Acoustic emission monitoring of marble specimens under uniaxial compression. Precursor phenomena in the near-failure phase

In the present study Acoustic Emission (AE) indices are used to characterize the damage process in marble rock specimen that is subjected to axial compressive stress. The Ib-value which depends on the amplitude distribution of the AE hits, shows a consistent trend of decreasing from the level of 70% of the ultimate compressive strength. A steep decrease is observed when the stress exceeds the 95% of the ultimate strength reaching a value near 1, a fact that can be considered as a warning of the coming event of fracture. Additionally, the average frequency of the AE hits shows a severe shift to lower values by about 200 kHz when the dominant failure mechanism is related to shear type of failure. Similarly, RA which is defined as the ratio of the waveform Rise Time over the Amplitude in jts/V, shows strong peaks at the moments near the fracture, indicating shear action. (C) 2018 The Authors. Published by Elsevier Ltd.