Issue 50

D. Triantis et alii, Frattura ed Integrità Strutturale, 50 (2019) 537-547; DOI: 10.3221/IGF-ESIS.50.45 545 stage A 0 10 20 30 40 0 20 40 60 80 mode mode II stage B 0 10 20 30 40 0 20 40 60 80 mode I mode II stage C 0 10 20 30 40 0 20 40 60 80 mode I mode II stage D 0 10 20 30 40 0 20 40 60 80 mode II mode I Mean value of AF [kHz] Mean value of RA [μs/mV] Mean value of RA [μs/mV] Mean value of AF [kHz] Mean value of AF [kHz] Mean value of RA [μs/mV] Mean value of RA [μs/mV] Mean value of AF [kHz] stageA stageB stageD stageC mode I Figure 10 : Classification of cracking modes according to the RILEM TC 212-ACD Recommendation (i.e., with the aid of the relation between the AF and RA parameters), in the four stages of the loading protocol ((a), (b), (c), (d) correspond to the stages A, B, C, D, respectively (see Fig.1)). The blue squares correspond to f<0.1, the green ones to 0.1≤f<0.5 and the red ones to f≥0.5. The red star in Fig.10d (i.e., for stage D) corresponds to the average values of AF and RA during the last 5 seconds before the catastrophic fracture of the specimen (see Fig.9). For f≥0.5 in all four stages, the mean values of RA and AF correspond to gradients k<0.5 kHz·mV/μs. This leads to the conclusion that, at least for the specific material, when the acoustic activity is intense, the shear type of cracking (Mode-II) is dominant. In contrast, when the acoustic activity is low (f<0.1), it is clear that all acoustic signals are classified as Mode- I ones. Particularly, during the last 5 seconds before the final fracture of the sample, the F-function shows a stabilization trend but with strong fluctuations (F=110±35) s -1 (see Fig.9), and the shear type of micro-cracking (Mode-II) is prominent. The mean values of RA and AF at this time interval are depicted Fig.10d with the aid of a red star. A more detailed description of the results of classification of cracking modes according to the characteristics of the AE hits in the four stages of the experiment discussed here is presented in Table 2. Again, it is seen from this table that when the acoustic activity is high (f>0.5) the Mode-II cracking is dominant (exceeding 50%) for all four stages of the loading scheme. On the contrary, for f<0.5 the majority of AE hits corresponds to Mode-I cracking. The shear mode exists also but it corresponds to a relatively low proportion ranging from about 6% to about 31%. Stage f>0.5 f<0.5 Mode-I Mode-II Mode-I Mode-II A 22% 78% 85% 15% B 25% 75% 94% 6% C 44% 56% 87% 13% D 35% 65% 69% 31% Table 2 : Classification of ΑΕ hits.