Abstract: Triaxial impact compression tests of coal specimens were carried out by using the split Hopkinson pressure bar (SHPB) test system. Based on the mechanical test results, the dynamic constitutive relationship of coal specimens was studied. The results show that: (1) The dynamic compressive strength and dissipated energy of coal specimens are positively correlated with the change of confining pressure and impact velocity (strain rate), and negatively correlated with the change of axial pressure. The failure mode of coal specimens under three-dimensional load is shear failure, and the fragmentation of coal specimens varies with mechanical properties. (2) Based on the statistical damage theory, the damage is described by introducing the dynamic Mohr-Coulomb strength criterion to represent the stress change of the micro-unit inside the coal specimens during the test, and the dynamic damage constitutive relation of the coal specimens is established. A genetic algorithm is used to fit the test curves, yielding a high goodness of fit that reflects elastic, plastic flow, strain softening, strain rebound, and other mechanical properties of the coal specimens. The constitutive relation is suitable to describe the dynamic stress-strain relationship of coal specimens under three-dimensional load. (3) Under dynamic loading conditions, the average cohesion and internal friction angle of coal specimens increase by 140.2% and 28.0% respectively compared with those under static loading conditions. Weibull distribution parameters m and F₀ strongly positively correlate with dynamic cohesion and dynamic internal friction angle, respectively. It shows that the brittle and hard rock mass has a high dynamic elastic modulus, and the higher the strength of the coal specimen, the greater the dynamic cohesion. (4) The applicability of statistical damage constitutive model and combined component constitutive model is compared and analyzed. In principle, although the constitutive model of combined component can also accurately reflect various mechanical properties of coal specimens during the loading process, the fitting parameters are too many and the model is quite complex. The statistical damage constitutive model has few fitting parameters and can reflect the complex dynamic mechanical properties of coal specimens under confining pressure.

Key words: SHPB, coal specimen, triaxial impact, dynamic Mohr-Coulomb strength criterion, constitutive relation, combined component