A method for the estimation of the CTOD fracture toughness δc from toughness results of bend specimens with a machined notch has been studied. The Weibull stress criterion is adopted as a brittle fracture criterion. The equivalent CTOD ratio β has been defined as the ratio of CTOD δ of the fatigue crack specimen to CTOD δ_M of the machined notch specimen (β = δ / δ_M) at the same level of the Weibull stress. CTOD tests are conducted for SM490A steel. It has been found that the equivalent CTOD ratio β is useful to convert the critical CTOD of the machined notch specimen to that of the fatigue crack specimens. The CTOD ratio β depends on the yield-to-tensile strength ratio YR; the steel with a high YR leads to a large β value than the steel with a low YR. Such dependence of β on YR has been verified with CTOD toughness data for various steel with different YR in the range YR=0.60-0.95 in FTC committee in Japan Welding Society.
