Extended Investigation of Test Specimen Thickness (TST) Effect on the Fracture Toughness (Jc) of a Material in The Transition Temperature Region as a Difference in the Crack Tip Constraint–what is a loss in Constraint in the TST Effect on Jc?

This paper is an extension of our recent work (Meshii et al., 2013), which demonstrated through experiments and elastic-plastic (EP) finite element analysis (FEA) that the test specimen thickness (TST) effect on the fracture toughness of a material Jc in the ductile-to-brittle transition temperature region, together with the bounded nature of Jc for large TST, had a correlation with the out-of-plane constraint parameter T33-stress. Because a definite measure of the crack tip constraint magnitude, especially for EP issues, does not exist, several well-known constraint parameters were tested from the standpoint of whether they have correlations to the decreasing and then the bounded nature of Jc for increasing TST. The results clearly indicated that Tz = σ33/(σ11+ σ22) at the specimen mid-plane could not be directly correlated with the TST effect on Jc, based on the observation that the highest Tz did not coincide with the fracture location predicted by the (4δt, σ22c) criterion, in which σ22c denotes the critical crack opening stress value at a distance ahead of the crack tip that is equal to four times the crack tip opening displacement δt. On the other hand, the results indicated that the well-known constraint parameter Θ = (hydrostatic stress)/(von Mises stress), measured at 4δt, has an ability to monitor the loss in constraint in the TST effect on Jc. Θ at 4δt had a linear relationship with the σ22 up to the fracture load Pc for thick specimens of thickness-to-width ratio of B/W = 1.0 and 1.5, while Θ at 4δt for thin specimens of B/W = 0.25 and 0.5 began to decrease before reaching Pc. In addition, Θ at 4δt for Pc exhibited a good correlation with the TST effect and bounded nature of Jc for increasing TST.