Principles of Variable Amplitude Fatigue Design and Testing

The proper consideration of variable amplitude loading by utilizing service spectra and appropriate Gassner-lines is essential for the design of light-weight components and structures by allowing loads in significant excess of the Woehler-line (S-N curve). This permits higher stresses than under constant amplitude loading and renders reduced component dimensions. Reliable reconstitution and simulation methods for service load-time histories require not only the rainflow matrices, but also information about the order of the cycles described by Markovian matrices, the power spectral density and, for multiaxial applications, the cross-correlations between the particular load directions as well as the phase relations. A major problem in numerical fatigue life assessment is still the fatigue life calculations for spectrum loading, because of the scattering of the real damage sum D over a wide range, which is not entirely understood. These findings demonstrate the need for experimental spectrum tests, which are indispensable for ensuring the safety of parts. With regard to safety and liability requirements, the failure probability resulting from the probability of occurrence of the spectrum, from the scattering of the fatigue strength and from the failure criterion (technical crack or propagation), must be taken into account.