The theory of the S-N fatigue damage envelope: Generalization of linear, double-linear, and non-linear fatigue damage models

A new theory for macroscopic fatigue damage estimation based on the S-N curve is presented. The proposed concept assumes that the area bounded by the S and N axes, and the S-N curve, can provide a characteristic damage map for each material. Finite element analysis is used to derive colored damage zones and non-linear isodamage lines within the S-N fatigue damage envelope. Implementation of this theory on two arbitrarily selected alloys under stepwise variable stresses is provided, and the successful prediction of the remaining life is discussed. The proposed S-N fatigue damage envelope theory is a generalization of most of the existing fatigue damage models.