13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- process, unmatched mechanical properties and discontinuous microstructures between base metal and weld area were induced at the welded joint, which weakened the fatigue strength of welded structure significantly. It was confirmed from the observation of S-N curves of base metal and HAZ that failure continued to take place right up to gigacycle range (109 cycles) as mentioned in [3,8,16]. But for FZ specimen, a horizontal platform could be observed beyond 5.6×106 cycles. Because solder fill process caused welding defects such as micro cracks, pores and inclusions at fusing zone, fatigue crack propagated directly from thus defects without crack initiate life. If the loading stress is over the threshold of propagation, existing crack extent until final fracture rapidly in low cycle fatigue range In other case, the specimen would not fatigue to failure because of the insufficient for a new crack initiation at lower stress level. So the fatigue strength limit of FZ specimen existed, because no failure occurred below the stress of 150MPa. 3.2 Fractography analysis Fig.4 SEM fractographs of tested specimen; (a, b) Base metal specimen; (c, d) HAZ specimen; (e, f) FZ specimen (a) (b) (c) (d) (e) (f) Striations Surface crack initiation Surface pit Pore Initation site Surface crack Manufacturing defect
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