The method is developed for calculating the fatigue damage caused by the combined high and low frequency loads, which are both narrowband Gaussian processes. For the combined high and low frequency process taken as a special case of the wide band Gaussian process, the rain-flow counting identifies the typical small and larges cycles. Some kinds of methods were developed for calculating the probabilistic distributions of stress ranges of small and large cycles, which may be very simple but conservative or accurate but complicated. In this paper, based on the rain-flow counting, a simple and accurate method is derived of calculating the damage caused by small and large rain-flow cycles. The method developed is benchmarked against the rain-flow damage estimate and compared with the existing ones. The numerical analyses show that the estimated damage by the method developed is very close to the rain-flow damage estimate.

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