Effect of Prefabricated Borehole Diameter on the Nonlinear Mechanical Response of Coal–Rock Composite Specimens Under Dynamic Loading

To investigate the effect of prefabricated borehole diameter on the dynamic response of coal–rock composite specimens, split Hopkinson pressure bar (SHPB) tests were conducted on specimens without a borehole and with borehole diameters of 4, 6, and 8 mm. Twelve specimens were tested, with three specimens in each group, under a nominal impact pressure of 0.3 MPa. The average strain rates ranged from 19.64 to 22.36 s−1, while the peak strain rates ranged from 29.79 to 33.10 s−1. The group-mean nominal dynamic peak strengths of the 0, 4, 6, and 8 mm groups were 92.87, 94.68, 78.34, and 77.60 MPa, respectively. The peak strain and peak secant modulus both exhibited non-monotonic variations with borehole diameter. The corresponding group-mean net absorbed energies were 33.96, 35.60, 32.16, and 33.05 J, respectively, also showing a non-monotonic variation. Across the four borehole-diameter groups, the group-mean fitted fragmentation parameter ranged from 1.95 to 2.06, while the group-mean characteristic particle size ranged from 6.95 to 7.76 mm. The Weibull statistical damage model provided a good descriptive representation of the nonlinear stress–strain response within the selected fitting interval, with group-mean coefficients of determination R2 ranging from 0.9895 to 0.9969. Overall, the load-bearing, energy, fragmentation, and nonlinear deformation responses varied among the tested borehole-diameter groups, and the different response indicators did not exhibit a uniform monotonic dependence on borehole diameter.