J's DISABILITY COULD EXPLAIN ALL THE TEST SPECIMEN SIZE EFFECTS OBSERVED FOR THE FRACTURE TOUGHNESS (J_c) OF A MATERIAL IN THE DBTT REGION

摘要

This paper summarized our recent studies on the test specimen size (TSS) effects on J_c of a material in the ductile-to-brittle transition temperature (DBTT) region. The validity of the deterministic approach to transfer the fracture toughness J_c obtained with different thickness specimens is demonstrated in these works. Based on the detailed finite element analysis results, it was found that the crack-tip stresses were different at the identical J in the test specimen thickness (TST) effect on J_c observed with both the non-proportional and proportional specimens. And adjusting loads to make the stress level equivalent showed increment in J that was equivalent to the J_c difference due to TST effect on J_c. This was similar with the past result obtained for the planar size effect on J_c (the difference in J_c due to the planar specimen configuration including crack length difference for the same specimen type or the specimen type difference). Thus, it was concluded that all of the TSS effects on J_c could be explained as due to J's disability to characterize the crack-tip stress field accurately, or in a more general explanation, due to the finite size effect. In addition, the (4δ_t, σ_(22c)) failure criterion (Dodds et al., 1991) was verified to transfer J_cs obtained for different specimen thicknesses and planar configurations. The critical value σ_(22c) varied for only a few percent. The fact that these critical values were always reached at the specimen mid-plane and the fact that cleavage always initiated at the specimen mid-plane supported the validity of the deterministic approach. Because the (4δ_t, σ_(22c)) failure criterion requires only "single" set of test data for J_c transfer and because σ_(22c) shows only a few percent scatter, it seems to have a possibility to replace what Weibull stress is expected to do.