13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- process (very common when drawing metals with bcc structure). With regard to the longitudinal section (Fig. 2), both decrease of interlamellar spacing and lamellar orientation in axial direction are observed. Figure 1. Microstructure in the transverse section: hot rolled bar (left) and prestressing steel wire (right) Figure 2. Microstructure in the longitudinal section: hot rolled bar (left) and prestressing steel wire (right). In both micrographs the vertical side is associated with the wire axis or cold drawing direction 3.2. Initiation of Fatigue Cracks from Surface Defects The surface quality of both commercial products, hot rolled bar and prestressing steel wire, is very different (Fig. 3). While in the first (which comes from a hot rolling process) some material losses and irregularities can be observed on its uneven surface, in the prestressing steel wire (heavily drawn) longitudinal grooves are observed (typical surface features in drawn steel wires [1]), the roughness being higher in the hot rolled bar than in the prestressing steel wire. The defects (pre-existent in the hot rolled bar) change the geometry with the drawing process, their depth decreasing up to the total disappearing in some cases [5]. In addition, the analyzed material (in the two forms as a hot rolled bar or a prestressing steel wire) has frequent inclusions (sulphides, oxides, silicates…), some of which can be found on the wire surface, provoking voids on the material (Fig. 4). Results show that fatigue cracks in pearlitic steels begin at the wire’s surface starting from some of these small defects (Fig. 5). The defect size decreases with the drawing process, as the cross sectional of the wire does. In the hot rolled bar the fatigue initiators are mainly the surface defects with small aspect ratio (material losses at the peripheral zones) while in the prestressing steel wire such initiators are principally the voids created by, probably, the existence of particles near the wire surface (cf. Fig. 5). Depth of maximum surface defects is about 120 μm in the hot rolled bar and about 25 μm in the cold drawn wire. Initiation of fatigue cracks from surface defects is due to the fact that the latter act as stress concentrators.
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