1 Introduction
During the excavation of the High Level nuclear Waste (HLW) disposal repository, the surrounding rock bear the periodic loading due to blasting vibration, mechanical excavation and redistribution of crustal stress. Meanwhile, in the using procedure of repository, periodic loading also formed due to mechanical vibration. The cyclic loading acts on the surrounding rock constantly, and causing a hazard to the safe and stable operation of the disposal repository. And, cyclic loading provide an effective way for better investigating rock damage and deformation characteristics (Zhou et al., 2019). The research on the mechanical behavior of granite under cyclic loading has a long history, and a series of achievements have been obtained.
Two types of cyclic loading tests are commonly used, which include fatigued and tiered cyclic loading. Fatigued loading is that loading cycled between a prescribed stresses interval, and it is widely used to evaluate the stability of tunnel under long-term dynamic loading. Under conditions of fatigued loading, damage of rock can be divided into three stages: initial damage, constant-velocity damage, and accelerated damage (He et al., 2019). Fatigued life increases as a power-law function with maximum stress decreasing, and the maximum stress also significantly affects the fatigued life (Momeni et al., 2015). And, the fatigued life of specimen subjected to discontinuous cyclic compression decreased sharply in comparison with conventional fatigued tests (Fan et al., 2019). Loading frequency affects the fatigued strength as well as the deformation of rock specimen (Arora et al., 2019). The level of confining pressure had a significant influence on the cyclic dynamic deformation and fatigued damage evolution of the rock samples (Liu and He, 2012). The stress lower limit can significantly affect the evolution of irreversible deformation, and the elastic modulus and Poisson’s ratio also increased significantly when the stress lower limit increased stepwise (Peng et al. 2019). The stress corresponding to the transition from volumetric compaction to volumetric dilation may be considered to be the threshold for fatigued failure (Wang et al., 2015). The crack propagation threshold appeared to gradually increase with each cyclic loading when the samples loading is beyond the crack propagation threshold (Ghazvinian and Diederichs, 2015).
However, in engineering practices, cyclic stress are not always of equal amplitude. Therefore, tiered cyclic loading-unloading should be investigated. Tiered cyclic loading is a special form of cyclic loading, where stress amplitude increases from one cycle to next cycle. Kaiser effect is that the absence of AE activity at stress levels less than the previously maximum axial stress (Kaiser, 1953), which can reflect the damage progress under cyclic loading (Rao and Ramana, 1992; Li and Nordlund, 1993). Felicity effect is the phenomenon that AE signal is more obvious before reaching the previous maximum axial stress, and Felicity ratio decreases with the increase of cyclic number (Meng et al., 2016, 2018). AE signal can also reflect the crack propagation under unloading process, under lower stress level, no AE signal can be monitored after unloading; however, AE signal can be monitored in the whole unloading process under higher stress level (Jiang et al., 2017). The propagation of crack can be divided into different stages, and the differences can be successfully identified according to the AE hits and AE events (Zhou et al., 2019). X-ray micro-CT, P-wave and photomicrographs were also used to investigate the effect of cyclic loading, and more branch crack can be observed in the specimen under cyclic loading than monotonic loading (Yang et al., 2015, 2017a). The development of the damage state can be briefly divided into three stages, which is as follows: constant gain rate during the pre-peak stage, increasing gain rate during the strain softening stage, and constant gain rate during the residual stage (Xiong et al., 2019).
Granite, as an excellent medium for high-level nuclear waste disposition, has lower permeability and better integrity (Yang et al., 2017b; Zhao et al., 2015). During the decay of radionuclides, a large amount of heat is released, which lead to transformation of granite mechanical behavior (Chen et al., 2017). Therefore, the failure behavior of thermal treated granite under triaxial tiered cyclic loading-unloading compression should be investigated to insure the safe and stable operation of HLW disposal repository. And, the evolution of elastic modulus, Poisson’s ratio and dissipated energy were analyzed in this work.