In the design of welded structures, knowledge of the cumulative fatigue damage under variable amplitude loading is still an important issue for reliable fatigue life assessments. Design codes for welded steel and aluminium structures still recommend use of the Palmgren-Miner linear cumulative damage rule, usually assuming a damage sum, which is, according to the present knowledge, in most cases not safe. For demonstrating this fact, recent results obtained under variable amplitude loading from welded joints are presented, and recommendations for fatigue design regulations will be suggested. Investigations of cumulative damage were carried out on two types of welded joints (butt welds and transverse stiffeners) of four structural steels, S355M, S690Q and S960Q in the thickness of t = 10 and 25 mm. Further investigations were performed with laser beam welded hollow hat specimens of an automotive steel DCO4 and an aluminium alloy AA 5083 with sheet thicknesses of 2 and 3 mm. The random spectra used were of Gaussian type. By applying the Palmgren-Miner rule modified by Haibach most fatigue life estimations with the conventional damage sum D = 1.0 were on the unsafe side. For cumulative fatigue design the evaluation of the results suggest the use of an allowable damage sum of D(al) = 0.5 with the slope of k = 3 up to the knee point of the SN-curve at 1 x 10 7 cycles and k' = 5 after the knee point. It is proposed, that this suggestion is suitable for spectra with constant mean stresses. However, for load-time histories with fluctuating mean stresses a lower allowable damage sum may be appropriate.
Fatigue life assessment of welded joints under variable amplitude loading - state of present knowledge and recommendations for fatigue design regulations
2004
16 Seiten, 17 Bilder, 8 Tabellen, 19 Quellen
Conference paper
English
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