PSI - Issue 41

M.Sh. Nikhamkin et al. / Procedia Structural Integrity 41 (2022) 759–765 Author name / Structural Integrity Procedia 00 (2019) 000–000

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a b Fig. 3. Change in the frequency of testing a specimen from the relative operating time N / Nmax: (a) - at a load level of 58.7% of σ 0 ; (b) – at the load level of 12.9% of σ 0 .

Fig. 4. An example of delamination of the specimen during testing

The described nature of the decrease in the rigidity of the material and the natural vibration frequencies of samples and structural elements with the accumulation of fatigue damage has been repeatedly observed both in the case of metals and in the case of composite materials [6, 9–11, 19–21]. A significant decrease (up to 30%) in the flexural rigidity of specimens made of orthotropic and quasi-isotropic laminates was experimentally found by the authors of [12]. Studies [12-15, 17, 22, 23] contain experimental data on the change in the elastic modulus of composite materials under fatigue damage. The results show a slight drop in the Young's modulus in the first stage of fatigue, followed by a stage with a stable slow decrease to the stage of its sharp drop and final destruction. Thus, the experimental data obtained in this work on the change in the natural vibration frequency of the samples are consistent with the known data obtained by other authors on samples of polymer composite materials, and the change in the loading frequency of the sample is an effective factor that makes it possible to determine the occurrence of a crack in the sample and indirectly determine the level of its damage. In order to find the most loaded zones with local stress concentrators, as well as to control the stress-strain state under loading, the deformation fields of the samples were determined. Before the beginning of fatigue tests for each specimen, it was statically loaded to the maximum load provided for in the cycle. In this case, the strain fields εxx, εyy, εxy were determined (the direction of the axes is shown in Fig. 1). In addition, the strain fields were recorded during cyclic loading and after stopping the tests in the loaded to maximum load and unloaded state in order to assess the level of residual deformation of the specimen. The destruction of the sample occurred when the cyclic operating time reached Nmax. Fig. 5 shows an example of the evolution of the strain field εxx in the specimen with increasing cyclic operating time with a maximum loading cycle stress 58,7 % of the static strength limit σ0. Figure 5(a) shows the strain field of an unloaded specimen. Under static loading of the specimen to maximum stress before cyclic testing (Fig. 5, a), zones of slight deformations εxx near the grips on the left side are visible in the specimen, caused by the clamping of the specimen in a testing machine. After 0,8 Nmax cycles under the same load, delamination appears in this zone (Fig. 5, c), and the tensile strain εxx almost doubles, which is associated with the appearance and opening of a crack. When the operating time is reached in Nmax cycles, an abrupt growth of the crack to a large length