Issue 28

M.Malnati, Frattura ed Integrità Strutturale, 28 (2014) 12-18; DOI: 10.3221/IGF-ESIS.28.02 12 A method for calculation of finite fatigue life under multiaxial loading inhigh-cycledomain M.Malnati RUAGAerospace Services GmbH - RUAGAviation, Claude-Dornier-Str., 82231Wessling, Germany mario.malnati@ruag.com , mario.malnati@yahoo.fr A BSTRACT . Amethod for fatigue life assessment in high-cycle domain under multiaxial loading is presented in this paper. This approach allows fatigue assessment under any kind of load history, without limitations. The methodology lies on the construction - at amacroscopic level - of an “indicator” in the form of a set of cycles, representing plasticity that can arise atmesoscopic level throughout fatigue process. During the advancement of the loading history new cycles are created and a continuous evaluationof the damage ismade. K EYWORDS . High-cycle multiaxial fatigue; Metal fatigue; Multiaxial rainflow counting; Crack initiation; Non- proportional loading; Continuous damage. I NTRODUCTION umerous methods for multiaxial fatigue analysis exist in literature (see e.g. [1]) and they are widely applied in different industrial contexts. Nevertheless many of them are limited to specific loading conditions - typically proportional loading: see [2, 3] for a review - or intended only for an evaluation of the unlimited fatigue life (see [1, 2, 4]). Themotivation of themethod presented in this paper comes from the need felt during years by the author in the fatigue assessment of industrial structures, to conceive a simple and easily applicable tool capable to estimate finite life crack initiation under any kind of loading, in the domain of High-Cycle Fatigue (HCF). Every type of variable amplitude multiaxial stress history canbe treatedwithout presenting incorrect filtering of significant cycling events. G ENERALPRESENTATIONOFTHEMETHOD t is a widely recognized and well accepted phenomenon [5] that the fatigue damage in theHCF domain is related to the amount of plasticity created at least in some grains under amacroscopically elastic stress history. The ref. [5] gives for instance a synthetic understanding: “… the common principal mechanism responsible of the crack initiation is the plastic strains and the damage developed in the grains due to irreversible dislocations motion. The essential difference between HCF and LCF regimes is that the scale of the plastic localization in amaterial volume is mesoscopic and respectively macroscopic ”. Some criteria for the calculation of the fatigue life are actually based on the evaluation of the plasticity amount. They can be either direct - as described for instance in [6] and [7], where a computational crystal plasticitymodel is used at a refined microstructural scale - or indirect, like for example the work of Jabbado and Maitournam [8] where a macro-meso relationship is used. N I