PSI - Issue 77

Miloslav Kepka et al. / Procedia Structural Integrity 77 (2026) 272–278

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3. Design load spectra Design spectra are created to simulate future loading conditions already at the design stage of a new structure or its component. These spectra are essential for ensuring the safety and reliability of the structure in cases where it is necessary to predict the fatigue life already at the design stage. Design spectra, sometimes also referred to as typical or standardised, are based not only on experience from the operation of various types of structures, but most often on the implementation and subsequent evaluation of massive measurement campaigns. The agreement of experts from the given field on the final result is essential. In various fields, for various machines, equipment or means of transport, various standardised load spectra have been proposed, some of them even have the status of a binding regulation, especially for the implementation of verification fatigue tests. Detailed information on standardised load spectra can be found in the professional literature. A comprehensive overview of load sequences and spectra accepted within Europe is provided by Berger et al. (2002), which includes references to primary sources. In general, it can be said that one of the parameters of the load spectra is their “aggressiveness”, which has an impact on the resulting fatigue life. The aggressiveness of the load spectra as one of the factors influencing the fatigue life has been intensively studied in the past, especially by the German scientific community, as presented, for example, by Heuler and Klätschke (2005). This involved experimentation mainly on material samples. The influence of the “shape” of the load spectrum is shown in Fig. 2.

Fig. 2. Influence of the shape of the load spectrum as one of the main factors affecting fatigue life, by Heuler and Klätschke (2005).

The potential shapes of the load spectra can be plotted on a graph, where the horizontal axis is the cumulative number of damaging cycles and the vertical axis is the ratio of a given load amplitude to the maximum amplitude caused by the load, see Fig. 3.

Fig. 3. Graphical representation of load spectra, by Heuler and Klätschke (2005).