Issue 40

I. Stavrakas, Frattura ed Integrità Strutturale, 40 (2017) 32-40; DOI: 10.3221/IGF-ESIS.40.03 37 gradual decay back to the background level occurs when the mechanical stress is relieved. It must be noted that the maximum value of the applied stress (i.e. 70% of the maximum stress) enters the specimen into the early nonlinear region regarding its stress-strain behaviour. During the second loading the applied mechanical stress reached approximately the level of the mechanical stress of the 1 st loading (i.e. 60MPa) while the recorded PSC reached a maximum of 10pA which is significantly lower than the recorded PSC of the first loading. Such a behaviour may be attributed to the underlying physical mechanisms of the PSC generation and specifically the Moving Charged Dislocations model [4]. Since the PSC variation is attributed to the damage generation and extension it is expected that when a brittle specimen, like marble, is subjected to a mechanical stress in sequential loading cycles new damages occur only during the initial application of the stress. During each following stress application only minor extension of the already created damages is produced and consequently only weak PSC emissions are expected. The third loading was scheduled to lead the specimens to failure, and the mechanical stress was applied at the same rate of the two initial loadings. During this third loading a strong PSC emission was detected reaching a peak PSC value of the order of 100pA. It is worth noticing that a short time (i.e. 5 seconds) before the stress drop (due to the specimen’s failure) the characteristic PSC decrease [5, 6] was observed clearly indicating the upcoming failure. The above experimental results are in good agreement with the corresponding results presented in other works [5, 6, 14-16]. Figure 6 : The temporal variation of the mechanical stress during the complete experimental procedure and the corresponding behaviour of the PSC emission. The inset figure constitutes a zoom in at the final stage before fracture making clear the characteristic PSC drop before the failure of the specimen. Fig. 7 demonstrates the behaviour of the PSC emission in logarithmic scale with respect to the mechanical stress during each loading cycle. Specifically, the blue, the green and the light grey lines correspond to the load increase from 0 MPa to 60 MPa of the first, the second and the third loading, respectively. The black line shows the behaviour of the PSC with respect to the mechanical load during the third loading, when the applied stress becomes higher than 60 MPa exceeding this way any previously applied level of stress. Observing Fig. 7 it is clear that during the three loadings and while the stress is below 60 MPa, the recorded PSC becomes significantly lower for each next loading. When the mechanical stress exceeded 60 MPa during the third loading, the PSC emission significantly increases reaching a peak of 100 pA, few seconds before fracture. The black line in Fig. 7 supports the dominance of the Kaiser effect during the experimental procedure and in addition that PSC may also be used to detect that a sample has suffered significant mechanical stress at earlier times and it “remembers” such a fact. Another observation is that during the second and the third loading the detected PSC and the corresponding PSC energy are both at the same low levels up to a stress value of 45 MPa. Beyond that point of stress and during the third loading the recorded PSC is maintained at a very low level. During the third loading and when stress becomes higher than 60 MPa strong PSC emissions and rapid increase is detected.