PSI - Issue 23

I.O. Sinev et al. / Procedia Structural Integrity 23 (2019) 565–570 I.O. Sinev/ Structural Integrity Procedia 00 (2019) 000 – 000

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The noted features of changes in the damage characteristics in the investigated steels can be a consequence of the different nature of the strain hardening process under loading, which develops in ferritic and austenitic steels. Thus, the early start of the process of microcrack merging during testing of stainless steel samples is a consequence of a higher coefficient of strain hardening of this steel, which leads to intense plastic deformation in the early stages of its development and decreasing k -criterion. In this way, the concentration criterion turned out to be a sensitive characteristic reflecting the influence of the structure on the development of damage. This is evidenced by Fig. 2, which shows the linear dependence of the index ( d ) in equation (1) on the coefficient of strain hardening of three steels. The higher the strain hardening coefficient, the lower the index d and, therefore, the lower the value of the k -criterion.

Fig.2. Dependence of the index d of the exponential function, approximating the curves of concentration k -criterion as a function of relative deformation ε* on the strain hardening coefficient n for smooth samples (1) and samples with stress concentrators (2) from low- ( ▲ ), medium carbon ( ● ) and stainless ( ■ ) steels To determine the effect of stress concentration on the damage characteristics during loading, a comparative analysis of the change in the damage parameters of low carbon steel samples with different grain sizes and geometries was carried out. Figure 3 presents the growth curves of the relative area of the damaged surface S * as a function of the relative deformation ε * for low carbon steel samples of various geometries. Zharkova et al. (2007) used samples with a lateral notch (stress concentration factor σ α ~ 2.9) 4 mm thick and 27 and 98 μm grain size, respectively (curves 1,2). Curve 3 in Fig. 3 corresponds to sample with thickness of 16 mm and a grain size of 97 microns. It has been established that an increase in the sample thickness and a decrease in the stress concentration leads to a later formation of surface microcracks in the plastic deformation zone, a quantitative reduction in the fraction of the surface occupied by microcracks and to later start of the defect merging process. Such results are due to changes of a stress-strain state of the sample material in the plastic zone. In addition, the effect of geometry leads to the reduction of other damage parameters, in particular, the density, average and maximum length of microcracks, indexes in exponential and power functional dependencies that approximate the accumulated damage curves. A decrease in σ α from ~ 2.9 to ~ 1 (curves 4, 5) leads to a decrease in the fraction of the surface occupied by microcracks.