In vibration fatigue three types of loads are typical; random, harmonic and impact. When dealing with real loads, any combination of these three types is possible. While the analysis of fatigue life in the cases of harmonic and random loads is well researched, the impact loads, or the combination of impact and any of the other two load types, were not yet investigated in detail in the frequency domain.

The main focus of this research is a theoretical study of the fatigue damage analysis in time- and frequency-domain of a single-degree-of-freedom dynamic system under well-separated half-sine impulse excitation. Based on this theoretical study a relation between the time- and frequency-domain damage is introduced, which is based on the dynamic properties of a flexible structure (natural frequency, damping), material parameters, and the loading characteristics. It was found that the frequency domain damage estimation differs significantly from the time domain damage; however, based on the introduced theoretical relation between the time- and frequency-domain damage estimation a frequency-domain correction can be applied. Besides theoretical, also experimental validation is presented.

This research provides the fundamental theoretical background for further research of impact loads, for instance for cases where the impacts are not well separated, or combination of different load types.

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