Abstract
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 is not yet clarified. In the present work, the dependency of the critical Weibull stress on temperature was investigated on a low carbon steel. The Weibull parameters of the critical Weibull stress were statistically evaluated with notched round bar tensile specimens. Three test temperatures of -196, -130 and -100 °C were selected at which fully cleavage fracture surfaces could be obtained. The 28 specimens were repeatedly fractured at each test temperature and the Weibull stress analyses were carried out based on the finite element stress solution. Both the shape parameter and the scale parameter for the critical Weibull stress which was defined with the original Beremin's formulation rarely depended on test temperature. The locations of the trigger points for cleavage in all fractured specimens were quantified by SEM observation. The triggers of the specimens at -130 °C and -100 °C located at the positions of the peak principal stress. But, the triggers at -196 °C were deflected from the peak stress position to the larger strained region.
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 stress. However, the present results suggest that the strain history for the micro-crack nucleation should be also considered for the cleavage fracture criterion. The formulation of the fracture probability, which covers both the micro-crack nucleation process and the propagation process, are also discussed in this work.
