PSI - Issue 16

Yevstakhiy Kryzhanivskyy et al. / Procedia Structural Integrity 16 (2019) 237–244 Yevstakhiy Kryzhanivskyy et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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Shear lips (ІІI). Shear lips for all investigated forms of incisions were commensurable in size. In general, with a decrease in the test temperature, their area was reduced, however, at -40 ° C, the area of cut lips of the BM was 48.6%, that of W1 was 12.7%, and that of W2 was 19.5%. Rupture area (IV). The conformity of plastic deformation and specimen appearance to the specimen fracture was observed. At high degrees of plastic deformation, the fracture surface was formed by the shear mechanism. This fracture zone was not informative enough for the analysis; therefore, an in-depth interpretation of the mechanisms of its formation was not done. Shear lips for BM. At a test temperature of 20 °C and 0 °C (Fig. 5a), a zone of mesoscopic scale, within which the localized plastic deformation develops under the scheme “translational – rotational vortex” (I), is formed at the tip of the Charpy specimen. It is the vortical nature of the plastic flow that enables the accumulation of a critical number of defects in this area, which provides the possibility of a macrocrack nucleation in zone (I). After its start, the further propagation of macrodefects is accompanied by the formation of a plastic “hinge” (II), which is accompanied by shear deformations in the surrounding material. They are the most active on the specimen surface, where they have the lowest energy intensity. Consequently, the propagation of the main crack (III) under impact loading is accompanied by the formation of shear lips. In the rear part of the specimen, due to the high rate of development of the fracture process and its significant localization, the region of pivot-shear deformation is formed, which is manifested at the stage of rupture (IV). At low test temperatures -20 °C , -40 °C , -60 °C , (Fig. 5b) plastic deformations are localized in a thin layer of the material, in the vicinity of the crack tip. In this case, plastic deformation develops at the micro-level in this thin layer (plasticity zone), and the rest of the specimen is not deformed, or only plastic deformations are localized. 4. Discussion and generalization of the results obtained

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Fig. 5. Mechanisms of ductile (a) and brittle (b) fracture of specimens and the formation of shear lilps under impact loading, Wilshaw, Pratt, (1966) and Panin et al. (2016): (a) fracture with the formation of a zone of plastic deformation; (b) fracture with the formation of a cascade of microshear plastic deformation sites; (c) fracture with the influence of the fusion zone; І – the area characterized by the translational-rotational vortex motion of the material in the crack start zone; II - plastic “hinge”; III – lines that “contour” the formed shear lips; IV - the area of deformation development according to the “shift + turn” scheme preceding the rupture; V - microshear plastic deformation sites; A - the influence of the fusion zone. Shear lips in welds. In case of welds, an incision in the zone of thermal effect limits the area of exhaustion of plasticity (local brittleness) significantly, Fig. 5c. However, for such specimens, there are significant differences in the shape of shear lips, which, in our opinion, are due to the influence of the weld on the zone of localized plasticity formed in the vicinity of the incision. At the same time, the mechanical properties in the zone of stress concentrators, taking into account the weld, change the stiffness of the stressed state (Gabetta et al (2008) and Nykyforchyn et al (2004)). In addition, it should be emphasized that in the operating conditions, micro-defects may occur in welds of gas mains. For specimens cut from steel of W2, a slight decrease in the impact toughness was found to be about 12%. This may be caused by the accumulation of structural defects due to reheating, or the structural heterogeneity of the weld. In general, the scatter of data on the impact toughness of the investigated welds is insignificant. Therefore, it should be noted that regardless of the presence of carbonitride-forming elements in the weld, which are quite numerous in the base metal, the share of niobium, vanadium and titanium in general is: Nb + V + Ti = 0.235%, and they partially