Issue 40

I. Stavrakas, Frattura ed Integrità Strutturale, 40 (2017) 32-40; DOI: 10.3221/IGF-ESIS.40.03 39   1 n PSC PSC i i CE E t    where n corresponds to the number of the recorded PSC values in the examined mechanical stress region. It may be clearly seen that during the first loading a significant amount of energy is released (blue line). Additionally it is clear that the PSC emission initiated from early mechanical stress levels which means that the PSC is emitted even in low stress levels. During the second and the third loadings (green and gray lines respectively) the PSC energy initiates only when stress becomes higher than 15MPa approximately, while their cumulative energy levels are significantly lower than the corresponding of the first loading. Finally, during the third loading and when the mechanical stress became higher than the 60 MPa a significant increase of the released cumulative energy is recorded. This fact that also manifests Kaiser effect dominates the PSC emissions and the corresponding PSC cumulative energy. C ONCLUDING R EMARKS uring the present experimental protocol two sequential compressive loadings and unloadings were applied on marble prismatic specimens up to a stress level that corresponds to early non-linear region regarding the stress- strain behaviour. During the third loading, the stress was not relieved but instead it was further increased until the sample mechanically failed. Concurrently to the loading AE and PSC emissions were recorded. The aim was to investigate and correlate the fingerprint of the Kaiser effect on the AE recordings and the PSC emissions as well as the corresponding cumulative energy. The tests were performed on series of marble prismatic specimens and representative experimental results of the carried out tests are presented herein. Additionally, regarding the AE recordings, it is experimentally verified that for marble specimens the Felicity Ratio (FR) obtains high values (i.e. 0.66 and 0.8 approximately) during the two loading/unlading cycles when the applied stress lays in the region where the material is still near the limits of linear region regarding its stress-strain behaviour. Regarding the behaviour of the AE events the number recorded is significantly large during the first loading while the number of AE events becomes fewer during the second load. AE events practically disappear during the third loading and more specific when the applied stress is in the range of the two previous loadings. When the stress was further increased significant AE events are recorded until the fracture of the sample. Regarding the PSC emissions it is observed that the maximum value of the PSC emission becomes significantly lower during each next loading while the applied mechanical stress varies in the same stress limits. It is also important to notice that detectable PSC emissions show up at higher stress levels during the second and the third loading. During the third loading and while the applied mechanical stress becomes higher than the corresponding values of the previous loadings, intense PSC emissions are detected. The characteristic drop of the PSC is detected when the sample approaches failure. All the above manifest that both AE and PSC emissions may be used to detect and qualitatively approach the Kaiser effect phenomena when Dionysos marble samples are subjected to mechanical stress. Combining the experimental findings after applying the AE and the PSC experimental techniques when marble specimens are subjected to compressive mechanical stress it is observed that both experimental techniques may be used in order to detect an upcoming specimen failure. Further on qualitative comparison shows that the use of PSC and AE provide similar information regarding the existence of the Kaiser effect. Specifically, it was seen that when applying sequential loadings and unloadings on marble specimens the emitted PSC becomes lower during each loading cycle and the recorded AE events rate start to increase only when the applied mechanical stress gets values higher than the corresponding stress values during the previous loadings. Quantitative comparison is not yet possible and further experimental work is required in order to attempt such an approach. R EFERENCES [1] Tonolini, F., Sala, A., Villa, G., General review of developments in acoustic emission methods, International Journal of Pressure Vessels and Piping, 28(1)(1987) 179–201. [2] Rao, M.V.M.S., Lakschmi, P.K.J., Analysis of b-value and improved b-value of acoustic emissions accompanying rock fracture. Current Science, 89 (2005) 1577-1582. [3] Enomoto, J., Hashimoto, H., Emission of charged particles from indentation fracture of rocks, Nature, 346 (1990) 641–643. D