Fracture Toughness in the Transition Region of a Carbon Steel and a Ferritic Nodular Cast Iron

Abstract

In order to characterize the fracture toughness in the ductile-brittle transition region for thick-walled cylinders of ASME SA350 Gr.LF5 carbon steel and JIS FCD300LT ferritic nodular cast iron, elastic-plastic fracture toughness tests were carried out. The specimens were fatigue precracked compact tension (CT) specimens of 25mm in thickness. The tensile testing machines used were Instron type, electrohydraulic type and drop-weight type ones.
In the static fracture toughness test on a FCD300LT cast iron. CT specimens were often fractured at somewhat higher loads after the initiation of pop-in cracks. Although the scatter of pop-in fracture toughness was small, the values of critical J-integral at the unstable brittle fracture scattered largely.
In the transition region of SA350 steel, the initiation of pop-in crack was not observed, and fracture toughness scattered largely.
At the propagation of the unstable crack near the transition temperature, the Weibull distribution provides good fits for the critical CTOD and the critical J-integral. This distribution can be mainly interpreted by the scatter of the distance between the precrack tip and the origin of unstable brittle fracture.