Comparative studies of the fatigue strength of bracket joints (AVIF-No. A 225)

Responsible research centre: Technical University Hamburg-Harbur (Prof. Fricke)

Time period: 01.07.2004 - 30.06.2006

Final report: CMT 9/2007


Project summary:

The fatigue strength of overlapped and inserted bracket connections at different stiffener types was analysed and compared by means of experimental tests and numerical calculations. The investigations were divided in a preliminary and a main study. Many tests have been performed to determine the fatigue life of the various types of bracket connections. Hereby the overlapped brackets turned out to be superior to the inserted brackets for a considered crack length of 10 mm. For the analysed bracket connections extensive hot-spot stress calculations were carried out. Therefore finite element models with different mesh size were created and the calculated stresses were compared to strain-gauge measurements performed at the fatigue test specimens. The existing recommendations for meshing and extrapolation of hot-spot stresses given by the “International Institute of Welding” (IIW) were applied and reviewed in terms of applicability especially to bracket connections with bulb profiles. Recommendations for hot-spot stress calculations at bulb profiles could be given. Besides that investigations with the effective notch stress approach were carried out with both simplified and realistic weld shapes. To record the real weld geometry a laser-based sheet-of-light measurement system was aquired within the research project. To incorporate the microstructural support effect, the real weld profiles were rounded additionally with a fictitious radius according to the concept of Radaj. In an S-N-diagram, the results showed substantial differences of the notch stresses for the various types of bracket connections. The fatigue strength can be assessed with the fatigue class FAT 200. However, the scatter of results is only slightly smaller than for the results based on hot-spot stresses. For the effective notch stress calculations with simplified weld shapes based on a 1mm notch radius, recommended by IIW, the fatigue class FAT 225 could be confirmed. By comparing the effective notch stress with the hot-spot stress calculations, the latter and the recommendations, derived subsequently, could be verified.


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