Evaluation of Plane Strain Fracture Toughness in the Ductile-Brittle Transition Temperature Region. 1st Report. Plane Strain Condition considered with the Crack Initiation Point.

Abstract

Fracture toughness tests were performed in the ductile-brittle transition temperature region using 65 three-point bend specimens which were cut out from SM41C carbon steel. Consideration of the probabilistic characteristics of fracture toughness, the crack initiation point of the brittle fracture and the plane strain condition gives the following results. (1) The probability distribution of fracture toughness shows the Weibull distribution. (2) The area of the plane strain condition is estimated from the crack initiation point and the shape of the crack front of the fibrous ductile fracture. The following equation giving the condition for plane strain in the center of the thickness is proposed : B≥0.0037{Kc(J)/σys}2+0.0092. (3) The thickness effect of the brittle fracture toughness is caused by the probability distribution of the weakest point in the crack front, namely, the Weibulltic volume effect.