The scatter of fracture toughness in the ductile-brittle transition temperature region was experimentally examined on 500 MPa class low carbon steel. The repeated toughness tests with the three points bend specimens were carried out at -60°C, -20°C and -10°C. The Weibull statistics of the fracture toughness K(Jc) at -60°C showed the single modal distribution with the shape parameter of around 4, which agrees with the analytical prediction from the conjunction between the Weibull stress analogy and the fracture mechanics. The Weibull distributions of the K(Jc)s at -20°C and -10°C, however, showed the typical bilinear pattern with the elbow point. The Weibull slopes below the elbow points were closed to the value of 4, but the slopes showed the transition to be smaller slope at the elbow points. The K(Jc)s above the elbow points induced the larger scatter of the data set at -20°C and -10°C. The fracture surface observations revealed that the K(Jc)s of the elbow points on the Weibull distributions are coincided with the stable extension of around 0.2mm or 0.3mm in length. This transition condition in the Weibull slope were also discussed, compared with the JIc value and the required conditions for δc or Jc prescribed in BSI and ASTM.
The bilinear Weibull distributions experimentally obtained were also simulated by the analytical model, in which the Weibull stress criterion was applied to the stably extending crack tip. In spite of the simplified model with the radical approximation, the analytical model could describe the trend of the bilinear Weibull distribution.
