PSI - Issue 81

Jesús Toribio et al. / Procedia Structural Integrity 81 (2026) 536–538

537

2. Materials and effect of cold drawing on microstructure The two materials used in this research were pearlitic steels with eutectoid composition. The first was a hot rolled bar (not cold drawn at all), whereas the second was a heavily cold drawn wire (commercial prestressing steel wire). Progressive cold drawing of pearlitic steel affects the microstructural arrangement in the form of slenderizing of the colonies, decrease of interlamellar spacing and orientation in the direction of cold drawing (wire axis) of both colonies and lamellae (Toribio and Ovejero, 1997, 1998a, 1998b, 1998c), as shown in Fig. 1.

Fig. 1. Microstructures of the hot rolled bar (left) and the cold drawn wire (right) in longitudinal sections. Vertical side of the micrograph is always parallel to the wire axis or drawing direction, whereas horizontal side is associated with the radial direction of the cylinders. 3. Fatigue crack paths Fatigue micro-cracks are transcolonial and translamellar (i.e., the crack path crosses the colonies and breaks the lamellae) with non-uniform crack opening displacement values, micro-discontinuities, branchings, bifurcations and local deflections, creating microstructural roughness. The micro-crack paths in the hot rolled bar and the cold drawn wire are given in Fig. 2.

Fig. 2. Fatigue micro-crack paths in the hot rolled bar (E0; left) and cold drawn wire (E7; right). Crack opening displacement (COD) and branches of the fatigue micro-crack path are shown in yellow colour, boundaries of pearlite colonies in green colour and some ferrite/cementite lamellae in red colour. 4. Discussion: globally isotropic versus locally anisotropic fatigue behaviour In both steel wires the fatigue behaviour is globally isotropic (crack advance in global mode I) and locally anisotropic (crack advance in local mixed mode). Thus the anisotropy of microstructure only produces locally anisotropic fatigue behaviour with no evidence of macroscopic crack deflection. The cold drawn wire exhibits a tortuous fatigue crack path with zig-zag shape consisting of alternating micro-defections , with more micro-deflections in the cold drawn wire than in the hot rolled bar (Fig. 3), the micro-deflection path being shorter and the micro-deflection angle being higher in the former than in the latter, i.e., and Thus the frequency of deflections in the fatigue crack path increases with the cold drawing degree (Fig. 3). This tortuous crack path is one explanation of the improvement of fatigue performance in the cold-drawn wire when compared to that of the hot rolled bar (Toribio and Toledano, 1999, 2000). (0) (7)    (0) (7) d d 

Fig. 3. Sketches of the fatigue crack paths in the hot rolled bar (left) and the cold drawn wire (right).