Abstractis article compares the results of a high cycle fatigue analysis obtained through the application of the two methods dedicated to analysis of response of the structure of the bi-modal type -the direct spectral method and the Fu-Cebon one. the compared parameter is the lifetime for an assumed material S-N curve and stress spectrum de ned in an article by Fu & Cebon.Keywords: fatigue analysis, stress cycle counting, spectral method, bi-modal process Streszczenie W artykule dokonano porównania rezultatów wysokocyklowych analiz zmęczeniowych wykonanych przy zastosowaniu dwu metod dedykowanych dla przypadków odpowiedzi struktury o charakterze dwumodalnym: metody spektralnej bezpośredniej i metody Fu-Cebona. Porównywaną wielkością był czas życia konstrukcji przy założonej postaci krzywej S-N materiału i zmienności naprężeń o charakterystyce wykorzystywanej w artykule Fu i Cebona.Słowa kluczowe: analiza zmęczeniowa, zliczanie cykli naprężeń, metoda spektralna, przebiegi dwumodalne

Vibrations of engineering structures are described usually by functions variable in time domain (displacements, stress). For real structures modes of vibrations existing in response of structure due to external excitation are determined by exciting frequency and the natural frequencies of the structure. In the paper, the general idea of method of stress cycle counting, based on the FFT analysis of the stress function response is discussed and compared with the rain-flow cycle counting method, for the case of bi-modal spectrum.

A new approach based on the direct spectral method for fatigue analysis of elements subjected to bimodal stress histories, including high compression effects, is proposed. A correction factor, taking into account the influence of the mean compressive stresses, is used in the proposed method. Equivalent amplitude is estimated, based on criteria proposed by Smith, Watson, and Tooper, and by Bergmann and Seeger. The method is presented with example of a thrust roller bearing. Two cases in which the rollers were subjected to constant force 206 N (where constant amplitude stresses occurred in the rollers) and cyclic force (where bimodal stresses with variable amplitudes occurred in the rollers) are studied. It is observed that multiaxial fatigue criteria (Crossland, Papadopoulos) do not include the influence of bimodal stresses and should not be used for such loading conditions. The proposed method includes both kinds of stress waveforms in the fatigue analysis and can be applied for the accurate identification of stress components and the determination of fatigue life. The damage rate calculated by the proposed approach for rollers subjected to a cyclic force (equivalent load equal to 151 N) was 0.86, which is in good agreement with the recommendations provided in the literature. The obtained accuracy of the proposed method is above 95%.