Experimental study of microscopic and mesoscopic damage features of limestone under cyclic loading and unloading

The dynamic property of rock is a crucial internal factor which influences the seismic stability of a rock slope. Cyclic loading induced by an earthquake can lead to rock damage, deterioration in rock performance, and a decrease in slope stability. To investigate the dynamic damage characteristics of rock in-depth, this study examines Permian limestone samples obtained from a rock slope. Uniaxial cyclic loading and unloading tests, combined with microscopic and mesoscopic analyses, are conducted to study the damage features and evolution rules of rock specimens. The effects of different loading conditions on the microscopic and mesoscopic damage of rock specimens are analyzed, and the correlation between macroscopic, microscopic and mesoscopic parameters is investigated. The results reveal that rock damage is progressive, primarily characterized by the generation, propagation, and connection of micropores, followed by a slight increase in macropores, which are present in smaller quantities. Compared to cyclic loading with a variable upper limit stress, loading with a variable lower limit stress promotes greater connectivity of micropores and the formation of macropores, resulting in a larger microfissure area in the rock specimen and a decrease in the average elastic modulus. Under loading with a variable upper limit stress, low stress amplitudes have a more significant degradation effect on the internal structure of the rock compared to uniaxial compression. Conversely, high stress amplitudes may compact the internal pores of the rock, leading to relatively low porosity and weakened deterioration effects. By considering the evolution trends of different microscopic damage variables, a steady crack development phase is observed, with the corresponding upper limit stress range being 0.4-0.6 times the static peak strength(sigma(f)), and the lower limit stress range being 0.3-0.5 times the static peak strength. These research findings enhance our understanding of the dynamic characteristics of rock at the micro- and meso-levels. They also provide theoretical explanations for the dynamic behavior of rock slopes and the initiation process of disasters under seismic conditions.