Issue 33

M.A. Meggiolaro et alii, Frattura ed Integrità Strutturale, 33 (2015) 368-375; DOI: 10.3221/IGF-ESIS.33.40 368 Focussed on multiaxial fatigue Shortcuts in multiple dimensions: the multiaxial racetrack filter M.A. Meggiolaro, J.T.P. Castro Pontifical Catholic University of Rio de Janeiro meggi@puc-rio.br , jtcastro@puc-rio.br H. Wu Tongji University in Shanghai wuhao@tongji.edu.cn A BSTRACT . Filtering techniques have been proposed for multiaxial load histories, usually aiming to filter out non-reversals, i.e. sampling points that do not constitute a reversal in any of its stress or strain components. However, the path between two reversals is needed to evaluate the equivalent stress or strain associated with each event. Filtering out too many points in such path would almost certainly result in lower equivalent stresses or strains than expected. To avoid such issues, it is important to consider how a measured multiaxial loading path deviates from its course using some metric, such as the von Mises stress or strain. In this work, a multiaxial version of the racetrack filter is proposed, which is able to perform efficient filtering even for 6D non- proportional histories. In the Multiaxial Racetrack algorithm, the stress or strain history is represented in a 6D space, only requiring from the user a desired scalar filtering amplitude r. For uniaxial histories, the proposed algorithm exactly reproduces the classic racetrack filter. The efficiency of the proposed Multiaxial Racetrack filter is qualitatively verified from a tension-torsion history example, showing the reduction in the number of data points for larger filter amplitudes r. The procedure can efficiently filter out non-damaging events but preserving the overall multiaxial path shape and multiaxial reversion points, which usually do not coincide with the reversion points of individual stress or strain components. K EYWORDS . Multiaxial fatigue; Racetrack filter; Non-proportional loading; Variable amplitude loading. I NTRODUCTION he racetrack filter, originally proposed by Fuchs et al. in 1973 [1] for uniaxial histories, aims to eliminate small amplitude load events that, although usually plenty, do not induce fatigue damage from a variable amplitude loading history. So, the resulting condensed histories can much accelerate both experiments and computations, focusing only on the few events that cause most or all of the damage. Fig. 1 illustrates the uniaxial racetrack filter concept. The original history of Fig. 1(a) is condensed into the history in Fig. 1(d), eliminating amplitudes smaller than a user- specified value r . The idea of the filter is to draw a “racetrack” of width 2r , bounded by upper and lower “fences” that have the same profile as the original history, see Fig. 1(b). If a “driver” racing in this racetrack needs to change its direction from upward to downward (or vice versa), then a reversal point is identified, as seen in Fig. 1(c), where the racer needed to change twice its direction, near points B and E. In this example, points C and D are filtered out, because the driver didn’t have to change direction to avoid the fences associated with them. Clearly, the track width 2r determines the number of counted reversals: wider tracks will filter out most of the original loading history, while narrow tracks will T