In certain biaxial fatigue problems, the stress components can be represented as a superposition of mean components, and proportionally applied alternating components. Such representation is the first step in applying some of the more popular methods for long-life fatigue analysis. It is commonly assumed, perhaps unwittingly, that the relative orientation of the mean and alternating principal stress axes does not influence fatigue behavior. It is explained how this assumption is inherent in several popular methods. It is pointed out that the validity of the assumption cannot be demonstrated by existing data.

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