Acoustic Emission RA-Value and Granite Fracture Modes Under Dynamic and Static Loads

Abstract

The RA-value characteristics and distribution versus rock acoustic emission average frequency, the peak frequency characteristics, and the evolution of rock fracture modes under dynamic and static loads were studied to understand the fracture modes of rock under dynamic and static loads. The Split-Hopkinson pressure bar system and MTS322 servo-controlled rock mechanical test system were used, respectively, to carry out impact-loading tests and uniaxial compression tests at different loading rates. The results indicate that the RA-value under impact loading is higher in the initial stage, decreases to below 1 ms v−1 through the failure process, and even the variation trend tends to horizontal lines with loading time, which demonstrates that the fracture modes are dominated by tensile failure. An opposite variation in RA-value under static loading results when the loading rate is lower, but the variation corresponds with the impact-loading tests when the loading rate is higher, which indicates that tensile fracture still dominates the failure process and the occurrence of shear failure, as loads peak when the loading rate is lower. The acoustic emission signals exhibit a higher peak frequency under impact loads than those under static loads. Furthermore, in impact-loading tests, the peak RA-value will increase gradually with an increase in strain rate. The RA-value can be used to classify the crack type and as a rock fragmentation evaluation index. In general, the peak frequency can be used to distinguish two typical signals under impact-loading tests; signals with a higher peak frequency (fp > 100 kHz) can be generated by rock fracturing, whereas those with a lower peak frequency and a higher RA-value can be generated by elastic wave propagation.