PSI - Issue 18

Tretyakov M.P. et al. / Procedia Structural Integrity 18 (2019) 816–822 Author name / Structural Integrity Procedia 00 (2019) 000–000

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500 °С (b), and in Fig. 4 (a) for a temperature of 600 °C. At shutter speeds with a fixed elongation, the initial stage of the relaxation curve is of the greatest interest, on which an intense decrease in the load on the sample is recorded. The dashed line on the graphs corresponds to the values to which the load on the sample decreases during exposure with a constant elongation for 250 s. In Fig. 4 (b) shows the characteristic appearance of the fractured specimens after tensile tests with exposures in position at different temperatures. With increasing test temperature, an increase in the relative residual strain after fracture and an increase in the degree of strain localization in the sample neck is observed.

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Fig. 4. Loading curves of a sample of steel X15CrNi12-2 with holdings on position at various elongations at temperature of 600 °С (a) and the specimens after tensile and relaxation tests at various temperatures (b).

On the given in Fig. 3 and Fig. 4 graphs show that increasing the degree of postcritical deformation leads to a decrease in the magnitude of the fall in absolute values of loads, which is most clearly demonstrated by the test at a temperature of 600 °C. When analyzing the obtained data, it is convenient to consider the reduction of the load from time in a dimensionless form ΔP/P p , where ΔP is the change (decrease) of the load during exposure with a fixed elongation, Pp is the load value by the time the exposure begins at each step. In Fig. 5 shows the dependences of the relative reduction of load ΔР/Р р on time at a temperature of 400 °С (a) and 500 °С (b), and on Fig. 6 (a) at a temperature of 600 °C. The blue lines in Fig. 5 and red in Fig. 5 (a) correspond to extracts at an elongation of 0.75 mm and 1.25 mm, which corresponded to the deformation at the stage of elastoplastic behavior. In this case, at a temperature of 400 °С (Fig. 5a), at the stage of elastoplastic deformation, a more intensive relaxation process is observed than at the subsequent postcritical stage. At the same time, at a temperature of 500 °С (Fig. 5, b), the intensity of the relaxation processes increases at the stage of postcritical deformation. When analyzing the obtained experimental data, it was generally noted that an increase in the degree of postcritical deformation achieved by the time of the beginning of exposure leads to a more intensive decrease in the relative load ΔР/Р р .

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Fig. 5. Curves of decreasing relative load at holdings under conditions of various achieved elongations at temperature 400 °С (a) and 500 °С (b).