Mechanical properties and energy evolution of unconfined and CFRP-confined coal samples with different height-to-diameter ratios

Using uniaxial compression tests and coupled FDM-DEM numerical simulations, the mechanical and energetic evolutions of unconfined and CFRP-confined coal samples were investigated under varying height-to-diameter ratio (HDR). The results indicate that peak strength decreases, while the elastic modulus increases nonlinearly with an increase in the HDR, and CFRP confinement significantly enhances the mechanical properties. The contact number increases, whereas the contact force decreases, with crack initiation and yield stress points identified through the second-order derivative of the crack number. For unconfined coal samples, damage propagation occurs from the center outward, while for CFRP-confined samples, it progresses from the ends toward the center. As the HDR increases, energy density and axial strain decrease, with geometric size differences and energy conversion emerging as critical factors for instability. CFRP-confined coal samples demonstrate greater energy storage and dissipation capacities compared to unconfined samples. The dissipated energy conversion for unconfined samples peaked at 10.76% at a HDR of 1.5 and was lowest at 5.53% at 3.0, while CFRP-confined samples peaked at 16.34% at 0.5 and dropped to 5.62% at 2.0. These findings reveal that an increasing HDR reduces ductility and raises instability risks, whereas CFRP confinement improves deformation resistance and energy dissipation, offering a theoretical basis for the reinforcement of coal samples.