PSI - Issue 41

G. Agalianos et al. / Procedia Structural Integrity 41 (2022) 452–460 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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the beginning of the experiment and it could be related to the bedding error (during the first linear segment of the load time curve), the other two regions are detected just before the two PSC plateaus mentioned previously. During the last plateau of the PSC and until the fracture of the specimen, acoustic hits of lower amplitude were recorded since the energy had been already exhausted. Similar behaviour is exhibited by the temporal evolution of the cumulative acoustic energy (Fig.4c) and the cumulative counts (Fig.4d). The maximum cumulative acoustic energy emitted was equal to about 5.07·10 4 aJ and the maximum cumulative counts recorded were 3618. Both values are lower than the respective maximum values obtained by the Class A specimen (i.e., 8.68·10 9 aJ and 43888 counts, respectively) despite the significantly larger number of the hits recorded in the Class B specimen. A summary of the values of some characteristic quantities is presented in Table 3. The above-described behaviour could be attributed to the limited number of capillaries in the Class B specimens, which in turn corresponds to fewer stress-concentration areas, restricting the high energy and high counts AEs. In addition, the specimens of lower porosity favour the transmission of the low amplitude hits, leading to higher number of acoustic hits. Furthermore, despite the fact that there are more acoustic hits, these are of low energy and counts because the elastic energy release is smoother due to the abovementioned bulk properties. Specifically, both micro- and macro-cracking processes take place leading to the formation of the cracking plane and thus the catastrophic failure is not followed by sudden high elastic energy release.

Fig. 4. The temporal variation of the PSC in juxtaposition to that of the (a) applied load; (b) AE amplitudes; (c) AE counts and (d) AE energy, for the case of the low porosity (5.85 %) specimen. The applied load is, also, plotted. The colored areas indicate the regions of intense acoustic activity.

Table 3. Summary of the characteristic quantities recorded for the two specimens. Class of specimens Three-point bending fracture load (kN) Porosity PSC maximum (pA)

Number of AE hits

Total AE counts

Totally released AE energy (aJ)

A B

13.7

22.86% 5.85%

7.14 14.5

59

43888 3618

8.68 · 10 9 5.07 · 10 4

18.83

326

In order to calculate the Ib-value a number of consecutive acoustic hits are selected and the a 1 and a 2 coefficients in Eq.1 are set equal to 1, as usual. Taking into account the small number of hits recorded in case of the Class A