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This paper aims at presenting a design methodology for in-servicecomplex variable fatigue loading on metallic engineering components. Whereas most ofthe approaches in the literature deal with variable loading by using counting techniques(for example, rainflow counting), the proposal at stake follows directly the load pathover time history, and thus avoids any loading processing (i.e. transformation and lossof information). In order to overcome a purely phenomenological description, a twoscaledamage model (macro – meso) integrates a multiaxial fatigue criterion and isformulated in the framework of thermodynamics of irreversible processes allowing tocapture as closely as possible degradation mechanisms at mesoscopic scale as well asphase shift effect and non linear fatigue damage accumulation. The incrementalformulation of the proposed model is an asset to deal with variable amplitude loadingsin future works.