The frequency domain estimate of fatigue damage of combined load effects based on the rain-flow counting

Based on the rain-flow counting technique, a frequency domain method is developed for calculating the fatigue damages caused by the combined loads, which can be linear or nonlinear and Gaussian or non-Gaussian loads. Firstly, the new method is developed for the fatigue damage estimates of the combination of high and low frequency Gaussian loads, by which the ranges of the small rain-flow cycles are obtained as high frequency cycles with considering the reduction effect of low frequency loads on them; the ranges of the large rain-flow cycles are determined by means of Turkstra's rule of load combination. Secondly, the method is extended to calculate the fatigue damages caused by the nonlinear Morison load which is the combination of linear inertial and nonlinear drag forces. The method is benchmarked against the rain-flow damage estimates and compared with the existing ones. The numerical analyses show that the damages estimated by the new method are close to the rain-flow damages.