Acoustic emission, damage and cracking evolution of intact coal under compressive loads: Experimental and discrete element modelling
Engineering Fracture Mechanics
In underground mining engineering, it is of considerable significance to focus on acoustic emission (AE), damage and cracking evolution inside coal. This article first performed a series of triaxial compressive tests on intact coal specimens simultaneously monitored AE signals; then evaluated the cracking behaviours of intact coal specimens under biaxial compression using a two-dimensional particle flow code (PFC2D). Before the specimens enter into the yield regime, it emits a small number of acoustic signals and generates stable crack initiation and propagation linked to Kaiser effect and modest damage. As increasing the compressive load upon to the ultimate strength, the strain energy in the simulation will reach the maximum value; subsequently, it decreases in the post-peak stage, which is consistent with the acoustic development in the experiments. Furthermore, confining pressure plays a vital role not only in strength and acoustic activity but also in crack growth and fracture patterns. The maximum AE count and maximum strain energy increase with the rise in confining pressure. Higher confining pressure will create a higher crack initiation threshold and hinder crack propagation and coalescence, thus contributing to a higher compressive strength. With increasing confining pressure, intact coal specimens are prone to shift from tensile-shearing mixed failure to shearing fracture. Additionally, once the axial strain exceeds approximately one per cent, brittle failure will occur accompanied by a strain-softening phenomenon. Unlike intact coal, tectonic coal typically exhibits a ductile failure with a strain-hardening phenomenon. Moreover, numerical results show that cracks within coal generally can be classified into six patterns. Tensile cracks, usually restricted to a ligament area, generate along the direction of principal stress in a relatively stable manner; whereas (primary, secondary, and quasi-parallel) shear cracks generally propagate and coalesce towards the boundary of specimens.
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Ministry of Education of the People's Republic of China