Influence of confining pressure on stress wave propagation character-istics in red sandstone

Confining pressure has a great effect on the internal microstructure of the rock,and results in a change in wave impedance,which affects the propagation attenuation laws of the stress wave in the porous rock subjected to high stress. In order to investigate the effect of confining pressure on the characteristics of stress wave propagation in rock, the relationships between both reflection coefficient and transmission coefficient with the wave impedance of rock were established based on one dimensional stress wave theory. Then the short and long rock specimens were made of red sandstone,and the experiments of stress wave propagation in specimens under different confining pressures were car- ried out based on the modified Split Hopkinson Pressure Bar (SHPB). Also,the experimental longitudinal wave veloc- ities,wave impedances,reflection coefficients and transmission coefficients of rock under different confining pressures were analyzed. Based on the empirical relationship between wave impedance and confining pressure,and the theoreti- cal relationship between wave impedance and transmission coefficient,the theoretical transmission coefficients under different confining pressures were obtained and compared with experimental values. The comparison shows that the differences between theoretical and experiment values were very small,and verifies the theoretical analysis of transmis- sion and reflection coefficients are valid. The influence laws of confining pressure on the stress wave propagation along red sandstone were obtained by the comprehensive analysis of longitudinal wave velocity,wave impedance,transmitted and reflected coefficients of rock under different confining pressures. The results show that both the longitudinal wave velocity and wave impedance of rock specimen increase with the increase in confining pressure. With the increase of confining pressure,the transmission coefficient of short specimen increases,and the absolute value of reflection coeffi- cient decreases,the transmission coefficient of long specimen,however,decreases gradually. The variation trends of both the reflection coefficient and trans-mission coefficient with confining pressure can be fitted with quadratic func- tion. All of the above variation tendencies show that the increase of confining pressure decreases the attenuation of its amplitude.