Issue 37

M. A. Meggiolaro et alii, Frattura ed Integrità Strutturale, 37 (2016) 138-145; DOI: 10.3221/IGF-ESIS.37.19 138 Focussed on Multiaxial Fatigue and Fracture A multiaxial incremental fatigue damage formulation using nested damage surfaces Marco Antonio Meggiolaro, Jaime Tupiassú Pinho de Castro Pontifical Catholic University of Rio de Janeiro, PUC-Rio, R. Marquês de São Vicente 225, Rio de Janeiro, 22451-900, Brazil meggi@puc-rio.br jtcastro@puc-rio.br Hao Wu School of Aerospace Engineering and Applied Mechanics Tongji University, Siping Road 1239, 200092, Shanghai, P.R.China wuhao@tongji.edu.cn A BSTRACT . Multiaxial fatigue damage calculations under non-proportional variable amplitude loadings still remains a quite challenging task in practical applications, in part because most fatigue models require cycle identification and counting to single out individual load events before quantifying the damage induced by them. Moreover, to account for the non-proportionality of the load path of each event, semi-empirical methods are required to calculate path-equivalent ranges, e.g. using a convex enclosure or the MOI (Moment Of Inertia) method. In this work, a novel Incremental Fatigue Damage methodology is introduced to continuously account for the accumulation of multiaxial fatigue damage under service loads, without requiring rainflow counters or path-equivalent range estimators. The proposed approach is not based on questionable Continuum Damage Mechanics concepts or on the integration of elastoplastic work. Instead, fatigue damage itself is continuously integrated, based on damage parameters adopted by traditional fatigue models well tested in engineering practice. A framework of nested damage surfaces is introduced, allowing the calculation of fatigue damage even for general 6D multiaxial load histories. The proposed approach is validated by non-proportional tension- torsion experiments on tubular 316L stainless steel specimens. K EYWORDS . Multiaxial fatigue; Variable amplitude loads; Non-proportional multiaxial loads; Nested fatigue damage surfaces; Incremental damage calculation. I NTRODUCTION ost fatigue crack initiation models need to properly identify load events before computing the damage induced by them. Hence their fatigue damage calculation routines need to include cycle counting algorithms like the well-known rainflow methodology for uniaxial loads. Cycle counting is necessary because traditional fatigue models are discrete in nature, since they only can accumulate damage after a load event (e.g. a half-cycle) is properly identified, detected e.g. from a load reversal or from a hysteresis loop that closes. However, the detection and counting of loading events can be a quite challenging task under multiaxial non-proportional (NP) histories. The existing multiaxial rainflow algorithms [1] are not trivial to apply. In fact, they are not even robust, since they can output very different half- M