Abstract
Horizontal layered composite rock samples composed of white and black sandstones with large differences in physical and mechanical properties were tested to explore the dynamic characteristics of layered composite rocks under impact load. Using the split Hopkinson pressure bar test system, the dynamic compression tests of two incident states of stress waves, that is, stress waves from white sandstone to black sandstone (W⟶B) and from black sandstone to white sandstone (B⟶W), were designed and carried out under different impact velocities. Combining the ultrahigh‐speed photography system and digital photogrammetry for deformation measurement (DPDM), we obtained the stress wave propagation characteristics, failure characteristics, and particle size distribution characteristics of broken rocks of the composite rocks under the two conditions. The experimental results were compared and analyzed, while stresses and strength conditions at the interface of the composite rock samples were theoretically assessed, yielding the following main findings. The energy dissipation pattern of composite rock had an obvious strain rate effect. The reflected energy and fragmentation energy density of composite rock increased approximately as quadratic functions of the incident energy. Affected by the wave impedance matching relationship, the W⟶B and B⟶W samples were significantly different in terms of the stress wave shape, energy dissipation, average particle size, and fractal dimension of the broken rocks at low impact velocities. However, with an increase in the impact velocities, the two gradually shared the same behavior. When composite rock samples deformed and failed, the macrocracks mostly initiated from the white sandstone. When the crack tip stress of the white sandstone at the interface exceeded the strength of the weakened black sandstone, the crack continued to develop through the two‐phase rock interface due to the difference in Poisson’s ratios. The damage degrees and failure modes of the two parts of composite rocks were different: black sandstone was prone to tensile splitting with local shear failure, while white sandstone exhibited shear failure with local tensile splitting. The damage degree of white sandstone exceeded that of black sandstone.
Highlights
Layered composite rocks are mostly natural engineering geological bodies consisting of multiple layers of rocks with different lithologies and thicknesses deposited in a certain order and manner [1,2,3]
In 2021, Xu et al [11] used the particle flow discrete element numerical simulation method combined with the uniaxial compression tests on a transversely isotropic rock with prefabricated fractures. e influence of cracks and weak surfaces on the rock fracture shape was analyzed, six failure morphologies were determined, and the crack evolution process of each morphology was studied based on matrix tensor analysis
Yang et al [16] combined acoustic emission technology and DIC technology to carry out conventional triaxial compression tests, confining pressure reduction triaxial tests, and Brazil disc splitting tests on transversely isotropic shales with different bedding angles. ey reported that the bending angle greatly influenced the elastic modulus, tensile strength, compressive strength, and failure morphology of the rock
Summary
Layered composite rocks are mostly natural engineering geological bodies consisting of multiple layers of rocks with different lithologies and thicknesses deposited in a certain order and manner [1,2,3]. In recent works on numerical simulation, Cao et al [10] used compressive-shear tests and PFC2D numerical simulation methods to analyze the influence of bedding angle on the mechanical characteristics, crack propagation patterns, and acoustic emission response behaviors of transversely isotropic rock with cuts at both ends. Triaxial compression tests were carried out on five sets of transversely isotropic samples with different bedding angles, and the evolution of mechanical parameters, fracture types, and failure morphology of inclined interbedded similar rock material was analyzed. More attention is paid to the failure characteristics of rocks under dynamic load To this end, Li et al [24] carried out a dynamic impact test study on single-joint red sandstone, which showed that the presence of joint weakened the dynamic compressive strength of sandstone. The law of energy dissipation, deformation, and fracture characteristics of layered composite rocks under the two stress wave incident states, that is, stress waves passing from white sandstone to black sandstone (W⟶B) and from black sandstone to white sandstone (B⟶W), were revealed. e research results can provide a certain reference for rock-breaking and support of composite rock projects
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