Mechanical Variability and Acoustic Emission Heterogeneity of Bedded Coal Under Uniaxial Compression

Bedding orientation affects coal strength and the interpretation of acoustic-emission (AE) signals, but detected activity and retrospective sequence descriptors have different monitoring meanings. Twelve coal specimens were tested under uniaxial compression, with three at each bedding angle of 0°, 30°, 60°, and 90°. Their mean peak stresses were 11.713 ± 1.412, 7.122 ± 1.064, 3.696 ± 0.667, and 10.357 ± 1.138 MPa, respectively (mean ± sample standard deviation). Detailed AE analysis remains limited to one specimen per orientation. For those four records, an event-index partition analysis at moment orders of −5 to 5 gives generalized-dimension ranges of 0.750, 0.552, 0.867, and 0.808 and spectrum widths of 0.925, 0.857, 1.088, and 0.987. Scale-range sensitivity, moment-range sensitivity, and temporal shuffling limit their interpretation to finite-scale heterogeneity. Independently estimated detrended-fluctuation slopes range from 0.597 to 0.775 and do not establish stationary long-range memory. Right-ended windows describe evolving AE structure without future observations, but no validated warning lead time is established. The mechanical results show an intermediate-angle strength minimum within the tested specimens; the AE observations support conditional mechanism hypotheses requiring replicate AE analysis and independent temporal and spatial validation.