Identifying Critical Damage Stages in Marble by Means of Natural Time Analysis of Acoustic Emission Cumulative Counts

The rate of change of the normalized Cumulative Counts of the acoustic hits, recorded while marble specimens are compressed uniaxially, and is analyzed in the Natural Time Domain. The analysis reveals the systematic presence of a characteristic plateau that could potentially serve as a precursor to fracture. Starting initially well below unity, the specific parameter increases towards a limit equal to one and is stabilized around this value with minor fluctuations. The starting “instant” of this plateau is linearly related to the loading rate applied. This “instant” and the respective load level are in very good agreement with the abrupt change of the average rate of generation of acoustic signals. These findings are juxtaposed to the respective ones drawn by analyzing data from previously published experimental protocols involving marble specimens of varying geometries subjected to various loading schemes and are found highly consistent with each other. The same holds true for the agreement between the data of the present study and recently published ones dealing with both plain and fiber-reinforced concrete beams. This consistency suggests that the conclusions drawn exhibit a kind of universality, highlighting the potential of this plateau’s onset to serve as a reliable index for Structural Health Monitoring purposes.