Fatigue assessment of complex welded steel structures

University dissertation from Stockholm : KTH

Abstract: To improve the usage of material in fatigue loaded complex welded structures a better calculation method (based on FEA) is needed than what is available on the market and an improved management of weld quality. Improved FE-procedures are directly applicable to design of generic structural components within virtually all industries using CAD and FEtools, irrespective of industrial sector. More and more welded structures are created as 3D models from the design offices and the FE-work will be easier but the requirement of being accurate from the beginning and the reduction of lead times create a great need to substantially reduce calculation time. The main objectives in this thesis is to develop novel procedures to extract design data from finite element stress analyses of complex fatigue loaded welded structures, to achieve a better understanding of the limits of the different fatigue design methods and to investigate the influence of fatigue strength due to the weld quality. An investigation of how weld geometry and different type of defects influence fatigue life of the weld has been made on a non-load carrying cruciform joint. The weld geometries were measured and the stress distributions were computed using FEA. Fatigue tests have been performed and the initial defects were analysed. Life predictions with LEFM based on the stress distributions and defects were performed with good agreement with the test results. The different fatigue design methods Nominal stress, Structural stress, Effective Notch stress and LEFM proposed by the International Institute of Welding (IIW) have been investigated and applied on a welded link. In order to verify the accuracy of the prediction methods, fatigue tests of the link were made. Structural optimisations based FEA, using different objectives regarding the fatigue properties of the welded link were also carried out to increase the performance of the link. Comparison and investigations of the advantages and disadvantage of the two different LEFM approaches, weight function technique and FEA solutions is made on welded structures. Based on the conclusions of the investigations an automatic 3D FE based LEFM program was developed. The program was successfully implemented and verified on two welded structures.

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