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.

The stay cables of the north bridge of the Galecopperbrug are reaching the end of their life span. Rijkswaterstaat, which is responsible for the bridges, needs a reliable Structural Health Monitoring (SHM) method to monitor the current state of the stay cables. At the moment, Rijkswaterstaat experiences issues with defining the state and the residual life span of the stay cables of the
Galecopperbrug. This study focuses on the technique of Acoustic Emission (AE) for monitoring the stay cables of the Galecopperbrug. The main research question of this study was: “Is the AE-system used in a fracture-based assessment suitable for structural health monitoring of the stay cables of the Galecopperbrug?”. In this study the following methods were used to investigate the AE behaviour of stay cables: a literature study on previous research and the current knowledge of AE and the structural behaviour of cables was done, a full-scale and two verification experiments were performed to investigate the AE behaviour related to wire breaks and to investigate the accuracy of linear source location techniques, an analytical model of the stay cables was made to investigate the influence of different parameters to the capacity of the stay cables and to predict the stress distribution in the wires during the experiment and finally, a SCIA model of the Galecopperbrug was used to investigate the load bearing distribution of the Galecopperbrug. In this study, it was found that wire breaks inside (stay) cables will generate elastic stress waves which can be captured and recorded by AE sensors. Based on an experiment where multiple wire breaks occurred, it was shown that wire breaks can be identified with the help of AE techniques. However, the identification of wire breaks is mainly depending on the correct choice of sensor type. Based on the experiments and assumptions that were made in this study the R6I-AST type of sensors are more suitable for wire break detection and the R3I-AST sensors are more suitable for AE signals due to impacts. Based on the assumptions and the experiments performed in this study it can be concluded that AE can be used in a fracture-based assessment for SHM of the stay cables of the Galecopperbrug.

Orthotropic Steel Decks (OSDs) are widely used in various types of steel bridges due to their benefits of light weight, high load bearing capacity and speedy construction. Although many improvements in aspects of design, fabrication, inspection, and maintenance have been achieved over the years for such bridge decks, fatigue remains a predominant problem, mostly because of the complexity of prediction methods. Many researchers have tried to investigate this component through experiments. However, performing only experiments may not lead to a cost-effective solution. Therefore, it is necessary to combine the experimental data with the numerical approaches. Particularly Linear Elastic Fracture Mechanics (LEFM) allows to model and analyse the crack propagation until subsequent failure, and significantly reduces the requirement of experiment. ABAQUS® provides an enriched feature, commonly referred to as the Extended Finite Element Method (XFEM) which incorporates two enrichment function namely the discontinuity function which represents the gap between the crack surface and asymptotic function which captures the singularity and thus can be used to model discontinuity independent to the finite element mesh. To evaluate the modelling efficiency and validate the simulation methodology, two XFEM-model based on LEFM and Virtual Crack Closure Technique (VCCT) are developed and the simulated results are compared with the experimental data. The first phase of the thesis deals with the numerical simulation to investigate the crack propagation rate in Compact-Tension (CT) specimen for different stress ratios. The results of two-dimensional (2D) model are found to be in good agreement (within 1.48%) with the fatigue coupon test results. As most of the work concentrates on 2D shell model, the extension to three-dimensional (3D) solid requires the investigation of related parameters to consider through-thickness effects. Nevertheless, the mechanism of 3D model is studied, and the simulated results match with the 2D results for fatigue crack growth (a, N). Moreover, a reliable technique of computing Stress Intensity Factor (SIF) is obtained by comparing with the ISO 12108 standard formulation. However, when the SIF and fatigue crack growth are combined, the crack propagation rate in 3D is overestimated (about 26%) when compared to the experimental data possibly because of the imperfection in the application of boundary conditions. The second phase deals with the numerical simulation in welded connection of OSD to determine the Paris law constants (C, m) by correlating the numerical result of fatigue crack growth with the beach mark measurements obtained in the fatigue experiments. Prior to automated XFEM simulation, a set of finite element analyses are performed to determine the vertical deformation, longitudinal stain distribution and hotspot stresses to validate the numerical model as per the test setup. The results of numerical analyses showed a good correlation (within 18%) with test data and Paris law constant C is predicted to be lower than the recommended value by IIW standard. The validated methodology is then applied on large scale to an existing bridge (Suurhoff bridge) structure which was built in 1971. In this case study, a crack length of 230 mm was detected in the deck plate originating from the root of the stiffener-to-deck plate welded connection between the cross-beams using TOFD measurements. To verify the problem, a numerical model is developed based on the dimension of the bridge to evaluate the crack initiation period and the crack propagation period. The crack initiation period is predicted using hotspot stress method and the crack propagation period is evaluated using automated XFEM simulation. Overall, the total fatigue load cycles are predicted to be 7.86 million which is equivalent to 48 years. A similar crack length was however detected after a service life of 44 years. This overestimation can be possibly explained as the model did not take residual stresses and other welding defects into account. The numerical model showed a good correlation with the real scenario and is therefore used to predict the permissible limit of deck plate crack length of 500 mm. The model predicted 8.02 million load cycles for a crack length of 500mm, which is equivalent to 34 years after the crack initiation period. Nevertheless, the fracture mechanics approach showed improvements in the assessment of fatigue life.