Recently, several works appeared in which temperature dependence of ductile fracture toughness of ferritic steels on the upper shelf of brittle-ductile transition curve was analyzed and Upper Shelf Master Curve concept was formulated. Generally, fracture toughness at different temperatures characterized by JIc or dJ/da should be proportional to the deformation work of unit volume evaluated from zero to the critical strain for ductile fracture. As in many other cases, critical strain for ductile fracture initiation may be identified with critical strain for initiation of shear bands, calculated for hyperelastic material with the corresponding stress-strain curve. This concept is successful, among others, in determination of the temperature dependence of fracture toughness of ferritic steels on the upper shelf. Two most important physical mechanisms controlling temperature dependence of constitutive behaviour of ferritic steels in the corresponding temperature range, hence the temperature dependence of both deformation work to initiation of ductile fracture and fracture toughness, are friction resistance to dislocation slip (Peierls stress) and dynamic recovery (dislocation annihilation). Predicted Upper Shelf Master Curve shape based on temperature dependence of constitutive parameters of different ferritic steels corresponds well to the published data.

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