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.