13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- Figure 3. Surface of the material in the two forms: hot rolled bar (left) and prestressing steel wire (right). In both pictures the vertical side is aligned with the wire axis or cold drawing direction Figure. 4. Inclusions in hot rolled bar: inside the wire (left) and on the wire surface (right). In both pictures the vertical side is aligned with the wire axis or cold drawing direction Figure. 5. Initiation of fatigue crack growth from a surface defect: hot rolled bar (left) and prestressing steel wire (right). Both images represent the transverse fatigue fracture surface (circular section perpendicular to the wire axis or cold drawing direction) and crack growth direction takes place from left to right in the fractographs With regard to fatigue crack initiation from the surface defects in the material supplied as a hot rolled bar and a prestressing steel wire, Fig. 6 shows the same information as Fig. 5 but includes arrows to identify the fatigue crack growth from the aforesaid defect. It is seen that the defect appearance is different in the two material forms. In the hot rolled bar the surface defect looks like an extremely shallow flaw with an approximate aspect ratio of 0.2 whereas the prestressing steel wire such a defect looks like a relatively small circumferential flaw with an approximate aspect ratio equal to the unity. In the matter of the initiation of fatigue crack growth, it is important to say
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