An analytical and experimental comparison of [formula omitted] for shallow through and part-through surface cracks

Abstract

The use of initiation fracture toughness ( J i ) values measured using standard deep crack single edge notch bend, SE(B), specimens in fracture integrity assessments of shallow part through surface flaws frequently leads to under predictions of structural reserve capacity against fracture. To determine if the ratio of the J- integral to the crack tip opening displacement ( m factor) can be used to judge the expected comparability of J i- Values appropriate to different crack geometries, dynamic J i- Values were measured using three different test specimen types. These included both laboratory specimens, shallow and deep crack SE(B), and a specimen having a shallow part-through semi-elliptical surface crack intended to represent that which might be encountered in service. Comparisons between the J i- Values measured using these three specimens were conducted for pre-initiation deformation conditions ranging from small scale to gross section yielding so that the effect of material toughness on J i similarity could be assessed. These comparisons indicated that specimen geometries which have similar m factors also have approximately the same resistance to crack initiation. Additionally, it was demonstrated that matching m factors was not needed to ensure similarity of J i- Values if specimen failure occurs under conditions approximating small scale yielding. This information suggests that the m factor may be useful for selecting test specimens which can provide fracture initiation toughness measurements appropriate for use in structural defect assessments.