PSI - Issue 55

Dimos Triantis et al. / Procedia Structural Integrity 55 (2024) 185–192 Triantis et al. / Structural Integrity Procedia 00 (2023) 000 – 000

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ing the titanium bars and covering the faces of the blocks in contact and finally yielding or fracture of the reinforcing bars themselves. Therefore, entrance into the critical stage is to be here understood as proximity to activation of at least one of these failure mechanisms and not exclusively to proximity to macroscopic fracture of one of the two marble blocks (which, in fact, was observed at a much higher load, equal to about 330 kN). Based on the above observation, attention is paid to the time interval around the critical instant corresponding to L cr ≈183 kN. The temporal variation of the normalized average energy of the acoustic events is plotted in Fig.4b, together with the respective evolution of the load applied. It is observed from this figure that several seconds before the bending load applied approaches the critical limit L cr (more specifically at the instant t ≈795 s) the normalized average energy of the acoustic events exhibits a rapid increase (from about 0.35 to about 0.47). From this instant on, its value tends to be stabilized (with minor fluctuations) in a narrow “window” in the in terval 0.47< * i E <0.53, forming a plateau (as it was the case of the elementary tests described in previous section). The plateau is terminated at the instant t ≈1025 s , namely the instant of the second slope change of the load-time plot. According to the analysis of the sets of all data available this instant corresponds to activation of an additional fracture mechanism (coalescence of micro-cracks within the marble blocks) indicating now proximity to the fracture of marble. In an attempt to comparatively consider (and further validate) the above findings, the temporal evolution of the Pressure Stimulated Current (PSC), recorded during the specific experiment is plotted in Fig.5, in juxtaposition to the respective evolution of the bending load. The term PSC designates low-level electric signals (in the form of very weak electric currents of the order of pA), which are emitted during loading of brittle geomaterials. The temporal evolution of the PSC provides reliable information about the damage level of the material. It is experimentally verified that the PSC exhibits a smooth increasing tendency during the period of generation of the networks of micro-cracks and a quite rapid increase when the networks of microcracks start coalescing to each other, leading to the formation of fatal macro-cracks (Li et al. (2021), Stavrakas et al. (2004), Saltas et al. (2018), Triantis et al. (2022b)). As a result, the temporal evolution of the PSC is considered as providing reliable pre-failure indicators, which have been found to be in very satisfactory agreement with the respective ones provided by the AEs (Triantis et al. (2022b)).

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Fig. 5. The temporal evolution of the Pressure Stimulated Current in juxtaposition to the respective evolution of the bending load. It can be seen from Fig.5, that at the time interval at which the normalized average energy of the acoustic events forms its plateau the PSC exhibits, also, a stabilization tendency (after a period of smooth, almost linear, increase). Moreover, it is to be emphasized that the plateau of the PSC is terminated abruptly at the instant t =1090 s and is followed by a quite abrupt increase, which is considered as a clear warning signal of upcoming macroscopic fracture (which was indeed observed about 40 seconds later, i.e., at the instant at t =1130 s). 3. Concluding remarks The temporal evolution of the average energy content of the acoustic events recorded during loading and fracture of marble specimens subjected to either direct tension or uniaxial compression was studied. It was highlighted that