Initiation and propagation of fatigue cracks in cold-drawn pearlitic steel wires

Abstract

In this paper the important role of manufacturing by progressive cold drawing on initiation and propagation of fatigue cracks in cold-drawn pearlitic steel wires is addressed by analyzing the initiation (from surface defects) and propagation of fatigue cracks in two pearlitic steels with very different cold drawing degree or cumulative plastic strain (and the subsequent very distinct microstructural arrangement): a hot rolled pearlitic steel bar and a commercial high-strength cold-drawn prestressing steel wire to be used asa component of prestressed concrete in structural engineering. Experimental results show that the initiation of fatigue cracks in pearlitic steels takes place at the location of small surface defects, many of them in the form of localized damage in the area of material surface (in the case of hot rolled bar) or voids created by tearing during drawing due to the existence of particles near the wire surface (in the case of the cold drawn wire). In both materials, fatigue cracks are mostly transcollonial and tend to fracture pearlite (ferrite/cementite) lamellae, so that different micro-mechanisms of fatigue damage appear in the material such as non-uniform crack opening displacement, micro-discontinuities, branchings, bifurcations and frequent local deflections, all creating a sort of microstructural roughness in the fatigue crack path that is different in the hot rolled bar and in the cold drawn wire, thereby determining their distinct fatigue performance.