Issue 78

A. Sancak et alii, Fracture and Structural Integrity, 78 (2026) 186-206; DOI: 10.3221/IGF-ESIS.78.11

Near-fracture cross-sectional microstructural characterization by SEM SEM examinations were performed on polished, nital-etched and platinum-coated cross-sections taken approximately 10 mm away from the fracture surface and parallel to the fracture plane. These images highlight the local ferrite–pearlite microstructure and microstructural heterogeneity in selected near-fracture regions. Because the examined regions were not located on the actual fracture surfaces, their spatial relationship with the crack path and their direct contribution to the measured absorbed impact energy cannot be established. Therefore, the SEM images were not interpreted as direct fractographic evidence or as definitive evidence of the fracture micromechanism. Fig. 13 shows the selected cross-sections, arranged as descriptive paired comparisons between lower- and higher-energy specimens within the same nominal conditions. These comparisons are qualitative and do not establish causation. The uncorroded reference specimens tested at 23 °C are shown in Fig. 13(a, b). Although both specimens belonged to the same reference group, their absorbed energies were markedly different: 22.19 J for Fig. 13(a) and 78.59 J for Fig. 13(b). The lower-energy specimen in Fig. 13(a) shows a more heterogeneous local microstructure, with pronounced grain boundary contrast, clustered pearlite regions and dark discontinuity-like areas. In contrast, the higher-energy specimen in Fig. 13(b) exhibits a more uniform ferritic matrix with less severe local discontinuity concentration, although isolated small dark features are still present. Because both specimens were not exposed to seawater corrosion, the within-group energy difference cannot be related to corrosion exposure. The selected SEM fields highlight local microstructural variability within the same nominal condition; however, they do not establish that the observed features caused the difference in absorbed impact energy. For the specimens exposed to seawater for 30 days, notched after corrosion exposure and tested at 0 °C, Fig. 13(c, d) compares specimens with absorbed energies of 6.56 and 16.95 J, respectively. The lower-energy specimen in Fig. 13(c) contains visible local heterogeneities, including pearlite-rich regions and dark angular discontinuity-like features. The higher energy specimen in Fig. 13(d) appears comparatively more homogeneous at the same magnification, with fewer severe discontinuity-like areas at the examined location. The two selected cross-sections therefore highlight different degrees of local microstructural heterogeneity. However, because the observations were made away from the actual fracture surfaces, the relationship of these features to crack initiation, crack propagation and absorbed impact energy cannot be determined.

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