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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Fig. 2. Time dependences of load and AE activity of SFRC samples with different fibers volume fraction: (a) plain concrete; (b) SFRC with 1.5% fibers; (c) 2% fibers; (d) 2.5% fibers.

It is known from the literature (Li et al. (2018), Logoń (2019)) that the propagation of cracks in the matrix and the interfacial bond, as well as the fracture of the filler are the sources of AE signals in concrete. For plain concrete (Fig. 2a), a sharp jump in the number of signals occurs at the subcritical stage of fracture. Adding 1.5% fiber to concrete significantly changes its AE activity. Fig. 2b shows that the number of AE signals generated long before the maximum load is reached, increases significantly compared to the plain concrete. This is due to the fact that new sources of AE appear in the concrete reinforced with hooked-end steel fibers: sliding and pull-out of fibers, cracking of the matrix from the deformed fibers. In particular, according to experimental data, the number of AE events registered during the plain concrete fracture was 36, and SFRC with 1.5% fiber – 422, with 2% – 760, with 2.5% – 1093. The identified patterns are consistent with those known in the literature (Li et al. (2018), Wu et al. (2000)). 5. Identification of types and mechanisms of SFRC fracture 5.1. AE signals analysis during plain concrete fracture To identify the mechanisms and types of fracture, the AE signals were analyzed by their CWT. Fig. 3 shows the CWT of the typical AE signals during fracture of plain concrete at the initial (Fig. 3a) and critical (Fig. 3b) stages of fracture. As it can be seen, the CWT of AE signals has local maxima preferably of one frequency range (Fig. 3a) at the initial stage of fracture. We observe local maxima of different amplitude at different frequency ranges (Fig. 3b), when the transition to the critical stage occurs. This change in the parameters of the CWT indicates that the fracture in the sample occurs mainly by one mechanism at low load, and different mechanisms alternate in a short time interval with the approach to its critical value that generate elastic waves of different frequencies. Using the algorithm for identifying of the fracture types (Stankevych and Skalsky (2016)), the main parameters of the local pulses of the CWT of AE signals (dominant frequency f max , radiation duration Δ t and energy parameter E WT ), which accompanied the fracture of the samples of plain concrete samples, were determined, and the types and mechanisms of its fracture (Table 4) were identified.