Impact-induced stress waves in low-permeability coal: Energy and damage analysis

The clarification of dynamic stress wave effects on coal damage behavior facilitates the development of water-free coal permeability enhancement methods. This study investigates the time-frequency and energy characteristics of stress waves generated by impact load and the corresponding damage effects on low-permeability coal. Dynamic impact tests were performed on coal specimens from Wuyang Mine (Shanxi Province) under various impact velocities. Stress wave monitoring equipment was employed to capture raw waveform data before and after the occurrence of coal damage Employing the adaptive optimal kernel time-frequency analysis (AOK-TFA) technique in conjunction with MATLAB-based numerical processing, the energy evolution of stress waves was systematically analyzed with respect to coal damage, thereby clarifying the damage mechanisms associated with the triaxial waves. The results reveal that as the impact velocity increases, the energy across all frequency bands of the triaxial stress waves consistently rises, indicating that energy accumulation in these bands is the primary factor driving coal damage. Moreover, the frequency bands predominantly responsible for damage differ from those associated with blast-induced stress waves in rock; for impact-induced stress waves, the damage energy is mainly concentrated within the medium-to-high frequency ranges of 39.06-312.50 Hz and 625-2 500 Hz. Additionally, the dominant spectral components of these stress waves are initially observed around 0.4 s, but after transmitting through the damaged coal, a time delay of approximately 0.05 s is consistently seen. Overall, the damage to coal inflicted by impact loading stress waves during the energy transfer and attenuation processes results from the combined effects of the triaxial stress waves (P, SV, and SH waves). These findings provide valuable insights into the mechanisms by which impact dynamic loads can enhance the permeability of coal.