Mechanical properties and energy damage constitutive relationship of coal samples confined by CFRP sheets

To investigate the confinement effect of carbon fiber-reinforced polymer (CFRP) sheets on the mechanical properties and energy damage behavior of coal samples, uniaxial compression tests were conducted on coal samples with varying layers of CFRP sheets. The stress-strain changes during the four stages of crack compaction, linear elasticity, yield and post-peak failure were analyzed, and the influence of CFRP sheets on the energy input, accumulation, dissipation, and release processes in coal samples was studied. The results show that the crack closure stress and crack initiation stress of coal samples first increased and then decreased with the increase in CFRP layers, but still higher than that in the unconfined state, demonstrating a significant confining effect and a marginal diminishing feature. The CFRP sheets effectively constrained the internal crack propagation in coal, improved stress distribution, made the failure process slower and more orderly, significantly enhanced the load-bearing capacity and ductility, and reduced the post-peak stress drop phenomenon. The CFRP sheets enhanced the energy input and distribution uniformity of coal samples, reduced the energy release rate, and improved the energy distribution and failure process. CFRP confinement significantly improved the mechanical properties and energy dissipation characteristics of coal samples. The proposed dissipated energy damage factor model verified the effectiveness of CFRP sheets in delaying damage accumulation and crack propagation. A mathematical relationship was derived between the layers of CFRP sheets and the equivalent coal cylindrical sleeve thickness. The research advances the understanding of coal pillar reinforcement mechanism and provides an economical and efficient new perspective for improving the recovery rate of residual coal resources in room-and-pillar mining.