Current defect assessment procedures based on fracture mechanics usually assume flaws to be infinitely sharp. While this assumption may be appropriate for fatigue cracks, for non-sharp flaws such as porosity, mechanical damage, or weld undercut, it can be an over-conservative assumption that can lead to pessimistic assessments of structural integrity and a significant underestimation of the true safety margin against fracture. Irwin studied notched and pre-cracked fracture toughness in the lower shelf region and suggested that notched KIc, which is linear-elastic plane-strain fracture toughness is proportional to the square root of notch radius ρ but is not continuous with pre-cracked KIc (i.e., when zero is substituted by ρ in the fitted KIc and ρ relationship, the obtained value differs from that of the pre-cracked specimen KIc). In contrast, Begley et al. conducted a similar study in the upper shelf region and suggested that JIcρ In addition, they showed that notched JIc is continuous with pre-cracked JIc. Very few studies have been conducted on this topic in the ductile-to-brittle transition temperature (DBTT) region. In this study, the effects of notch acuity on notch fracture toughness in the lower shelf and DBTT regions were studied for 0.55% C steel JIS S55C with 0.5TSE(B) specimens. The notch sizes ρ were selected as 50, 150, and 375 μm. Fatigue pre-cracked specimens were also studied. The experimental results showed that notched KIcρ1/2 but is not continuous with the pre-cracked KIc at a lower shelf temperature of −166 °C. The DBTT notch fracture toughness KJcρ1/2 and is continuous with the pre-cracked KJc. By conducting elastic-plastic finite element analysis (EP-FEA), the mid-plane crack-opening stress distribution on the x1-axis, was shown that the scaled stress distribution at fracture load was identical for pre-cracked and notched specimens. Thus, notched and pre-cracked KJc has a reason to be continuous. The reason for notch size effect on Jc was explained as the difference in load for notched specimens to reach the stress level of the pre-cracked specimen.

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