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Extension of the fatigue strength verification of the FKM guideline Computational strength verification for application to multi-channel non-proportional loads

2024, 109 pages

Project No. 618
Final report
Abstract:
In the course of the research project, extensions to the algorithm of the FKM Guideline "Computational Strength Verification" (Edition 7 of 2020) have been developed. The focus is on the fatigue strength verification.
Two shortcomings of the existing algorithm are identified: Firstly, the calculation results are not coordinate invariant, i.e. they depend on the choice of coordinate system for the evaluation of local stresses. Secondly, the effect of multi-axial non-proportional stresses can only be taken into account by conservative estimates, as the phase relationship between the individual stresses is not taken into account. In order to identify potential for improvement, the extent of coordinate variance and the conservatism of the assessment of non-proportional stresses are analyzed as part of the project.
Based on this, two proposals for an improved algorithm are developed. For this purpose, equivalent stresses are introduced with which the temporal course of the stress tensor is converted into the course of a scalar reference variable. These reference variables are a modified form of the scaled normal stress on the one hand and a modified form of the Findley parameter on the other.
In order to be able to continue to take into account the differences in the evaluation of normal and shear stress (e.g. different Wöhler line inclinations) contained in the current FKM guideline, the strength parameters are adjusted depending on the stress multiaxiality. To take non-proportional stresses into account, the critical section plane method is introduced for both equivalent stresses. In addition, a proposal is developed on how to calculate support numbers for non-proportional stresses.
Finally, the accuracy of the new algorithms is verified. The accuracy of the previous algorithm can be reproduced almost unchanged for proportional stresses with one of the newly developed procedures; however, the coordinate invariance is established.
For non-proportional stresses, the new procedure results in a similarly good accuracy as for proportional stresses. The latter represents a significant improvement on the current edition of the guideline.
The project thus achieves a significant improvement in the applicability of the FKM guideline. The new algorithms outperform the existing ones to a high degree in terms of robustness and accuracy. Considerable additional potential has been made available for component design, particularly in the verification of multi-axial non-proportional loads.