1993 年 11 巻 4 号 p. 570-575
Discussion has been made on the influence of strain hardenability of structural steels on the resistance to brittle fracture initiation. Two high strength steels, YR90 steel and YR70 steel, with different strain hardening properties but with the same chemical composition were tested by 3-point bend CTOD method. The yield-to-tensile strength ratio, YR, for these steels are about 90% and 70%, respectively. The critical CTOD value of YR90 steel at the onset of cleavage fracture without appreciable crack growth was apparently lower than that of YR70 steel. This was addressed by 3-dimensional FE-analysis on the stress distribution in the vicinity of the crack tip. YR90 steel exhibits more elevated crack tip stress and larger extent of highly stressed region than YR70 steel, which leads to lower resistance to cleavage crack initiation. FE-analysis indicated that the near-crack tip stress intensity is controlled by the strain hardening exponent n rather than by the yield ratio YR. The larger the strain hardening exponent n, the more the near-tip stress increases. This implies that the material with a larger n should retain more redundant fracture toughness for assuring structural integrity.