Effect of CFRP Layers on the Energy Evolution of Axial Compressed Cylindrical Coal Based on Particle Flow Software

To investigate the effects of different layers of carbon fiber reinforced plastic (CFRP) on the mechanical properties and energy evolution of axially compressed cylindrical coal samples, the finite difference method-discrete element method (FDM-DEM) coupled numerical simulation and laboratory uniaxial compression tests are combined in this paper. The test results show that both unconfined cylindrical coal samples and CFRP-confined samples undergo four stages in the stress-strain curve, namely, compression-tightness, elasticity, yielding, and post-peak. The CFRP-confined samples show obvious ductile damage in the yielding and post-peak stages, and their average peak stresses, peak strains, and elasticity modulus are about 2, 2.5 and 1 times higher than those of the unconfined samples, respectively. Numerical simulations show that the peak strain and peak stress increased to 733% and 548%, respectively, with the increase in the number of CFRP layers. The elastic modulus does not increase monotonically, indicating that a balance between strength and stiffness is required when designing the CFRP layers. In addition, the increase of CFRP layers leads to the change of the damage mechanism from tensile damage to shear damage, indicating that it has a significant effect on the stress distribution and damage process of the cylindrical coal samples. The total and dissipated energy of the cylindrical coal samples significantly increased with the increase of CFRP layers, and the energy absorption efficiency reaches up to 10.51 times, showing a significant enhancement of their destabilization resistance. To quantify the confinement effect of CFRP sheets, the concept of “equivalent thickness” is introduced. It is found that the equivalent thickness increases nonlinearly with the number of CFRP layers, and at 6.78 layers, the equivalent thickness approaches infinity, which emphasizes the importance of CFRP sheet in improving the stability of cylindrical coal sample structure, and provides an important reference for future research.