Equivalent stress concepts for transforming of variable amplitude into constant amplitude loading and consequences for design and durability approval

This paper compares two methods for transforming a stress or load spectrum into a damage equivalent constant amplitude loading, i.e. the modified equivalent stress (MES) and the required fatigue strength (RFS) concepts. Because of the common basis, i.e. the modified Palmgren-Miner hypothesis, the resulting MES- and RFS-amplitudes are related to each other, but not equal. The MES-method results in a damage equivalent stress, which depends on the selected position of the Woehler-line. However, the RFS-method delivers a damage equivalent fictitious Woehler-line indicating the lowest necessary strength quality. This line also allows an experimental durability proof for any amplitude-cycle combination along it.