Welding is a widely used process in various industries for joining materials, but it can introduce several complexities that affect the mechanical properties of the welded components. In particular, the presence of pores within the welded material can have significant implications for fatigue resistance, residual stresses, and overall structural integrity. The primary objective of this thesis is to develop and analyze thermal and mechanical models to understand how various parameters, including geometry, mesh, material properties, boundary conditions, and heat input, influence the behavior of a welded thick plate containing pores. The simulations are conducted with different pore configurations and multiple welding passes to comprehensively explore their impact. Two Finite Element (FE) models were designed. In the first model (Model 1), the objective was to replicate the experimental setup detailed by Qiang et al. in their work "Through-thickness welding residual stress and its effect on stress intensity factors for semi-elliptical surface cracks in a buttwelded steel plate" [41]. This endeavor was accomplished utilizing Abaqus as software. A Dflux subroutine, written in Fortran, was incorporated to effectively emulate the welding procedure. The model was constructed to account for multiple welding passes (up to 7). By extrapolating transverse and longitudinal stresses, a comparative analysis was conducted across five distinct models and the experimental data [41]. The intention was to elucidate variations and similarities in the results. Subsequently, the second model (Model 2) was conceived to exploit the more favorable boundary conditions present in Zhang’s model as described in "Effect of Welding Sequence and Constraint on the Residual Stress and Deformation of Thick Welded Butt Joint Made of Q345qD Steel" [58], when contrasted with the initial experiment. Following a similar procedure, the FE model was developed, with the added feature of an increased number of welding passes (up to 12). Distortions along three distinct paths and transverse and longitudinal residual stresses within two specific regions were extrapolated and subsequently compared. The investigation then progressed to the introduction of pores into Model 2, in the simulation characterized by the lowest number of passes. The aim was to assess the impact of these pores on temperatures, distortions, and residual stresses. By comparing the outcomes of the models both with and without defects, valuable insights were gained into the effect of pores during welding processes. The main findings of this research can be summarized as follows: • Thermal data extracted from a welding experiment conducted at four distinct positions within a 250mm x 200mm x 32mm plate, as documented in Zhang et al.’s publication [58], exhibited inconsistencies when compared to the results obtained through Abaqus simulations in this thesis. These disparities predominantly stem from the absence of accurately specified thermocouple placements in the reference paper [58]. Despite conscientious endeavors to pinpoint the precise thermocouple coordinates, discernible variations of 100°C and a maximum divergence of 25% between the outcomes of the simulations and the reference findings were identified. • Residual stresses obtained in Abaqus (Model 1) trying to replicate Qiang et al. paper [41] in a 700mm x 400mm x30 mm showed good agreement with the reference simulation but deviated from the experimental data. The obtained numerical values peaked around 150 MPa, while the experimental values reached more than double that magnitude (305 MPa), for specific simulations. The number of welding passes significantly influenced the compression at the sides of the plate, with an increasing trend as the number of passes increased. • The simulations (Model 2) created to replicate Zhang et al. experiments [58] demonstrated better agreement with the experimental data for both longitudinal and transverse residual stresses. The number of passes played a crucial role in the results, with the eight to twelve passes simulation showing the closest match to the experimental data. • The presence of the studied configuration of pores in the material did not affect significantly the shrinkage, distortions, and residual stresses in the simulations. Closer to the pores, higher values for residual stresses were found. Based on the findings of this thesis, several recommendations for future research can be made. Laboratory experiments should be conducted to validate the numerical results obtained in this study. Sensitivity analysis considering modifications in the dimensions, boundary conditions, and pores locations of the model can provide further insights. Additionally, incorporating thermally independent properties can reduce the computational time of the simulations. Overall, this thesis contributes to understanding the impact of pores on the thermal and mechanical behavior of thin and thick plates and provides a foundation for future research in this area.

Over the past few decades, the Netherlands has built numerous steel railway bridges to improve its infrastructure. The repeated loading from railway traffic makes steel bridges susceptible to fatigue damage. The stress ranges at fatigue-prone locations are generally higher in railway bridges than in road traffic bridges. Due to the limited space available due to road and rail alignment, the connection between the main girder and cross-girder is often identified as a critical fatigue location.

The main- to cross-girder connection can be designed for fatigue using different design principles. Designing the connection with a certain rotational stiffness leads to higher fatigue stresses at this location. Meeting the fatigue requirements at the connection can often be achieved by local adjustments, such as welding extra steel plates to the fatigue-induced location to evenly distribute stresses from the cross-girder to the main girder. This can be a costly solution. Designing a connection that is flexible could reduce stresses at the main- to cross-girder connection. Solving fatigue issues can be done by making global adaptations to the structure. The question arises: which aspects can be adjusted such that slight changes can notably improve fatigue resistance in a cost-effective manner?

