Issue 37

C. Riess et alii, Frattura ed Integrità Strutturale, 37 (2016) 52-59; DOI: 10.3221/IGF-ESIS37.08 52 Focussed on Multiaxial Fatigue and Fracture The non-proportionality of local stress paths in engineering applications C. Riess, M. Obermayr ZF Friedrichshafen AG, DTGS1, 88038 Friedrichshafen, Germany christian.riess@zf.com, http://orcid.org/0000-0002-8784-5297 martin.obermayr@zf.com, http://orcid.org/0000-0002-7672-8685 M. Vormwald Technische Universität Darmstadt, Materials Mechanics Group, Franziska-Braun-Str. 3, D-64287 Darmstadt, Germany vormwald@wm.tu-darmstadt.de , http://orcid.org/0000-0002-4277-785X A BSTRACT . A scalar measure, which describes the non-proportionality of local stress paths in engineering applications, is introduced. For this purpose the moment of inertia approach by Meggiolaro is modified in a way that the stress time history is evaluated in a tresca-stress-space. This modification makes the non-proportionality factor invariant with respect to the coordinate system. An optimization procedure is implemented to derive a test set-up for new component tests with 2 load channels. The aim of the planned tests is to get a high non-proportionality at the potential crack initiation site. It is not possible to obtain a high non-proportionality factor at the failure location without selective weakening of the component (housing of a rear axle steering). Therefore specific areas of the structure are cut out and the optimization procedure is repeated. As a result of the optimization a test set-up with high local non-proportionality at the potential crack initiation site is achieved for the weakened structure. Another set-up with slightly less non-proportionality but with a very localized damage is derived. This set-up is preferred, because of the robustness in the physical test. K EYWORDS . Non-proportional fatigue; Multiaxial testing. I NTRODUCTION any components in engineering applications are subjected to multiple and uncorrelated loads during service-life. Thus multiaxial stress states with rotating principal directions may occur. It is therefore useful to introduce a scalar measure (e.g. in the range of 0 to 1), which describes the non-proportionality of a local stress path. There are many approaches to characterize the non-proportionality in literature. One possibility is to directly consider the non- proportionality and the additional non-proportional hardening in an incremental plasticity model (see e.g. [1]). In this case, the non-proportionality is evaluated at every time step and transient effects may be taken into account. A second group of approaches determines the non-proportionality of a single cycle. An example for this group is the rotation factor according to Kanazawa [2]. A last group of non-proportionality factors (NPF) was developed to efficiently describe the non-proportionality of a whole stress time history [3-6]. M