Transition from Ductile Crack Growth to Brittle Fracture of Heavily Deformed Steel Plate

The tensile and bend tests were carried out using cracked specimens to clarify the transition behavior from ductile crack growth to brittle fracture. The brittle fracture initiation which occurred after the large plastic deformation was evaluated by comparing such fracture mechanical parameters as a stress intensity factor K and a CTOD δ obtained using the initial crack length with those calculated using the total length include ductile crack extension at the brittle fracture. The latter parameters were denoted as a K mod and a δmod. Although the values of K, δ, and δmod at the brittle fracture depended on the loading mode and/or the crack length, the Kmod gave almost constant value. In order to clarify the significance of Kmod, that dependence of K and δ on specimen geometry, in which the stress distribution ahead of crack was equal to that at the brittle fracture initiation under large deformation, was investigated using FEM analysis. Although the values of δ depended on specimen geometry, the values of K were almost constant. It was considered that Kmod was afected by plastic constraint theoretically. Kmod, however, was hardly affected by plastic constraint when the ductile crack grew enough to form a strain distribution peculiar to materials ahead of the crack. Kmod could be used as the criterion for the transition from ductile crack growth to brittle fracture.