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Abstract
Industrial structures are often subjected to multiaxial fatigue loadings. If the multiple stress signals are not synced the loading is said to be non-proportional. Most of the multiaxial fatigue criteria give highly inaccurate lifetime predictions when used in the case of such loadings. The scalar equivalent stress defined by the criteria does not take into account the non-proportional nature of the multiaxial loading and leads to non-conservative predictions. Moreover a multiaxial fatigue criterion can only be applied on a stress cycle which has no clear definition when multiple unsynced signals are to be considered. This study addresses these issues by proposing a correction of an invariant based multiaxial fatigue criterion through the definition of a non-proportional degree indicator. A definition of multiaxial cycle is also given based on the Wang-Brown method. Finally a complete chain of invariant based lifetime prediction for non-proportional multiaxial fatigue is validated.
Highlights
A large number of multiaxial fatigue criteria are proposed in literature to determine the high cycle fatigue lifetime of a structure
Most of these criteria are calculated on specific stress cycles extracted from the loading signal using a cycle breakdown algorithm
In this study a new invariant based NP indicator is defined and integrated in a full high cycle fatigue lifetime prediction chain based on Sines criterion
Summary
A large number of multiaxial fatigue criteria are proposed in literature to determine the high cycle fatigue lifetime of a structure. Most of these criteria are calculated on specific stress cycles extracted from the loading signal using a cycle breakdown algorithm. In the case of proportional multiaxial signal, meaning it can be represented by a time dependent scalar, cycles can be extracted using the Rainflow counting [1]. Once a NP cycle is defined, most multiaxial fatigue criteria do not give accurate predictions Citeria such as Sines [3] make use of the second invariant of the alternate deviatoric stress tensor J2a which can be defined as the radius of the Minimum Enclosing Ball (MEB)
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