PSI - Issue 30

Kirill Kurgan et al. / Procedia Structural Integrity 30 (2020) 53–58 Kirill Kurgan et al. / Structural Integrity Procedia 00 (2020) 000–000

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It is known that the staging on the stress–strain curves is associated with the nature of the localization of plastic strains. Four stages can be distinguished on the  (  ) curves of the studied samples. Their boundaries are highlighted by dashed vertical lines (Fig. 2). The first stage ( T ) was the transitional one and represented the transformation of the strains from elastic to plastic. The initial stage of parabolic hardening ( II ) was reflected in the distribution patterns of the stain fields in the central part of the stretched steel sample and the localization of the plastic strain spot from the lower grip side (Fig. 3, pattern 3), which then enhance in size with increasing stress. At the same time, the sides in the central part of the stretched sample became stress concentrators with the development of the plastic strain processes. As a result, the small spots of the plastic strains transformed into sources of localized strain macrobands at angles of 45  and –45  to the longitudinal axis (Fig. 3, pattern 4) that increased to mutual contact with the process developing. The combining of the macrobands into one large central area (Fig. 3, patterns 5 and 6) was reflected on the  (  ) curve (Fig. 2, a) by the transition from the stage ( II ) to ( III ). As a result of this process, the steel sample thinning was formed under the tensile load, which was characteristic of the pre-fracture stage prior its failure. It follows from the analysis of the stress–strain curve of the steel sample (Fig. 2, a) that its tensile strength was 392 MPa, after which the pre-fracture stage ( IV ) began. The transition to this stage was accompanied by the formation of a localized region with high values of the plastic strains in the central part of the sample. The plastic strains in this region were more than twice higher than the average for the entire sample (Fig. 3, patterns 7 and 8).

Figure 3. The distribution patterns of the  yy longitudinal strain fields on the steel sample surface: 1)  =0.2%; 2)  =0.7%; 3)  =3.7%; 4)  =8.7%; 5)  =11.4 %; 6)  =24.0%; 7)  =31.0%; 8)  =38.0%. These patterns correlate with the points 1  8 on the stress-strain curve shown in Figure 2, a