Fatigue strength of out-of-plane gusset welded joints subject to springing and whipping superimposed wave loadings is examined by using Plate-Bending-Vibration (PBV) type fatigue testing machines developed in the previous reports [Osawa, N. et al. (2013) Proc. OMAE2013, Paper OMAE2013-11582, Osawa N. et al. (2013) Proc. PRADS2013, pp.550–556]. Springing vibration is superimposed by attaching an additional vibrator to the test specimen, and whipping vibration is superimposed by intermittent hammering. ‘Enlargement counting’ method, in which the stress history is approximated by a waveform with the low frequency component’s period and the enlarged total amplitude, is proposed. Fatigue damages and equivalent stress ranges are calculated by enlargement and rainflow cycle counting methods. It is found that the fatigue life under high frequency superimposed loads can be predicted with acceptable accuracy by the modified Miner rule when enlargement or rainflow stress counting is performed and the S-N curve is modified so that it fits the equivalent stress range’s Ps = 50% curve. Based on test results, a simplified assessment method for high frequency effect on fatigue strength of ship’s welded joints is proposed. The validity of the proposed assessment method should be further examined by carrying out fatigue tests with realistic stress histories which emulate intermittent occurrence of springing and whipping in ship structure.

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