Abstract
Earthquakes cause damages by fatigue at welded joints of steel structures. Structures which were not seriously damaged would be continuously used even after earthquakes. The fatigue strength of those structures, however, might have been decreased due to the cyclic straining during earthquake. In order to clarify the effect of large cyclic straining on the fatigue strength of the welded joint, low cycle fatigue crack propagation test under large straining as well as high cycle fatigue tests after large cyclic straining were performed on welded joint made of 780 MPa class high strength steel. Experimental results showed that the high strength steel welded joint exhibited an excellent fatigue limit characteristic in the case of reversal stressing. However, the fatigue limit is quite sensitive to mean strain. The application of positive mean strain substantially decreased the fatigue limit. Moreover, the cyclic application of large strain caused fatigue crack, which caused the substantial decrease of fatigue strength. The amount of reduction in fatigue limit was larger in the case of high strength steel compared to that in the case of low strength steel.
