Using local approaches to fracture to quantify the local conditions during the ductile-to-brittle transition in ferritic steels

Ferritic steels, which are typically used for critical reactor components, including reactor pressure vessels (RPV), exhibit a ductile-to-brittle transition. The fracture process has been linked to the interaction between matrix plasticity and second phase particles. Under high-enough loads, a competition exists between particles rupturing to form micro-cracks and particles decohering to form micro-voids, which are prerequisites for cleavage and ductile fracture, respectively. Currently, there is no sufficiently adequate model that can predict the competition between these two failure mechanisms and hence fracture toughness values in ferritic steels. In this work, failure probability has been estimated using a local approach to cleavage fracture incorporating the statistics of micro-cracks. It is shown that changes in the deformation material properties are not enough to capture the significant changes in fracture toughness. Instead, a correction to the fraction of particles converted to eligible for cleavage micro-cracks is proposed. The method is developed for the RPV steel 22NiMoCr37 and using experimental data for a standard compact tension C(T) specimen. The proposed approach offers more accurate calculations of fracture toughness in the ductile-to-brittle transition regime using only decoupled models, which is attractive for engineering practice.