Acoustic emission responses and damage estimation of coal with carbon fiber-reinforced polymer confinement under uniaxial compression

Abstract

Fiber-reinforced polymer (FRP) wrapping is a potential technique for coal pillar reinforcement. In this study, an acoustic emission (AE) technique was employed to monitor coal specimens with carbon FRP (CFRP) jackets during uniaxial compression, which addressed the inability to observe the cracks inside the FRP-reinforced coal pillars by conventional field inspection techniques. The spatiotemporal fractal evolution of the cumulated AE events during loading was investigated based on fractal theory. The results indicated that the AE response and fractal features of the coal specimens were closely related to their damage evolution, with CFRP exerting a significant influence. In particular, during the unstable crack development stage, the evolutionary patterns of the AE count and energy curves of the CFRP-confined specimens underwent a transformation from the slight shock–major shock type to the slight shock–sub-major shock–slight shock–major shock type, in contrast to the unconfined coal specimens. The AE b-values decreased to a minimum and then increased marginally. The AE spatial fractal dimension increased rapidly, whereas the AE temporal fractal dimension fluctuated significantly during the accumulation and release of strain energy. Ultimately, based on the AE count and AE energy evolution, a damage factor was proposed for the coal samples with CFRP jackets. Furthermore, a damage constitutive model was established, considering the CFRP jacket and the compaction characteristics of the coal. This model provides an effective description of the stress–strain relationship of coal specimens with CFRP jackets.