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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occurs (Figure 5, d) and further growth with opening with violation of the speckle structure.

a d Fig. 5. Dynamic deformation fields ε xx obtained by digital image correlation during operating time: a – 0 cycles without load; b – 0 cycles with load; c – 0.8N max cycles; d –N max cycles. Thus, the analysis of the strain fields can serve not only as an indicator of damage to the specimen, in particular, the appearance of delamination, but also predict the likely places of their appearance, as a result of a gradual increase in stress and strain near some zones as the operating time. During cyclic loading, the thermal state of the specimens was recorded by the non-contact method using a thermal imager. A local increase in the temperature of the specimen indicates the intensification of heat release, as a consequence of the accumulation of damage in this zone. In addition, thermography makes it possible to assess the general temperature state of the specimen. Nevertheless, the thermal state of the specimens even in tests with the highest load (58,7% of σ0) remains satisfactory - the maximum temperature in the heating zone does not exceed 40° C, which does not lead to a decrease in the strength properties of the polymer matrix. However, just before the destruction of the sample, local heating was observed at the site of subsequent destruction to a temperature of 91°C. In the present study, during fatigue testing, acoustic emission parameters were recorded in order to assess the possibility of using them to identify the early stages of fatigue failure. Before and after the fatigue tests of the specimens, they were statically loaded to stress values corresponding to the maximum stresses σmax in the test cycle. In this case, the following acoustic emission parameters were recorded: the number of acoustic emission events per unit time, and the total number of all acoustic emission events over the loading history. Comparison of the response of the specimen under static loading by the number of acoustic events per unit time before and after fatigue loading allows us to give a qualitative assessment of the development of damage after fatigue loading. A record of the total number of acoustic emission events throughout the entire process of cyclic testing of the specimen allows you to visually estimate the change in the level of the specimen damage during the test. Fig. 6 a, b, c show the changes in the resonance frequency of the tests, the total number of acoustic emission events and the number of acoustic emission events per unit time over the entire duration of the fatigue test, until the sample reaches the established operating time. It is seen that each abrupt decrease in the resonance frequency of the tests (Fig. 6, a), indicating the appearance of an interlayer crack, coincides in time, and, therefore, is accompanied by a decrease in the rate of accumulation of the total number of acoustic emission events, expressed in a decrease in the angle of inclination of the curve (Fig. 6 b). The change in the rate of accumulation of the total number of acoustic emission events occurs due to a decrease in the number of acoustic emission events per unit time, as occurs with operating times of ~ ~15% of Nmax and ~65 % of Nmax (Fig. 6, c). b c