Ductile-Brittle Fracture Transition as a Result of Increasing In-Plane Constraint in a Medium Carbon Steel

Abstract

Crack-tip opening displacement (CTOD) tests and large deformation finite element analyses have been carried out for double-edge notched tension [DE(T)] specimens with a/W = 0.9 [DE(T)9] and 0.5 [DE(T)5], single-edge notched tension [SE(T)] specimens with a/W = 0.5 [SE(T)5], three-point bend [SE(B)] specimens with a/W = 0.5 [SE(B)5] and 0.1 [SE(B)1], and center-cracked tension [M(T)] specimens with a/W = 0.5 [M(T)5]. The results of the CTOD tests show that the fracture toughness of the material decreases, and a ductile-brittle fracture transition takes place as the in-plane plastic constraint of the specimens increases. In M(T)5 and SE(B)1 specimens with low constraints, fracture is ductile and no transition occurs. In DE(T)5, SE(T)5, and SE(B)5 with higher constraints, fracture initiates by ductile tearing and then changes to cleavage. In the DE(T)9 specimens with the highest constraint, fracture initiates by brittle cleavage. The results of the finite element analyses show that the maximum achievable tensile stress outside the finite strain zone ahead of the crack tip increases with increasing plastic constraint in the order of M(T)5, SE(B)1, DE(T)5, SE(T)5, SE(B)5, and DE(T)9. The ductile-brittle transition is due to the maximum tensile stress ahead of the crack tip reaching the critical stress for cleavage fracture as the plastic constraint increases.