Fatigue Analysis of Welded Structures Using the Finite Element Method

Licentiatavhandling, 2012

Fatigue design and analysis of steel and composite bridges is generally based on the notion of the nominal stress using the classified S-N curves with corresponding fatigue classes for typical details. Such an approach can yield unrealistic the estimation of the load effects for structure components because of an ever increasing number of structural details and loading situations resulting in a limited number of possible treatable design cases. The hot spot stress method has been developed to enable an accurate estimation of the load effects for the fatigue strength of welded steel structures, in cases where the nominal stress is hard to estimate because of geometric and loading complexities or in cases where there is no classified detail that is suitable to be compared with. Although this method has been used in fatigue design and analysis in tubular structures for several decades, the method has not been applied on a larger scale on steel and composite bridges. The essential of adopting the finite element method for determining the design stresses for fatigue life calculations has been increased recently especially when utilizing the advanced fatigue assessment methods for welded steel structures. However, the result from finite element analysis can be highly sensitive to modelling technique since the stresses obtaining from these advanced methods such as the structural hot spot stress method and the effective notch stress method are often in an area of high strain gradients, i.e. stress singularities. The resulting stresses may differ substantially depending on the type and size of elements. In this study, this was recognized by evaluating the hot spot and effective notch stresses obtained from the finite element analyses with the fatigue test data collected from the literature. Orthotropic steel bridges have both complex geometry and loading conditions producing complex bridge deck behaviour which is hard to estimate and analyse using the traditional fatigue assessment methods. The effects of the loading and geometrical conditions, i.e. decks components which are working in a group, need to be considered accurately in the stress calculations. Assuming overall elastic behaviour in such complex structure systems, in which the stress raising sources that have decisive effects on the fatigue strength capacity are partly included, can yield over/underestimated stress values to be evaluated in fatigue design. The application of the advanced life assessment methods using the finite element method studied in this thesis produce more accurate stress results, including stress raising effects at welded details that are prone to fatigue.