Active ultrasonic monitoring of damage progression in coal under uniaxial compression

Coal burst is a sudden and violent failure resulting from the formation and accumulation of microcracks caused by high overburden stresses, posing a significant safety risk to underground mining operations. Conventional stress-strain methods, often used for fracture assessment, are prone to subjectivity and errors, especially in highly fractured coal samples. This study investigates the feasibility of utilising ultrasonic through-transmission techniques as an alternative approach to determine the crack damage stress thresholds for coal. During uniaxial compression, changes in ultrasonic wave amplitude, wave velocity, dominant frequency, and Root-Mean-Square Voltage (VRMS) are actively monitored using 1 MHz and 100 kHz excitation frequencies. Distinct characteristics of the transmitted waveform were observed, marked by pronounced attenuation, reduced transmission velocity and a lower dominant frequency. Both excitation frequencies yielded significant insights when examining velocity changes during loading, with the highest velocity observed near the crack initiation stress threshold. In terms of amplitude, VRMS, and dominant frequency changes, the 1 MHz excitation proves to be more effective for coal specimens, displaying a consistent trend across all three samples, enabling the identification of crack closure, crack initiation and crack damage stress thresholds.