A multiaxial incremental fatigue damage formulation using nested damage surfaces

Multiaxial fatigue damage calculations under non-proportional variable amplitude loadings still\udremains a quite challenging task in practical applications, in part because most fatigue models require cycle\udidentification and counting to single out individual load events before quantifying the damage induced by them.\udMoreover, to account for the non-proportionality of the load path of each event, semi-empirical methods are\udrequired to calculate path-equivalent ranges, e.g. using a convex enclosure or the MOI (Moment Of Inertia)\udmethod. In this work, a novel Incremental Fatigue Damage methodology is introduced to continuously account\udfor the accumulation of multiaxial fatigue damage under service loads, without requiring rainflow counters or\udpath-equivalent range estimators. The proposed approach is not based on questionable Continuum Damage\udMechanics concepts or on the integration of elastoplastic work. Instead, fatigue damage itself is continuously\udintegrated, based on damage parameters adopted by traditional fatigue models well tested in engineering\udpractice. A framework of nested damage surfaces is introduced, allowing the calculation of fatigue damage even\udfor general 6D multiaxial load histories. The proposed approach is validated by non-proportional tensiontorsion\udexperiments on tubular 316L stainless steel specimens