Acoustic emission (AE) characteristics of granite after heating and cooling cycles

Understanding the mechanical behaviour of rock under heating and cooling cycle condition is necessary for safe and effective deep geo-engineering applications, including geothermal energy extraction, deep geological disposal of nuclear waste and tunnel fire. In this experiment, the temperature rises from room temperature to predetermined temperature followed by rapid cooling. The rock failure test of uniaxial compression is carried out, and the corresponding crack propagation mode is studied by AE system. The AE and b-value in the process of rock fracture are studied with the variation trend of different stress levels and temperatures. According to the results, once the rock mass is subjected to higher thermal stresses, strength characteristics are significantly reduced, mainly due to thermal damage of mineral particles and preferential expansion and cracking between quartz particles or quartz and another mineral (Wang et al., 1989 [30]; Heard and Page, 1982 [17]). The stress-time response revealed that the failure mode of granite is changed from brittle to quasi-brittle fracturing with increasing temperature. The following cooling causes the strength characteristics of the granite to be further decreased through the enhancement of crack density, which is evidenced by the AE results (Browning et al., 2016 [4]; Griffiths et al., 2018 [15]). The AE signal has a high degree of consistency with the stress-time curve. As the number of heating and cooling cycles increased, the AE became more active, but accumulated AE counts gradually decrease. The b-value reflects the initiation and propagation of microcracks in rock failure process. The b-value fluctuated during the rock fracturing process. When the stress reached the peak intensity the b-value began to decrease rapidly, until a minimum is reached. The b-value increases with the increase of heating and cooling cycles.