Experimental investigation into damage and failure process of coal-rock composite structures with different roof lithologies under mining-induced stress loading

Coal burst is essentially a destructive manifestation of stratigraphic structure. Although the failure mechanism of the structure has been extensively studied using experimental and numerical methods, few studies have considered the critical role of the mining stress environment and the roof stratum in structural instabilities. In this paper, uniaxial static-cyclic loading tests were performed on a series of composite specimens with different roof lithologies. The objective is to investigate the damage and failure process of composite specimens under mining-induced stress conditions, particularly the mechanical response and damage evolution to coal and rock under cyclic disturbance. The results indicate that the mechanical behaviour of the composite specimen is closely related to the mechanical properties and interactions of the components, and the coal and rock show various characteristics of deformation and damage under cyclic loading with increasing static stress. A dynamic elastic strain energy (DESE) index based on the energy conversion of the specimen was proposed, which is more consistent with the trend of the P-wave velocity and thereby can well reflect the damage state of the specimen. The variation in the acoustic emission (AE) energy rate implies that the composite specimens experience progressive damage before failure, along with AE events spatially clustered in the failure plane of the weak roof and near the upper and lower boundaries of the coal body. The failure pattern of the composite specimen changes with increasing roof strength from initial structural instability due to tensile splitting or shear failure of the roof to the ejection of localized coal chips near the interface, which is attributed to the lateral tensile stresses on the coal near the interface caused by the mechanical difference and frictional constraint under cyclic unloading.