A literature study and a finite element investigation of the main- to cross-girder connection are performed. A reference model is made after which parameters are altered to investigate their impact on the fatigue response of the connection. The reference model is based on the design of the bridges from project Oostertoegang. However, instead of considering a connection with a certain rotational stiffness, the design is made more flexible for this study. Ansys 2022 R2 is used to create a finite element model of the bridge with shell elements. The hot spot stress method is used to conduct the fatigue assessment. In total four critical fatigue locations are researched for six parameters. The parameters researched are: the center-to-center distance between the cross-girders, the height of the cross-girder and main girder, the thickness of the inner web plate of the main girder, the diaphragm, and the steel deck plate.

From the analysis, it can be concluded that for one detail (M3) local measures should be applied to meet fatigue criteria. The three other details can satisfy requirements within feasible limits. The most cost-effective and realistic way is to increase the thickness of the inner web of the main girder. Other cost-effective but less feasible solutions to optimize the flexible design for fatigue are: decreasing the thickness of the diaphragm and increasing the height of the cross-girder. These parameters show the best ratio between the costs needed to alter the parameter and the total fatigue damage change of the critical detail.

Furthermore, it is determined that the fatigue assessment using finite element analysis with shell elements can be optimized by using the hot spot stress method in combination with modeling the weld using an increased thickness.

In the past years, the safety of steel railway bridges has been questioned due to the coped beam connection sensitivity to fatigue and fatigue crack appearance. To ensure the safety of these railway bridges, the propagation of the fatigue crack needs to be analysed by performing regular inspection on the bridges. The frequency of these is required by the defined inspection intervals, which are based on the critical crack length and the number of loading cycles leading to this crack length. Laboratory tests and measurements on the bridges have been performed in the past to define inspection intervals, but had given only an approximation of the real situation. By creating a finite element model of a coped beam, a crack initiation and propagation analysis can be performed. This analysis provides an indication after how many loading cycles the crack will reach a critical length and guides towards revised inspection intervals.

In this thesis, the objective is to take the first steps towards the revised inspection interval by performing a finite element analysis of the crack propagation of steel railway bridges. More specifically, this means an finite element analysis of the coped beam with extended finite element method.

To achieve this goal, first, the location of the fatigue crack initiation has been investigated to understand where exactly in the cope the crack starts. This was performed by creating a local 3D linear elastic FE-model of the coped beam based on the boundary conditions, geometry, and loading from laboratory tests performed by Michael C.H. Yam and J.J. Roger Cheng. The the longitudinal stresses of the model were compared with stresses measured during the laboratory test to validate the crack initiation model. After the model was validated, the location of crack initiation was determined by identifying the location of the peak stresses.

After completion of the crack initiation analysis, the crack propagation analysis can commence. For this, the FE-model was transformed from the crack initiation to the crack propagation model. In the same manner as with the crack initiation model, the propagation model has been based on the boundary conditions, geometry, and loading from the laboratory test. Since no path of the crack was registered during these laboratory tests, and thus could not be implemented in the model, a method for crack propagation analysis called extended finite element method (XFEM) has been used. The accuracy of this method has been confirmed by validating the stress intensity factor (SIF) values obtained for stationary cracks with three different mesh topologies and comparing the results with results from two established methods; J-integral and VCCT.

After the validation of the crack propagation model, a sensitivity study was performed to understand the potential influence of modelling decisions on the number of load cycles versus crack length relation. With the mesh sensitivity analysis, a linear trend has been obtained for models with matching number of elements through the thickness. For the initial crack size sensitivity analysis, the effect of the number of elements through the thickness and the effect of mesh topology have been investigated. One element through the thickness and parallel mesh topology led to lower variation in the results.

Additionally, a new value for the previously assumed Paris law coefficient C has been obtained by calibrating the number of cycles versus the crack length curve from FE-model to match the laboratory test results.

In conclusion, this research provides recommendations for modelling crack propagation in a coped beam. The results are more reliable when keeping at least five finely meshed elements around the crack tip, using a parallel mesh topology, and using one element through the thickness. New value for the C coefficient from the Paris law has been suggested based on this research.

The next step in this research is a continuation of the crack propagation model analysis to reduce the variation in the results. Furthermore, different loading positions and expansion from a simply supported beam model to a model with multiple spans should be analysed to bring us one step closer to the ultimate goal of redefining the inspection interval for coped beam steel bridges based on the models.