PSI - Issue 33

Tatyana Tretyakova et al. / Procedia Structural Integrity 33 (2021) 1082–1088 Author name / Structural Integrity Procedia 00 (2019) 000–000

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3.2. Estimation the effect of short stops on the PLC effect When testing specimens under monotonic kinematic loading, the critical values of deformation corresponding to the occurrence of intermittent plastic flow due to the PLC effect are determined. The minimum elongation value at which intermittent deformation occurred was 1.9 mm (corresponding to a strain level of 3.8%). According to the received data, the strain levels at which the loading process was stopped for one second were selected (see the Table 2). These levels were below the critical values of the initiation of the Portevin-Le Chatelier effect. After short-term stops, the tension was resumed at the same loading rate. As a result, deformation curves were obtained, shown in Fig. 7. The curves are characterized by the manifestation of the jerky flow, numerous drops are observed. Fig. 8 shows the initial section of the tension diagrams, where vertical dash-lines mark the moments of realization of the short-term stops and the PLC effect. It was shown that during a short-term stop, a drop in the stress is observed, after which the initiation of jerky flow occurs at lower levels of deformations as compared to monotonic quasi-static loading. The obtained data on the critical deformation at which the Portevin-Le Chatelier effect begins to manifest itself has been evaluated, and deformation bands of localized plastic flow are recorded on the surface of the samples by using DIC technique.

Fig. 7. Curves of quasi-static tension of specimens at a loading rate of 2.4 mm/min with short stops at various elongations (correspond to Table 2)

3.3. Influence of the non-monotonic loading on the regularities of the jerky flow To assess the effect of non-monotonic loading on the spatial-time inhomogeneity of the jerky flow in the Al-Mg alloy, mechanical tests with a change in the loading rate at the stage of intermittent plastic deformation were carried out. Figure 5.5 shows a diagram of the deformation of a sample with a change in the tensile rate and the corresponding dependence of elongation on time, implemented in the test. The rate has been reduced by one order of magnitude from 2.4 mm/min to 0.24 mm/min. The curve is shown on Fig. 8. According to the results of previous tests, it was revealed that at the indicated rates of stretching under conditions of quasi-static tension, various types of intermittent deformation are realized, which is reflected in the different shape of the drops on the curves (Fig. 3, Fig. 4). The onset of the processes of inhomogeneous discontinuous deformation also corresponds to different values of elongation, depending on the rate of stretching. The change in the tensile rate occurred at an elongation 5 mm (marked by a vertical line in Fig. 8), at which type A discontinuous plastic deformation was already observed. A change in the tensile rate led to a change in the type of discontinuous yield.