Lightweight-structural durability design by consideration of variable amplitude loading

The technical importance of variable amplitude (spectrum) loading is not only given by obtaining much longer fatigue lives compared with those determined under constant amplitude loading with the maximum value of the spectrum. It also enables the lightweight potential resulting from proper consideration of the exceeding of the Woehler-lines (constant amplitude loading) by the Gassner-lines (variable amplitude loading) for a required duration. The degree of exceeding at the required service duration allows higher design stresses than under constant amplitude loading and is decisive in determining to what extent cross sections and hence weights can be reduced. However, the experimental determination of Gassner-lines is time and cost consuming, while their estimation by cumulative damage, i.e. fatigue life, calculations, is subject to underlying large uncertainties. Therefore, especially for safety–critical parts, experimental verifications and service inspections are indispensable.