Issue 33

M. Cova et alii, Frattura ed Integrità Strutturale, 33 (2015) 390-396; DOI: 10.3221/IGF-ESIS.33.43 390 Focussed on multiaxial fatigue Fast assessment of the critical principal stress direction for multiple separated multiaxial loadings M. Cova, P. Livieri, R. Tovo University of Ferrara, Italy roberto.tovo@unife.it A BSTRACT . The critical plane calculation for multiaxial damage assessment is often a demanding task, particularly for large FEM models of real components. Anyway, in actual engineering requests, sometime, it is possible to take advantage of the specific properties of the investigated case. This paper deals with the problem of a mechanical component loaded by multiple, but “time-separated”, multi- axial external loads. The specific material damage is dependent from the max principal stress variation with a significant mean stress sensitivity too. A specifically fitted procedure was developed for a fast computation, at each node of a large FEM model, of the direction undergoing the maximum fatigue damage; the procedure is defined according to an effective stress definition based on the max principal stress amplitude and mean value. The procedure is presented in a general form, applicable to the similar cases. K EYWORDS . Multiaxial Fatigue; Principal Stress directions; Critical Plane. I NTRODUCTION ulti-axial fatigue theories usually address problems characterized by different degrees of complexity. The simplest problems are typically the proportional loading conditions. In these situations, the principal stress directions are constant; they can be calculated at any time value and the variations (mean values, amplitudes or ranges) can be easily computed since, along each principal direction, principal stress is a known time-variable scalar. In this condition, many fatigue criteria based on principal stress value can be used; the most important is probably the Sines criterion [1] which is actually based on octahedral shear stress and such a value requires the calculation of the principal stress direction. In a less specific way, under non-proportional loading, principal directions change in time; this variation is one of the most tricky problem in the investigation of the multi-axial fatigue damage, since there is not an intrinsic or natural frame of reference where defining the fundamental stress quantities. Several research efforts, in multiaxial fatigue, deal with the definition and computation of critical plane direction or main principal frame of reference; for instance, just few examples can be found in [2-5]. Specifically, the main principal stress directions can be used to evaluate the consequent shear based critical plane [2]. Otherwise, the principal stress can be directly used for fatigue damage assessment [6], in this case the principal stress amplitudes and mean values are used in the criterion for equivalent stress amplitude definition. Moreover, the variability of the principal directions, which is null under proportional loading, can be caused by several type of non-proportionalities. In the most complex cases, several independent time-variable component are superimposed. In these cases, the numerical iterative procedures are necessary. M