Effects of residual stresses on the fatigue crack propagation of an orthotropic steel bridge deck

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Abstract

Orthotropic steel decks (OSDs) are susceptible to fatigue failure due to cyclic loading by heavy traffic passing the deck. These cyclic loads can cause fatigue cracks in the rib-to-deck joint. In and around the joint residual stresses are present due to the welding process. In this thesis, the effects of residual stresses on the fatigue crack propagation rate is evaluated. This research is based on finite element analysis and verified against experimental data. First, a thermomechanical weld simulation analysis is made in order to define and verify the residual stress field of the OSD-specimen. This analysis consists of two parts starting with a thermal model in which the temperature field during the welding process is analysed. This temperature field output is introduced into the mechanical model in which the residual stress field is modelled and verified. This data is subsequently used in a crack propagation model based on the extended finite element method (XFEM) in which the effect of the residual stresses on fatigue crack propagation is quantified. Next to that the residual stress component in each direction is analysed separately to obtain the effect of the residual stress per stress direction. The fatigue crack simulation including residual stress field shows good correlation (R2=99%) compared to the experimental data. The simulation without residual stress field shows less correlation (R2=95%), which shows that including the residual stresses improves the results significantly. Introducing only one residual stress component does not significantly improve the results compared to the model without the residual stresses. The effects of the residual stresses are relatively large as the tensile transversal residual stresses increase the crack propagation, while the tensile longitudinal residual stresses decrease the crack propagation rate.
In the case study the obtained knowledge from the research will be applied on the Suurhoffbridge of Rijkswaterstaat. On this highway bridge fatigue cracks occurred and these are measured using Time Of Flight Diffraction (TOFD) measurements. Firstly, the geometry and boundary conditions of FEM-model are verified using literature data based on hot-spot stresses. Secondly, the critical load position of the truck load is determined by obtaining influence lines based on the stress intensity factor. These lines show that critical position is reached when the wheel load is positioned between the webs of the stiffener. Also, from the influence lines can be concluded that the residual stresses in the transverse direction increase the stress intensity factors, while the influence of the residual stresses in longitudinal direction is negligible. At last, a fatigue crack propagation simulation with and without residual stresses is performed. The transversal tensile residual stresses are essential to initiate crack propagation, as at the weld root, compressive transversal stresses are present due to the traffic load which prevent crack propagation. Overall, the fatigue crack simulation including the transverse residual stresses corresponds relatively well compared to the measured data as the difference is approximately 14% also considering that two wheel load that have limited effect have been ignored in the analysis. If these two wheel loads would be included the results will improve. However, there are many assumptions made that can effect the result, such as the Paris law properties and load parameters. A slight change in these parameters would effect the results greatly. In order to verify the results, the assumptions made in the case study, should be verified using measurement data of the structure.