There is a hot spot stress (H.S.S.) method where a unified S-N curve is used instead of many curves corresponding to joint category for the estimation of fatigue strength of welded structures. In this paper, the hot spot stress method, which is more reasonable and simpler than before, is being discussed for the welded plate structures, ship hull structures, the bogie frame of the rolling stock, etc. First, stress analysis of simple fillet welded models were carried out, by means of boundary element method (BEM) to investigate a simpler calculating method of hot spot stress. As a result, it was found that stress concentration due to weld bead is nearly reduced to zero at the point about 30% thickness of main plate off the weld toe, so long as the plate thickness (t) is within the range used for welded plate structures (3mm<__=t<__=18mm). The authors propose, herein, a simplified H.S.S. calculating method by defining H.S.S. as a stress at 0.3t from the weld toe (0.3t method) for, mainly, ship hull structures. In order to verify the validity of this calculating method of hot spot stress (0.3t method), the fatigue tests have been carried out for two types of the fillet welded joints, i.e., cruciform-tension-type and T-bending-type fillet welded joints. Each type has global stress concentration or no global stress concentration due to the structural discontinuity. But each type has the same local stress concentration due to the weld bead shape. As a result, all S-N data based on hot spot stress range by the 0.3t method gathered within a narrow band independent of structural stress concentration factors. And it has been confirmed that the 0.3t method is applicable for unified fatigue strength estimation of fillet welded plate structures.
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