Dynamic Mechanical Properties and Constitutive Model of Coal Rock Under Direct Tension

As one of the hotspots in rock mechanics research, coal rock’s dynamic tensile properties are crucial to the parameter selection of blasting engineering, mechanism studies of rock burst disasters, and stability control. The stress states of coal rock during indirect tension with Brazilian splitting and direct tension with split Hopkinson tensile bar (SHTB) were compared using numerical simulations, revealing the superiority of the direct-tension test. On this basis, systematic dynamic direct-tension tests of coal rock under the strain rates (124 to 247 s−1) were carried out utilizing the improved SHTB, and quasi-static tensile tests were performed for comparison. The results reveal that the connection mode of using high-strength adhesive paste specimens and gradient reinforcement with steel wire mesh is reliable for SHTB tests. However, the stress equilibrium is no longer satisfied if the strain rate exceeds 300 s−1, and the layered fracture of the specimen will occur. With an increase in strain rate, the dynamic elastic modulus increases linearly, the dynamic tensile strength tends to increase as an exponential function, and the dynamic increase factor (DIF) tends to increase linearly in two stages. The DIF grows slowly once the strain rate is over 180 s−1 and the dynamic ultimate tensile strength of the coal rock lies around 8 MPa. Finally, based on the improved Zhu–Wang–Tang (ZWT) model and the introduction of dynamic damage factor, the unified damage constitutive equation that can describe the strain rate effect under dynamic tension of coal rock is constructed. Moreover, the rationality of the constitutive parameter value is verified.