock composite structure, to achieve the expected reinforcement effect. Compared with pure concrete or rock structures, composite structures have different mechanical properties, energy evolution and damage characteristics. Understanding the dynamic mechanics and damage characteristics of composite structures under impact loads is important to the safety of underground engineering construction. First, dynamic compression tests on different grades of mortar-sandstone composite specimens were carried out by using a Hopkinson pressure bar system to discuss the effect of impact velocities on the mechanical properties and failure modes of composite specimens. The results show that the dynamic peak stress, strain energy density and fragmentation block of the mortar-sandstone composite have an obvious strain rate effect. With an increase in impact velocity, the internal cracks of the sample do not have sufficient time to expand, and the energy absorbed by the sample leads to the continuous initiation of new cracks in the sample, resulting in an increasing damage degree of composite specimens. The increase in mortar strength can significantly increase the peak stress, improve the load-bearing capacity of the sample, and reduce the degree of sample damage. At the same impact velocity, the fragmentation degree of the mortar sample is more severe than that of sandstone, and the mortar sample is broken into powder debris, while the sandstone is broken into fragments. In addition, according to the failure properties of the composite sample, the mortar and sandstone samples are described by different damage models, and the dynamic damage constitutive model of the mortar-sandstone composite is established based on the overlay principle. The dynamic test results were fitted. The research shows that the fitting curves of the model are well fitted with the test curves and that the model accurately reflects the changes in the mechanical parameters of the composite specimens.

In combination with high-speed photography, a 3D digital image correlation (DIC) experimental system suitable for an in-plane blasting experiment is established to effectively explore the mechanism of blasting. The dynamic response of the specimen under in-plane blasting loading is analyzed on the basis of the model experiment, and the effect of out-of-plane deformation on the 2D-DIC measurement under a blasting stress wave is studied. First, the out-of-plane displacement and strain evolution law of the specimen under blasting load are studied by using a 3D-DIC system. Second, an analytical model of the effect of out-of-plane curved displacement on 2D-DIC measurement under the blasting load is proposed on the basis of pinhole imaging theory. Finally, the specific error value and error percentage of out-of-plane displacement to the 2D-DIC measurement results are quantitatively analyzed on the basis of the established theoretical method. Results show that the theoretical error of out-of-plane curved displacement to 2D-DIC accounts for 10.8% of the measured vertical strain. In the polycarbonate plate specimen, the effect of out-of-plane displacement caused by the blasting experiment on 2D-DIC measurements cannot be ignored. This novel, to the best of our knowledge, experimental system provides a wealth of experimental data, including in-plane full-field strain evolution and out-of-plane displacement changes.

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.