Spectral Content of Acoustic Signals of Artificial Sandstone Samples under Uniaxial Loading

The study analyzed the spectra of acoustic signals obtained during uniaxial compression of artificial sandstone samples under continuous loading through the postultimate state up to failure. The authors attempt to reveal a shift towards lower values in the characteristic acoustic signal frequency as the load approaches the critical load state. The experiments were performed on a lever press in two modes: simple continuous loading and continuous loading with periodic impact on the sample by direct current. This was done in order to indicate the effect of electric current on the acoustic emission parameters. In both cases, when a load reached close-to-ultimate and postultimate states, the amplitude of the signals in the 10–20 kHz frequency range begins to significantly exceed the amplitude of the signals in frequency ranges above 20 kHz. At the final stage, immediately before the onset of an avalanche-like rise in acoustic emission activity, an increase in signals with frequencies in the 5–10 kHz range is also noted. Based on the identical behavior of the samples, the authors have concluded that the electric impact did not significantly affect the spectral acoustic signal characteristics. The shift of the frequency interval, which is accounted for the maximum RMS amplitudes of the acoustic signal, towards low frequencies may indicate either the formation of larger cracks or the appearance of additional cracks of a different mode than at lower loads (e.g., shear cracks). By itself, this phenomenon may indicate impending macrofailure.