Experimental study on acoustic emission evolution characteristics and response mechanism of damaged rocks

The evolution of rock internal damage is one of the causes of geotechnical engineering disasters, and revealing the response characteristics of key parameters during the rock failure could provide a basis for the identification instability state after rock failure, which is of great importance for the early warning and prevention of rock engineering disasters. Herein, the siltstone specimens with different damage degrees were prepared by cyclic loading-unloading tests, and uniaxial loading tests were carried out with the damaged specimens. On this basis, the relationship between the damage degree of specimen and the wave velocity, as well as the acoustic emission (AE) evolution law, was analyzed, and the AE response mechanism of rocks in different damage degree was discussed. The results show that: (1) The P-wave velocity of damaged specimens decreases linearly with the increase of damage degree, but the AE ringing counts change from periodical increasing to rapid increasing during the whole loading process. The AE energy increases rapidly from a small amplitude at the yield stage. (2) The peak value of AE b value reflecting the trend of development of internal fractures at different scales, transfers from the post-peak failure stage to the compaction stage, resulting in the unobvious instability characteristics of post-peak failure stage. The S value, reflecting the concertration and energy scale of AE sources within the rock mass, varies from medium amplitude fluctuation at low value to small amplitude fluctuation at high value in the compaction to yield stage, indicating that unstable failure occurs in the pre-peak stage. (3) The breaking process of damaged specimen changes from the dominant type of intergranular slip to the crack development with the increasing of damage degree. Thus, the AE high-frequency and low-energy signals (400-800 kHz, 0-250 aJ) vary from sporadic appearance to intensive distribution during the whole loading process, and the high-frequency signal band becomes wider. (4) The different development degrees of cracks in the damaged rock is the basic reason for the differentiation of AE response mechanism. The density of microcracks in rock increases as the damage degree increases, which lead to the transition of progressive stable failure mode to burst unstable failure mode during the loading process, and the enhancement of AE signal activity and intensity. Generally, the characteristics of AE parameters of rocks with different damage degrees were obtained in this study, which could provide theoretical basis for the damage identification of engineering rock mass.