P 1028 – Fatigue strength analysis of bonded steel joints under multiaxial loading conditions


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P 1028 – Fatigue strength analysis of bonded steel joints under multiaxial loading conditions

A reliable fatigue life analysis of bonded steeel joints under cyclic loads has a high priority in structural car body engineering. Currently there are no concepts which are recognised as generally valid and which are easy to use at the same time. Therefore, the solution of most engineering tasks with steel adhesive joints requires experimental effort. Aggravating this situation, the general case with loads acting in different spatial directions simultaneously is faced in practice. In the case that there is e. g. a phase shift within the load-time signals, the local stress state is no longer proportional to the applied loads. This so-called non-proportional loading can have a considerable effect on fatigue life and has therefore to be accounted for in the fatigue resistant design of bonded steel joints. Up until now, this general case of loading has not been considered with bonded steel parts.
Studies on specific adhesive joints subject to single channel loading have shown that a fatigue life assessment on the basis of the initial stress state and the S-N curve of the adhesive is possible if rules for damage accumulation and mean stress correction are known and as long as the loads are small and the bond line remains elastic. The advantage as well as disadvantage in applying the stress-life method to adhesive joints is the negligence of stress evolution. Thanks to this simplification, calculations take only little time, thereby enabling the consideration and evaluation of many load cases within whole-body simulations. The disadvantage is the associated deviation between theoretical and actual lifetime.
More accurate results can be expected if the progressive stiffness loss of the bonded steel construction would be considered in the model. This is possible in the framework of continuum damage mechanics. The higher accuracy is opposed to the higher demand of computing power, which has up until now prevented a broad application of damage mechanical models in the fatigue resistant design of bonded steel structures.
The objectives of the present study were to answer questions about the behavior of bonded steel joints under multiaxial loads and to develop and verify numerical approaches. A stiff, high-strength adhesive and a toughened adhesive for body-in-white applications were investigated on steel surfaces.   Considered methods for fatigue life prognosis were either based on local stresses followed by postprocessing or on a damage mechanical approach. For both approaches, the validation methodology was further developed and approved. The results were intended to contribute to the improvement of  fatigue life predictions for multiaxially loaded steel adhesive joints.

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Prof. Dr. B. Mayer, Dr. Ch. Nagel, Dipl.-Ing. A. Fiedler, Dipl.-Ing. G. Rybar, Prof. Dr.-Ing. G. Meschut, Dipl.-Ing. S. Cavdar, Dr.-Ing. D. Teuenberg, Prof. Dr.-Ing. Matzenmiller, Dipl.-Ing. U. Kroll