Effect of Microstructures of steels on Fracture Toughness and Its Scatter

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For the assesment of reliability or safety of structures, it is very important to know the probabilistic distribution of fracture properties. In the present work, based on the view that the criterion for the cleavage fracture strength of steels is given by the maximum tensile stress, experiments were performed on several steels to study the probabilistic nature of the cleavage fracture stress and fracture tough-ness. Furthermore, quantitative correlation batween the cleavage fracture stress/flow properits andthe fracture touhgness has been investigated for several steels which have different microstructures, grain size and chemical composition.The cleavage fracture stresses in the smooth and notched specimens obey the weibull distribution, but the fracture stress in notched specimens takes higher values than the one in smooth specimens and the weibull parameters are different in both specimens. Such effect of notch or stress gradient on the cleavage fracture can be quantitatively explained by statistical FEM analysis based on Weakest Link Theory. This statistical approach has also been applied to predict the probabilistic distribution of the cleavage fracture toughness.It was assumed that the cleavage fracture at the crack tip would be initiated when the fracture stress is exceeded over a certain characteristic distance. Based on this Criterion good correlations have been obtained between the fracture toughness and flow/fracture properties of the materials for several steels which have various microstructures.