Application of T-Scaling Method to Account for the Effects of Notch Acuity on Notch Fracture Toughness in the Ductile-to-Brittle Transition Temperature Region

Abstract

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 JIc ∝ ρ 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 KIc ∝ ρ1/2 but is not continuous with the pre-cracked KIc at a lower shelf temperature of −166 °C. The DBTT notch fracture toughness KJc ∝ ρ1/2 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.