Experimental Study Of Rock Mechanical Properties Under Coupled Static-Dynamic Loads And Dynamic Strength Criterion

Aiming at the characteristics of the deep hard-rock metal mines and the relative shortage status of experimental and theoretical research on deep rock mechanics, it is essential to study and understand the mechnical properties of deep rock under coupled static-dynamic loads. For deep rocks with different pre-stress state, the dynamic experiments were conducted with impact loads by using an innovative testing system, and some theoretical researches were also carried on in this paper. The main contents and conclusive results are as follows:(1) Two indexes, the stress wave peak-value factor and stress wave rising-edge time factor, were defined to analyze the propagation rules of stress wave (rectangular waves, triangular waves and half-sine wave) in 5 different diameters SHPB. For large diameter (>50mm) SHPB device, the results show that half-sine stress wave is able to meet the one-dimensional assumptions in the propagation process.(2) According to the transmission-reflection laws of stress waves between elastic bar with rock sample, a look-up table to obtain the relative stress difference value was given by using the reflection coefficient and the back-forth communication number of stress wave in rock. The quadratic function relationship between the wave velocity of rock with the maximum length of sample was obtained, which provides a reference method to determine the sample size for specific test condition.(3) The dynamic uniaxial compression tests show that the compressive strength of rock increases with the magnitude of strain rate when the strain rate is less than 102/s and the upward trend in the compressive strength is proportional to 1/3 power of strain rate when the strain rate is larger than 102/s. There is a critical incident energy value when the shock wave incidents in the rock. When the incident energy is less than the critical incident energy, the rock sample does not participate in the energy absorption.(4) In the dynamic triaxial tests with different strain-rate, the compressive strength increases with the magnitude of strain rate when the confining pressure is the same and the strain rate is less than 102/s, and the compressive strength increases with the 1/3 power of strain rate when the strain rate is larger than 102/s. When the strain rate is a constant, the compressive strength of the rock will linear increase with the increases of confining pressure. For the dynamic triaxial impact test, the critical incident energy and the critical strain rate were obtained.(5) In the one-dimensional dynamis tests coupled loads, when the strain rate is a constant, the impact compressive strength will first increase and then decrease as the axial static pressure increases and also reach the maximum value when the axial static pressure is about 60%-70%of static compressive strength. When the axial static pressure is a constant, the impact strength will increase with exponential function when the strain rate increases. The ratio axial compressive pressure will influence the response of rock specimen to impact energy and the specimen will experience three phases "energy absorption-energy release-energy absorption" when the ratio axial compressive pressure increases. The transformation mechanism of dynamic strength increase, rock burst and induced fracture of rock under high in-situ stress can be explained better by the above three phases.The results can also provide a theoretical guide for the deep rock engineering practice.(6) When the strain rate is a constant in three-dimensional coupled static-dynamic loading tests and the confining pressure is also a constant, the impact strength of rock will increase firstly and then decrease with the axial compression pressure increases. When the confining pressure is 5MPa, there is no obvious difference between the impact strength with that of one-dimensional coupled tests. But when the confining pressure increased to lOMPa, the impact strength of rock will be greatly enhanced. In the situation with the axial compression is a constant, the impact strength will show an increasing trend as the confining pressure increases.(7) Combined the theoretical clastic solution of disc on cardiac force and the physical parameters obtained in actual measure of experiments and based on the principle of calculus, two analytic algorithms to estimate the static and dynamic tensile modulus in Brazilian disk splitting tests were presented. The Brazilian tensile test results show that the tensile strength of rock will increase with the magnitude of strain rate when the strain rate is less than 102/s and the upward trend in the tensile strength is proportional to 1/3 power of strain rate when the strain rate is larger than 102/s. The splitting processes in static and dynamic tensile tests were photographed by using high-speed camera and confirmed that the split starting position is the center of the specimen in loading direction from the loading beginning. The splitting destruct developing mode under impact loading is summarized.(8) Combined the analysis of data in dynamic uniaxial tests, dynamic triaxial tests and dynamic tensile tests and the principle of Mohr-coulomb criterion, Hoek-Brown criterion and Griffith criterion, the dynamic Mohr-coulomb criterion and dynmaic Hoek-Brown criterion were given. The experimental results show that dynamic Mohr-coulomb criterion and dynmaic Hoek-Brown criterion can be used to estimate the dynamic strength in low strain rate range. In high strain rate range, the dynamic strength can be estimated by using dynamic Mohr-coulomb criterion, and the Griffith criterion can be used to estimate the dynamic uniaxial compressive strength and dynmaic tensile strength.