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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synchronized with time. The AE measurements were performed by two wide-band sensors the operating frequency range of 0.2…2.0 MHz. Signals detected by the AE transducer were passed through pre-amplifiers of 40 dB gain with a band pass of 200 – 600 kHz. To reduce the effect of false AE signals from friction at the point of contact of the surfaces of the beam sample with the testing machine supports, antifriction gaskets were installed on them. Previously developed techniques based on the continuous wavelet transform (CWT) (Stankevych and Skalsky (2016)) and discrete wavelet transform (DWT) (Skalskyi et al. (2020)) of AE signals were used to analyze the AE signal peculiarities. The fracture type of composite materials can be identified by the criterion parameter E WT calculated according to the parameters of the CWT of AE signals and the correlation of the fracture mechanisms and spectral characteristics of AE signals can be determined by the energy distribution based on the DWT. 3. Stress distribution in composite reinforced with steel fibers One of the most important stages in predicting the strength of composites is to determine the stress-strain state in the areas of the highest probability of initiation and development of fracture. To do this, the finite element method is widely used. The results of modeling of the stress-strain state for various composite systems are known in the literature (Kumar and Mohanty (2012), Chen (2015)). In this study, the stress state in a composite reinforced with rectilinear fibers was evaluated under a tensile-compressive load. A sample of a composite made on the basis of concrete (matrix) and reinforced with rectilinear steel fibers was considered. The mechanical characteristics of the matrix and fibers are given in Table. 2. The mechanical stresses in a loaded sample of cubic shape with two reinforcing fibers were calculated by the finite element method using ANSYS software. The lower face of the cubic matrix was considered to be rigidly clamped, and the opposite face was loaded with a normal tensile force of intensity of 1 MPa evenly distributed on its surface (Fig. 1). The fibers were considered to be rigidly bonded to the matrix and placed at a sufficient distance from the parallel faces of the sample to neglect the effect of these edges on the distribution of mechanical fields in the composite. The following three cases of fiber orientation are considered: parallel to each other and placed at an angle of 45º and 90º. The composite region was divided by standard quadrilateral finite elements of 10 knots each. The calculation was performed for different partition densities, gradually thickening the mesh of finite elements, until the difference between the previous and next calculation became insignificant. In this case the accuracy of the obtained numerical results will be maximal. Then, the average total number of finite tetra-elements was about 46000, and the number of nodes – 75000. As you can see from Fig. 1, the highest values of the maximum shear stresses are reached in the areas near the contact boundaries of the matrix with the reinforcing components in all three cases of their mutual location. Comparison of the values of shear stresses at different orientations of the fibers shows that the most dangerous is their location at an angle of 90  . The obtained results are consistent with those known in the literature (Chen (2015)). Therefore, considering the zones adjacent to the contact boundary between the filler and the reinforcing element, structurally the weakest point of the composite, based on the obtained calculations, we can expect that these zones will be the most probable places of initiation and development of fracture. Table 2. Mechanical characteristics and geometric dimensions of composite components. Material Jung’s module , MPa Poisson's ratio Density, kg/m 3 Dimensions, m Concrete (matrix) 2.5  10 4 2.0  10 5 0.33 0.33 7850 2300 1  1  1 (Cube) Structural steel (fiber) Diameter d = 0.06