Local Approach to Strength Mis-Match Effect on Cleavage Fracture of Notched Material

This paper discusses the strength mis-match effect on cleavage fracture of notched materials based on the local approach. The aim of the study is to verify the independence of the critical Weibull stress at fracture on the strength mis-match condition. Diffusion bonded joints were made with two materials S and H different in strength level. Two types of 3-point bend specimen were extracted. One had a notch in the lower strength material S near the bonded interface, and another in the higher strength material H. Homogeneous specimens were also made for each material. The critical CTOD value at cleavage fracture for the overmatched specimen with a notch in the material S was smaller than that of the homogeneous specimen of the material S. By contrast, the undermatched specimen with a notch in the material H showed apparently larger critical CTOD value than the homogeneous specimen of the material H. The strength mis-match effect on the CTOD results was explained in terms of the constraint effect on the near-tip stress field. Namely, the near-tip stress is elevated by strength overmatching and relaxed by strength undermatching. The Weibull stresses for the homogeneous and mis-matched specimens were evaluated in the light of the near-tip stress fields. The critical Weibull stress at fracture did not depend on the strength mis-match condition near the notch. By using the critical Weibull stress obtained for the homogeneous specimen, the strength mis-match effect on the CTOD results can be predicted. Close agreement was observed between the critical CTOD value predicted and that obtained by the experiment.