On the cleavage stress promoting crack path deflection and anisotropic fracture in cold drawn pearlitic steel

Abstract

Crack path deflection is a phenomenon appearing in cold-drawn pearlitic steel wires as a consequence of the pearlite microstructure orientation induced by the manufacturing procedure, the deflection angle being approximately 90º, thereby representing a crack propagation step (fracture wall), a signal of splitting, delamination or debonding in the form of axial cracking (or micro-cracking) by means of a microscopic fracture mode in the form of enlarged and oriented cleavage in the direction of the wire axis (cold drawing direction). The described phenomenon is a signal of anisotropic fracture behaviour (strength anisotropy) associated with mixed-mode propagation. This paper demonstrates that, in addition to the oriented microstructure of the cold drawn pearlitic steels (at the two microstructural levels of pearlite colonies and lamellae), a sufficient level of stress triaxiality (constraint) is required, as in the case of cracked specimens in which a cleavage stress appears as a consequence of the singular stress distribution in the close vicinity of the crack tip (crack tip stress-strain field).