13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- that, apparently, there is no single initiation point over the defect boundary, but instead of it a line (i.e. a set of points can be viewed) defining the emerging fatigue crack front at the early stages of cracking and diverging fatigue propagation lines emanating from the defect: lack of material previously lost during the manufacturing process in the case of the hot rolled bar and voids created by a previous inclusion in the prestressing steel wire. Figure 6. Initiation of fatigue crack growth from a surface defect: hot rolled bar (left) and prestressing steel wire (right), including arrows to identify the fatigue crack growth from defect In high strength eutectoid steels fatigue crack growth can start from defects caused by the drawing process itself. Fig. 7 shows a surface defect possibly generated by a hard particle in the wire surface during the drawing process, because a longitudinal track, aligned in the drawing direction, appears in the wire surface. A sort of plastic deformation can be observed surrounding the hole (Fig. 7; left) with a very regular geometry. Fig. 7 also includes a magnification (right) of the surface defect (and consequent fatigue crack growth) viewed from different angle, showing the longitudinal scratching marks, an evidences of the pre-damage created by an inclusion (hard particle) during the plastic straining as a consequence of the manufacturing process by cold drawn. Figure 7. Surface defect caused by cold drawing producing transverse cracking in the prestressing steel wire (left) and magnification of the surface defect (and consequent fatigue crack growth) viewed from different angle (right) Fatigue cracks created from defects exhibit a fractographic appearance consisting of ductile microtearing events (Fig. 8), which can be classified as tearing topography surface or TTS [14]. Such microtearings are less rough in the initiation period (Fig. 8) than in the propagation phase (Fig. 9) and exhibit a spacing remarkably lower in the prestressing steel wire than in the hot rolled bar, so that their size decreases with cold drawing as the steel microstructure does, and a sort of materials science relationship appears between microstructural unit size and fatigue microfracture event. The aforesaid TTS microfracture mode has been associated with hydrogen embrittlement in
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