The fatigue strength of friction welded joint with flash decreased remarkably with the increase in upset pressure. This is caused by stress concentration with flash, as described in the previous report. In this study, the effect of flash onprevention of decrease in fatigue strength of friction welded joints was examined by means of rotating bending fatigue test using as welded joint specimens of S15C, S35C, S45C and S55C carbon steels under various friction welding conditions.The following experimental results were obtained:(1) The reduction ratio of fatigue strength was lowered with the decrease in upset pressure. It was small in the case of high carbon steel under the same pressure.(2) The decrease in fatigue strength of S45C and S55C welded joints with flash was not observed when upset pressure was 64 MPa.(3) Hardness of toe of flash increased remarkably by a friction weld thermal cycle in the case of high carbon steel.(4) The stress concentration factor of welded joint with flash became large with the increase of upset pressure. It was high in the case of high carbon steel under the same pressure.(5) The effect of hardening on the fatigue strength was larger than that of stress concentration in the case of high carbon steel.
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