Prediction of fracture toughness for heat-resistant steels considering specimen dimensions. Report 3. Brittle fracture

A model has been developed for predicting fracture toughness characteristics on the lower shelf of the fracture toughness temperature dependence considering the influence of specimen dimensions. The model is based on the physicomechanical model of brittle fracture governed by microcleavage critical stress which in its turn depends on plastic prestraining. This model is also based on the assumption that the change in the body dimensions influences fracture toughness as much as the change in the stress state caused by it, which is evaluated by the introduced parameter χ, and affects plastic deformation preceding brittle fracture by cleavage. According to the given model, in some cases an increase in specimen thickness can lead only to a shift of the brittle-to-ductile transition temperature, and in other cases to both a shift of the brittle-to-ductile transition temperature and a decrease in the critical SIF value at which this transition occurs. The paper also presents further elaboration of the probabilistic approach to the prediction of size effect for the cases of brittle fracture based on Weibull's three-parametric distribution. A distinctive feature of this approach is the determination of one of the parameters of the cumulative distribution function, namely, the parameter of location K_{c min} , directly in the experiment and not by conventional statistical methods. This reduces appreciably the requirements for the size of the sample and simplifies the body of mathematics. In this case, the parameter of location K_{c min} corresponds to the critical SIF value during the first unstable jump of a fatigue crack K _fc ⁽¹⁾ which, according to a large number of experimental data, is minimal and the most invariant among all other critical SIF values obtained experimentally. Translated from Problemy Prochnosti, No. 2, pp. 21–31, March–April, 1997.