Acoustic Emission Characteristics of Fracture and Damage in Coal Samples under Overstress Loading and Unloading Paths

Abstract

Abstract As coal mining depth is over 1000 m, an overstressed effect is commonly observed around working face which shows a higher stress than the ultimate strength of the coal mass. Aiming at the problem of dynamic disasters induced by overstress loading and loading, a true triaxial AE monitoring unit was designed to systematically investigate the acoustic emission (AE) characteristics of fracture and damage. The AE results show that the ring counts of coal sample under true triaxial loading are influenced by three factors: loading level, individual differences, and loading rates. When under lower true triaxial loads, the coal sample damage represented by ring counts appears largely in loading phase, and the quantity is few. When true triaxial loads increase, the number of ring counts, especially in packing phase, increases significantly, which indicates that the damage process is transferred from loading phase to packing phase. The study on evolution laws of microcracks, wave velocity, and energy indicates that, during the initially true triaxial loads, the primary microcracks and the initial loading on primary dense areas contributes to a small amount of lower and higher velocity regions in the sample. Along with the increase of true triaxial loading, the closure of the primary microcracks and the fracture of the dense area result in a transfer and expansion of higher velocity regions and the formation of velocity anomaly regions. When approaching failure, the macrocracks throughout the sample present a large area of lower velocity band. The formation and closure of cracks, the fast transfer of high velocity region, and the velocity anomaly region indicate the unstable status of the sample. During the packing phase of true triaxial loads, the cracks are compacted, and the inner structures are thoroughly damaged in the sample, which eventually presents a continuous damage in a form of the shrink of the lower velocity region and the increase of integral velocity of the sample.