On the local conditions for cleavage initiation in ferritic steels

The present study is concerned with an investigation of the local conditions for initiation of cleavage fracture in ferritic steels which have been analyzed in the context of a research program directed to different methods of cleavage fracture assessment. The investigations are performed experimentally and numerically using a German nuclear grade 22NiMoCr3-7 (similar to A 508 class 2) pressure vessel steel as a material example. In the experimental investigation, a variety of standard and non-standard fracture mechanics specimens has been tested including SE(B), C(T) and CC(T) specimens with different sizes and crack depths. The cleavage mechanism and location was determined in a fractographic analysis. Subsequently, the tests were simulated numerically by means of the finite element method in order to determine the local mechanical field quantities at the cleavage initiation spots. Based on the results, a local failure criterion is proposed, accounting for both the nucleation of micro defects and their possible instability due to overloading.