Dynamic rock tensile failure under hydrostatic pressure analyzed with wavelet transform of acoustic emission signals

We develop a novel experimental apparatus, integrated into a dynamic testing system, for the real-time monitoring of the failure process of deep rocks. Four groups of Brazilian disc (BD) granite specimens subjected to various hydrostatic confinements (0, 10, 20, 30 MPa) were tested with different loading rates. A strain gauge and an acoustic emission (AE) transducer were attached to each specimen to obtain the real-time failure signals during the impact. The continuous wavelet transform was utilized to analyze the recorded AE signals. It was observed that tensile crack initiation and propagation effectively absorb AE signals produced by stress waves above 50 kHz, which indicates that post-failure AE signals are predominantly generated by the cracks themselves. It is also found that the proportion of AE signals within the 100–150 kHz range increases with the loading rate, suggesting that higher loading rates encourage the emergence of smaller cracks. Moreover, higher hydrostatic stress enhances the dynamic tensile strength of the rock, thereby restraining crack initiation. When confining stress is applied, the AE signals exceeding 100 kHz are delayed in their occurrence post-stress failure, contrasting with those within the 50–100 kHz range that appear simultaneously with the strain drop that precedes stress failure. This pattern indicates that cracks generating AE signals within the 50–100 kHz range are instrumental in the tensile failure observed in Brazilian Disc tests, while those with frequencies above 100 kHz are byproducts of tensile crack propagation following the failure.