Fracture toughness variability of structural steel

The weakest-link model of brittle fracture initiation has had substantial success in describing the inherent variability (scatter) in fracture toughness values for steel samples failing by cleavage. The model predicts a Weibull distribution of fracture toughness with slope 4 when plotted in the conventional fashion [E 1921-02. Standard test method for determination of reference temperature, T0, for ferritic steels in the transition range. Annual Book of ASTM Standards, vol. 3.01. PA, USA: American Society for Testing and Materials; 2002]. However, the Weibull slope for samples of a structural steel tested at CANMET has been found to be 1.86, significantly less than the expected value of 4. Possible reasons for the discrepancy are discussed.