Correlation between fracture toughness and the distance (X) from the tip of a fatigue pre-crack to the initiation site of cleavage crack

摘要

The extent of cleavage process zone and its effect have been studied using mild-steel materials and large power- generation- type rotor materials having different levels of strength, and the cleavage crack initiation sites in the temperatureregion under ductile-to-brittle transition temperature were examined. In particular, a detailed examination was performed on the distance from the tip of a fatigue pre- crack to the cleavage crack initiation site, and on the distance from the tip of theductile crack to the cleavage crack initiation site for the case of a progressing fibrous ductile crack. Also the relationships between the distance (X) and the value of fracture toughness were examined and the criteria for cleavage fracture as proposedin the past were studied. The following results were obtained.(1) The distance X is distributed in a random manner between 100 and 1000μm in the case of SM400C steel and between 80 to 600μm in the case of NiCrMoV steel, regardless of whether the fibrous ductile crack extension (Δa) exists or not.(2) The distribution range of the distance X increases with the increasing temperature. And the maximum value of the distance X goes toward the larger value as the temperature increases.(3) Since no correlation could be observed between the distance X and the value of Kc(J), the site of crack initiation does not necessarily originate at the point of maximum stress. An explanation could therefore not be given using the conventionalcriteria for cleavage fracture.(4) The initiation of cleavage fracture is governed by the relative relationships held by the maximum weak point in the toughness distribution with the R curve and with the cleavage process zone of the crack tip. If the maximum weak point happens to liewithin the cleavage process zone of the tip of the fatigue pre-crack, the cleavage fracture will occur on the blunting line before the occurrence of fibrous ductile crack growth. On the other hand, if the maximum weak point exists outside this cleavageprocess zone, the ductile crack will progress while searching for the maximum weak point, and the cleavage fracture will occur at the intersection of the R curve and the toughness distribution curve.