Mechanical response and acoustic emission characteristics of damaged mudstone

Mudstone, which is a representative weak rock mass, often experiences mechanical deterioration because of repeated stress disturbances in underground engineering. This study elucidates the mechanical and acoustic emission (AE) characteristics of damaged mudstone. Cylindrical mudstone samples with damage levels corresponding to 0%, 20%, 40%, and 60% uniaxial compressive strength (UCS) were prepared and tested under uniaxial compression with real-time AE monitoring. The results reveal that the peak strength decreased with increasing damage level, whereas the elastic modulus decreased from 4.861 to 3.871 GPa. An accelerated reduction in both the peak strength and elastic modulus occurred at damage levels of 40% and 60% UCS, corresponding to a transition from slow microcrack initiation to localized crack coalescence. Energy analysis revealed that the energy inflection point appeared earlier with increasing predamage level. Both the total input energy and elastic strain energy before the peak decreased significantly, whereas the proportion of dissipated energy increased from 13.45 to 55.42%. The highly damaged mudstone exhibited step-like surges, indicating cascading crack-cluster propagation and shear‒slip localization. In terms of the AE behavior, higher predamage levels resulted in earlier activation of the AE counts and more distinct multistage bursts before the peak. AE analysis indicated that while tensile failure was dominant, the proportion of shear failure increased from 21.36 to 35.10% with increasing damage level, as indicated by a decrease in the wave velocity and a shift in the AE parameters. Moreover, the increased microcrack density and bedding plane weakness resulted in an increase in the shear failure percentage. Furthermore, the AE b value decreased from 1.089 to 0.680, and the AE S value increased from 0.202 to 0.281, confirming a shift from distributed small-scale cracking to a concentrated, large-scale fracture. These findings provide crucial quantitative insights into the mechanical degradation of damaged mudstone and valuable AE-based precursors for failure, which have important implications for hazard prediction in engineering practices involving mudstone.