Based on the rain-flow counting technique, a frequency domain method is developed for calculating the fatigue damage caused by the combined drag and inertial loads. Firstly, by observation of the combined signal simulated, the combined load can be considered as the oscillation of the high frequency inertial force around the low frequency drag force with the random amplitudes, which makes it possible to identify the rain-flow large and small cycles. The cyclic range of rain-flow small cycles are determined by considering the reduced effect of the low frequency drag forces on the cyclic range of high frequency inertial forces. The cyclic ranges of rain-flow large cycles are determined by means of Turkstra’s rule of load combination. The numerical analysis show that the damages estimated by the developed method are very close to the rain-flow damages.

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