Ductile-brittle fracture transition due to increasing crack length in a medium carbon steel

A phenomenon of ductile-brittle fracture transition with increasing normalized crack length in CS1030 steel has been observed in notch bend specimens. It is found that for stationary cracks in three-and four-point bend specimens the transition occurs at about a/W = 0.2. For propagating cracks in four-point bend specimens, this transition occurs at larger a/W ratios in some specimens but there is no transition in three-point bending. The Ritchie-Knott-Rice (RKR) critical stress model for cleavage fracture, in combination with finite element analyses of crack tip stress fields, successfully explains the ductile-brittle transition with relative crack length. The model also successfully predicts critical values of the CTOD for cleavage fracture.