Energy characteristics of impact-induced stress waves and their damage effects on coal

Clarifying the effects of dynamic stress waves on coal-damage behavior supports the development of water-free coal-permeability enhancement methods. This study examines the time–frequency and energy characteristics of stress waves generated by impact load and the resulting damage to low-permeability coal. Dynamic impact tests were conducted on coal specimens from Wuyang Mine (Shanxi Province) under various impact velocities. Stress-wave monitoring equipment captured raw waveform data before and after coal damage. Using the adaptive optimal kernel time–frequency analysis (AOK-TFA) technique combined with MATLAB-based numerical processing, the energy evolution of stress waves was systematically analyzed to elucidate the damage mechanisms associated with triaxial waves. The results show that with increasing impact velocity, energy across all frequency bands of the triaxial stress waves rises consistently, indicating that energy accumulation in these bands is the primary driver of coal damage. The frequency bands most responsible for damage differ from those of blast-induced stress waves in rock; for impact-induced stress waves, damage energy is mainly concentrated in the medium-to-high frequency ranges of 39.06–312.50 Hz and 625–2 500 Hz. The dominant spectral components of these waves appear initially at about 0.4 s, but after passing through damaged coal, a consistent time delay of roughly 0.05 s occurs. Overall, coal damage caused by impact-loading stress waves during energy transfer and attenuation results from the combined effects of triaxial stress waves (P, SV, and SH waves). These findings offer valuable insights into how impact dynamic loads can enhance coal permeability.