Extended investigation of the test specimen thickness (TST) effect on the fracture toughness (Jc) of a material in the ductile-to-brittle transition temperature region as a difference in the crack tip constraint − What is the loss of constraint in the TST

This paper answers a question that was raised from our recent work [1] for the non-standard single-edge notched bend (SE(B)) specimens, which exhibited the test specimen thickness effect on Jc (TST effect on Jc) together with the bounded nature for increasing TST in the ductile-to-brittle transition temperature region. The question was “what is the loss of constraint in the TST effect on Jc, because the crack opening stress σ22 distribution scaled with CTOD at fracture load was approximately same for different TSTs?” wer this question, several well-known constraint parameters were investigated to determine their correlations with the TST effect and the bounded nature of Jc for increasing TST. Based on the elastic–plastic finite element analysis results, it was demonstrated that the well-known stress triaxiality factor Θ = (hydrostatic stress)/(von Mises stress), measured at 4δtc (crack-tip opening displacement at fracture load Pc), exhibited a good correlation with the decreasing and subsequently bounded nature of Jc for increasing TST. Θ started to decrease at some load level before fracture for relatively thin specimens, and this was the loss of constraint in the TST effects on Jc.