PSI - Issue 36

Olena Stankevych et al. / Procedia Structural Integrity 36 (2022) 114–121 Olena Stankevych, Valentyn Skalskyi, Bogdan Klym et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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were divided into two types (type I and II), and the number of AE signals of each type was the same.

Fig. 4. (a) Time dependence of the load for SFRC with steel fibers volume fraction of 2% under three-point bending test; (b) Distribution of the number of AE events that accompanied the different fracture mechanisms at different stages of loading.

It is known that the amplitude of the AE signal is directly proportional to the size of the newly formed defect (Nazarchuk et al. (2017)). Comparing the amplitudes and energy parameters of the AE signals (Table 4 and Table 5), it can be seen that they are much higher in the second case. This indicates that under the action of load in SFRC, the newly formed defects are larger than in plain concrete. In section A-B, AE events appear in the higher frequency range and with small values of the energy parameter, which accompany the sliding of the fiber in the matrix, which is demonstrated by the results of identification of types and mechanisms of fracture in Table. 6. Table 6. Parameters of local pulses of CWT of AE signals corresponding to different fracture mechanisms of SFRC with steel fibers volume fraction of 2% on the area of load curve A-B. Fracture mechanisms Type of fracture f max , kHz Δ t , μ s E WT Pull-out / Sliding of fibers Ductile 450…500 10…14 0.003…0.0052 Microcracking Brittle 140…150 15…30 0.042…0.095 Macrocrack growth Brittle 110…170 16…45 0.16…1.44 Most AE events at this stage of loading are characterized by the propagation of micro- (38%) and macrocracks (60%) (in the matrix and at the matrix/fiber interface), and fiber sliding – 2%. The energy distribution of the DWT in this area is similar to the previous one. Table 7 shows the results of identifying the fracture mechanisms on the area of load curve B-C. It can be seen that AE signals with higher energy parameter are generated at this section than at the previous stages, which indicates the formation of new fracture surfaces of larger dimensions. The percent of AE events accompanying fiber pull-out/sliding increases to 10%, although the mechanisms of micro- (40%) and macrocracking (50%) dominate. Table 7. Parameters of local pulses of CWT of AE signals corresponding to different fracture mechanisms of SFRC with steel fibers volume fraction of 2% on the area of load curve B-C. Fracture mechanisms Type of fracture f max , kHz Δ t , μ s E WT Pull-out / Sliding of fibers Ductile 350…400 10…12 0.007…0.01 Microcracking Brittle 90…230 12…42 0.012…0.091 Macrocrack growth Brittle 110…170 20…50 0.193…3.65 According to the results of the DWT, in this area there are three types of AE signals: type I (82%) – maximum energy concentrated at the level of D4 (125…250 kHz) and A4 (0…125 kHz) with a dominant level of D4, type II (14%) – maximum energy concentrated at the level of D4 and A4 with a dominant level of A4, type III (4%) – maximum energy concentrated at the level of D3 (250…500 kHz) and D4 (125…250 kHz) with a dominant level D4. The obtained results are consistent with the identification of mechanisms and types of fracture of the AE