PSI - Issue 36

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

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composites. The acoustic emission (AE) method is an effective tool for monitoring the development of damage to fiber reinforced composites and structures made of them (Nazarchuk et al. (2017). The formation of defects in the structure of composites is accompanied by the generation of elastic waves with different characteristics, which allows us to study the process of accumulation of damage and the development of such materials fracture. It is important to determine the type of fracture, because it allows us to choose the optimal design of reinforcement to ensure its resistance to external loads. In addition, since the development of shear cracks is preceded by tensile cracks propagation, their classification using the appropriate AE parameters will help to assess the material state, and hence the structure as a whole. A number of methods for identifying the mechanisms of fracture of fiber reinforced concrete, which use different parameters of AE signals, are presented in the literature. The most popular among them is the technique developed by Ohno and Ohtsu (2010). The authors proposed to classify cracks according to the distribution of RA parameter (rise time / maximum amplitude) and average AF frequency (number of events / generation duration), depending on the values of which the tensile and shear cracks are identified. On the basis of the general theory of AE the automated method of crack detection, establishment of its type and orientation is developed. This approach has been actively used in various studies of SFRC (Li et al. (2018), De Smedt et al. (2020), Bian et al. (2021)). Other methods of AE signals analysis are also known: by the AE activity (Wu et al. (2000)) and cumulative AE energy (Su et al. (2019)), multiparameter analysis (Triantis et al. (2020). The frequency parameters of the AE signals during the fracture of the SFRC require separate studies, because it is known that each mechanism of fracture in the material generates elastic waves of a certain frequency range. To study the features of macrofracture of composite materials, the wavelet transform of AE signals is an effective method, which allows a local analysis in the spectrum of signals to distinguish the mechanisms of materials fracture (Skalskyi et al. (2018)). The aim of this study is to develop a method for identifying the mechanisms of SFRC fracture under the action of quasi-static loading according to the parameters of wavelet transform of AE signals. 2. Materials and methods The materials employed for the fabrication of concrete mixture are following. Portland cement M400 was used as the binder. The mineral composition of cement is presented in Table 1. Pre-washed crushed gravel stones of sizes of 5-10 mm and natural river sands of sizes of 0 – 5 mm were respectively used as the coarse aggregates and fine aggregates. The composition of the concrete mixture: cement – 520 kg/m 3 , sand – 700 kg/m 3 , gravel – 1038 kg/m 3 , water – 200 kg/m 3 . Hooked-end steel fibers with a profiled surface were used in these investigations (diameter 1.2 mm and length 50 mm), which had the best reinforcing properties and good adhesion to the concrete matrix. The typical features of steel fibers are: the tensile elongation under break – 2%, density – 7.8 g/cm 3 , tensile strength – 1100 MPa, elastic modulus – 190 GPa. Prismatic beams with a cross-section of 40×40 mm and a length of 160 mm with different volume fractions (0%, 1.5%, 2%, 2.5%) were employed in three-point bending tests. For each concrete mixture, five specimens were prepared in order to obtain an average value. A forced mixer was used for the concrete mix. The detailed procedure includes the following steps: 1) adding dry fine aggregates and cement into the mixer, and starting the mixer for 5 min; 2) pouring water into the running mixer, then mixing for 5 min; 3) layer-by-layer filling of the plastic form with the concrete mix and laying of fiber in rows evenly in volume. The specimens were kept in the laboratory condition for 24 h. They were then demoulded carefully and kept in a standard curing room with a constant temperature of 20°C and humidity of 95% until the 28 -day strength was achieved. The three-point bending test was performed on a universal mechanical testing machine UME-10TM. A load was monotonically applied to the specimens with the rate 4×10 -7 m/s. The AE signals were recorded using the AE detection system SKOP-8M with a sampling rate of 4 MHz. The recordings of AE and mechanical load were Table 1. Type M400 cement constituents, in mass percent (%). СаО SiO 2 Al 2 O 3 Fe 2 O 3 Other elements 67 22 5 3 3