Laboratory evidences on accelerated fatigue failure in brittle granitic rock by coupled thermal and mechanical cyclic loading: Acoustic emission monitoring and implication to underground hydrogen storage

Abstract

In underground hydrogen storage (UHS), injecting and recovering hydrogen in response to fluctuating energy supply and demand introduces unique challenges associated with thermal and mechanical cyclic loadings, leading to potential fatigue failure of the host rock. However, the effect of thermal cycles on the fatigue of crystalline rocks remains poorly understood. This study presents laboratory evidences on accelerated fatigue failure in brittle granitic rock by coupled thermal and mechanical cyclic loading, using acoustic emission (AE) monitoring. A series of uniaxial compression tests were conducted to characterize fatigue damage evolution under cyclic loading at a constant temperature of ∼8°C (CYC) and cyclic loading with temperature variations of ∼8–80°C (CYC-T). The results show that thermal cycles accelerate fatigue damage, with CYC-T specimens failing after an average of 3.3 cycles compared to 5.8 cycles for CYC specimens. This accelerated fatigue damage was corroborated by their systematically different increasing patterns of cumulative AE counts, which indicates that thermal cycles facilitate coalescence of fatigue microcracks leading to more abrupt and catastrophic failure. The shift in AE peak frequencies reveals the formation of a new group of cracks that differ in size and location, highlighting the complex interactions between thermally induced and mechanically induced microcracks. Young's modulus exhibits only a slight decrease while Poisson’s ratio increases markedly under both loading conditions, indicating significant dilative behavior and crack propagation. The findings in this study underscore the necessity of incorporating thermal stress management strategies in the design and operation of UHS systems for long-term stability and operational sustainability.