13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- For high strength steel, it is known that the crack initiation and early propagation of interior-initiation fracture surface in VHCF range exhibit a pattern of “fish-eye” [17], a relatively rough region with fine granular morphology often presents surround the inclusion inside fish-eye. However, the fracture morphology of Q345 specimens in this test was different from high strength steel. Fig.5 illustrates the fatigue crack initiation modes based on SEM characterization. Due to the better plastic properties, the elongation of Q345 was much better than high strength steel. In addition, the stress at the surface was higher than internal, owing to the stress concentration effect caused by arc in the middle of the specimen. Local plastic deformation occurred more easily at the surface and fatigue cracks prone to start from surface of specimen for Q345 BM, which can be certified by the fatigue striations observed in Fig.4a and Fig.4b, around crack initiation site at the surface. The fatigue crack initiations of HAZ specimens were located at the surface (Fig.4c and d), which can be correlated to the local stress concentration at the retained surface defects caused by machining. For FZ specimens, cracks initiated from the welded defects such as pores and inclusions, as shown in Fig4 (e) and (f). Many researchers have reached an agreement that fatigue crack initiation process of base metal occupied over 99% of total fatigue life in very high cycle fatigue, but the fatigue cracks of FZ specimen begin to expand directly from existing welding defect, which reduced the crack initiation life. Therefore the fatigue strength of FZ specimen was much lower than that of base metal specimen and the crack propagation life in VHCF range will be discussed in the following sections. 3.3 The varying surface temperature of BM specimen Fig.5 Temperature profiles of base metal specimen during testing. Fig.6 The rapidly increasing process of surface temperature for BM specimen
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