Issue 33

V. Anes et alii, Frattura ed Integrità Strutturale, 33 (2015) 309-318; DOI: 10.3221/IGF-ESIS.33.35 309 Focussed on multiaxial fatigue Random accumulated damage evaluation under multiaxial fatigue loading conditions V. Anes, L. Reis, M. de Freitas IDMEC, Instituto Superior Técnico, University of Lisbon, Av. Rovisco Pais, 1049-001 Lisbon, Portugal. E-mail: luis.g.reis@tecnico.ulisboa.pt A BSTRACT . Multiaxial fatigue is a very important physical phenomenon to take into account in several mechanical components; its study is of utmost importance to avoid unexpected failure of equipment, vehicles or structures. Among several fatigue characterization tools, a correct definition of a damage parameter and a load cycle counting method under multiaxial loading conditions show to be crucial to estimate multiaxial fatigue life. In this paper, the SSF equivalent stress and the virtual cycle counting method are presented and discussed, regarding their physical foundations and their capability to characterize multiaxial fatigue damage under complex loading blocks. Moreover, it is presented their applicability to evaluate random fatigue damage. K EYWORDS . Fatigue; Multiaxial random loading; Cycle counting method; Fatigue life; Damage accumulation. I NTRODUCTION sually, structures and mechanical components are subjected to random fatigue loadings. Car suspensions, wind towers for energy harvest or offshore structures are three examples, among others, that are subjected to this type of loading during service. Random loadings are quite different from the ones usually used in lab to evaluate fatigue strength, because they have unknown time histories. Therefore, damage assessment of these loadings is quite difficult, because exists an aleatory behaviour that is difficult to simulate. Nevertheless, in service conditions nothing is totally random; for instance, when a car suspension travels in the same path every day, the random forces of the car suspension will have similar patterns each day, but they do not have the same pattern. Thus, typical loading spectra are used in design stages of components and structures for reliability, where it is required reliable fatigue criteria. Multiaxial random fatigue is the ultimate stage in fatigue life assessment; it takes into account several multiaxial fields that usually/traditionally are studied separately. In this stage, it is needed to cover a broad number of concepts related to multi- axial fatigue. In fact, it is not possible to deal with random accumulated damage without considering all of them. Fig. 1 shows the fatigue pyramid concept for random fatigue characterization. In this pyramid, each level depends of all levels below and each level deals with a wide number of material science concepts. This is a very complex issue that has been handled in a segmented manner, i.e. each level of the fatigue pyramid has been studied and analysed without an evident interconnection between them. In literature, it can be gathered information related to each subject inherent to each fatigue level, but they are usually studied and presented separately. Therefore, issues like damage accumulation, damage parameters, cyclic properties, multi-axial cycle counting among others, are concepts that capture partially fatigue phenomena when considered separately, however they have not been U