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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3. Results

The analysis of images obtained during loading of samples with stress concentrators, as well as the data from studies Bovina and Soldatenkov (2017), Zharkova et al. (2007), and Tyutin et al. (2018), made it possible to estimate the effect of stress concentration and the structure of the material under study on the change in damage characteristics. Figure 1(a) shows the loading curves, changes in the concentration k -criterion and the relative area of the damaged surface S* as a function of the relative deformation ε* , which allow to distinguish the stages of microcracks development (I, II, III) and establish the specific changes indicated above characteristics at these stages. Stage I is associated with the process of nucleation of microcracks; its completion, as shown by analysis of microcrack pattern, corresponds to the beginning of microcracks merging and the transition to stage II. From Fig.1 (a) it follows that when testing samples of stainless steel stage II begins at less deformation compared to mild steel. This leads to a shift in the deformation dependence of the k -criterion towards smaller strains, the early onset of the microcrack formation process, and then their merging characterized by a value of the k -criterion close to 2.6. When testing specimens from carbon steel, the k -criterion varies over a wider range and the process of coalescence of microcracks begins at the k -criterion close to 3. The k - ε* curves in Fig. 1 (a) are well described by equation: * d k Ae    (1), where index d depends on steel structure. Intensive growth of the damaged surface area begins at the stage II. During the transition to stage III, the curves of the cumulative distribution of the concentration of microcracks along their length (Fig. 1b) reflect changes in the damage accumulation mechanism. The appearance of the linear part of this distribution indicates the transition from the exponential function approximating these curves to the power one.

Fig. 1. Deformation curves, changes in concentration k -criterion and relative area of damaged surface S * as a function of relative deformation ε* (a) and curves of cumulative distribution of concentrations of microcracks along their length (b) for low carbon samples (top) and stainless steel samples (bottom)