Assessment of the role of micro defect nucleation in probabilistic modeling of cleavage fracture

The present study is concerned with an experimental and numerical investigation of the local conditions for initiation of cleavage failure in ferritic steels. In the experimental analysis, a variety of SE(B), C(T) and CC(T) specimens has been tested. The cleavage triggering sites and the fracture mechanisms were determined in a subsequent fractographic investigation. In a finite element analysis, the local mechanical field quantities at the cleavage initiation spots at the instant of fracture were investigated. Based on the results, an enhanced local concept for cleavage assessment is proposed, accounting for both, the nucleation of critical micro defects and their possible instability due to local overloading. Together with a comparison to the prediction of previous probabilistic models for cleavage failure assessment, the results reveal the importance of considering the nucleation of possibly critical micro defects due to plastic straining as a second necessary criterion for cleavage fracture.