The Weibull stress criterion has been accepted as a fracture criterion for cleavage fracture of steels and has been applied to the analysis of the fracture toughness. However, the effect of the temperature and/or the yielding scale on the critical Weibull stress has not yet clarified. In the present work, the dependency of the critical Weibull stress on the yielding scale was investigated on a low carbon steel. The Weibull parameters of the critical Weibull stress were statistically evaluated with notched round bar tensile specimens. The 28 specimens for both 1mm radius notched round bar and 50mm radius hourglass bar were repeatedly fractured at -196°C and the Weibull stress analyses were carried out based on the finite element stress solution. Although the constant shape parameter was successfully obtained independent on the specimen configurations, the scale parameter for the critical Weibull stress which was defined with the original Beremin's formulation was higher in the sharp notch specimen. The locations of the trigger points for cleavage in all fractured specimens were quantified by SEM observation. The local stress and strain conditions at the trigger points were also estimated by means of the FE solutions. The local tensile stress at trigger was not constant and showed the dependency on the local plastic strain.
In the original formulation of the Weibull stress proposed by Beremin, enough numbers of the nucleated micro-cracks are assumed and the fracture probability from micro-cracks is defined by only the maximum principal stress. However, the present test results suggest that the straining history for the micro-crack nucleation should be also considered for the cleavage fracture criterion. Modified Beremin model, in which the probability of the microcrack nucleation is also considered, was applied to the present results. The variation of the critical Weibull stress with the plastic constraint was almost disappeared in the modified Beremin model. The contribution of plastic strain on the cleavage fracture was discussed.
