Study on dynamic failure mechanism and constitutive model of fractured coal-rock mass in rock burst working face

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To investigate the mechanism of rock burst occurrence at coal mining faces in rock burst coal mines, the split Hopkinson pressure bar (SHPB) test system was adopted to study the failure laws of fractured coal-rock masses with different prefabricated fracture angles under high-frequency dynamic loads, and a high-speed photography system was used to analyze their failure modes. The results show that the dynamic stress-strain curves of coal-rock masses under different impact loads can be divided into four stages, namely the compaction stage, elastic deformation stage, plastic deformation stage and failure stage, and fractured coal-rock masses exhibit better ductility than intact ones. Under dynamic loading, the fracture angle and load magnitude exert a significant influence on the failure morphology of coal-rock masses; both the dynamic peak stress and dynamic elastic modulus of coal-rock show a trend of first decreasing and then increasing with the increase in prefabricated fracture angle, with the minimum values observed at a fracture angle of 45°. The fragmentation energy consumption density of coal-rock with different fracture angles increases first and then decreases as the fracture angle rises, and the higher the load, the more severe the fragmentation of coal-rock masses. By introducing the energy proportion coefficient, it is found that the proportion of absorbed energy increases first and then decreases with the increase in fracture angle; the coal-rock mass with a fracture angle of 45° has the highest proportion of absorbed energy and undergoes the most severe fragmentation. The average particle size of impact fragments of fractured coal-rock masses is smaller than that of intact coal-rock masses; as the impact load increases, the cracks in coal-rock masses become more complex and the fragment size decreases continuously. Based on the Zhu-Wang-Tang (Z-W-T) model, a damage variable D was introduced to characterize the damage degree of fractured coal-rock masses, and a damage evolution constitutive equation for fractured coal-rock masses under impact loads was established to carry out fitting analysis of the stress-strain curves of coal-rock masses with different fracture angles. It reveals that fractured coal-rock masses are highly sensitive to high strain rates, and the experimental curves are in good agreement with the theoretical curves, which verifies the rationality of the proposed dynamic damage constitutive model for fractured coal-rock masses.

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