The orthotropic steel deck (OSD) is nowadays commonly used due to its advantages, such as its lightweight, short construction time, and high capacity to bear the loads. However, fatigue cracks occur in different welded connections in the OSD. In this study, the rib to-deck-plate connection at the crossbeam conjunction has been analysed. This connection has been considered as a very critical connection due to the high-stress concentration under local wheel loading. In addition, the crack initiating at the weld root and propagating through the deck plate thickness cannot be visually inspected until surface cracks appear when the overlay is removed for the investigation. In the Netherlands, deck plate cracks at rib-to-deck plate connections at the crossbeam constitute a significant proportion of investigated cracks. In this thesis, a numerical investigation of the aforementioned cracks is carried out.
First, XFEM is applied to study fatigue crack propagation using compact tension (CT) specimens. Commercial FE software package Abaqus® is used to build 2D and 3D CT models. The corresponding fatigue crack propagation is calculated in the direct cyclic step with strain energy release rate calculated using the virtual crack closure technique (VCCT). A good match is found between analytical calculation and 2D XFEM modeling. For 3D fatigue crack propagation, the number of cycles calculated from the automated crack propagation approach, using the low cycle fatigue step provided by Abaqus®, shows high sensitivity to element sizes, specifically the used element sizes through the thickness of the CT.
Second, 3D crack propagation is applied to OSD with a 20 mm deck plate. For this detail, the fatigue crack initiation under compressive stress states is caused by the local loading on the deck plate. A hypothesis of changing the loading sign from compression to tension to subject the initial crack zone to tension is proposed so that this type of crack can propagate using automated XFEM. This assumption is made because high residual stresses normally exist in the welded connections and fatigue crack can propagate even under compressive stresses induced by loading. Before carrying out crack propagation calculation using VCCT, the stationary crack states were analysed using FEM and XFEM for cracks inserted with different angles. The SIFs obtained from FEM and XFEM are the same; in addition, that the SIF obtained when the detail is loaded in tension or compression has the same absolute value. Subsequently, the propagating crack analysis is carried out as a small initial crack with a depth of 0.5 mm is inserted and Paris’ law parameters C and m of 1∙ 10-13 and 3.0 were selected respectively. The obtained results of strain evolution as a function of loading history came close to that from the experiment with a difference of 7.75% for the initial strain value and 10.95 % for the maximum one. Moreover, the predicted crack shape, angle, and rate were all validated with similar crack shapes obtained from experiments in various literature. As for the crack propagation behaviour in the thickness direction, the crack arresting took place at around 75% of the deck thickness and an angle with the vertical axis of around 30o.
Finally, the deck thicknesses of 16 mm and 10 mm were considered. Their results converged in terms of the crack propagation behaviour with real test specimens from the literature. It is also found that the 16 mm and 20 mm cases had a common crack propagation behaviour in the thickness direction as the crack arrest occurred at around 75% of the deck thicknesses, while for the 10 mm deck case the crack developed more dangerously by penetrating through the deck plate leading to a through-thickness crack. The approach proposed by this thesis proved to be valid for predicting the crack behaviour for the considered OSD deck plate thicknesses. Thus, the structural integrity of the detail can be assessed. This proved that for the 16 mm and 20 mm deck plates, the structural integrity is acceptable, while the 10 mm deck safety is not confirmed.

In the course of this thesis, a research has been conducted to the application of fracture mechanics on an othotropic steel deck (OSD). Obtaining accurate predictions of crack path and fatigue life assessment were the main objectives for this thesis. As 3D crack propagation modelling in orthotropic steel decks is an arduous task, simpler studies were examined first in order to obtain confidence for the task at hand. These studies are comprised of a through thickness crack in an asymmetrical four point bending specimen and a semi-elliptical crack in a centre cracked plate. Furthermore, studies on computational fracture mechanics and the integration of Python scripting language with Abaqus were conducted for numerical analysis of crack propagation in complex structures. The extended finite element method was used for evaluation of contour integrals of a crack with a constant mesh throughout the propagation analysis. Post-treatment of the results was applied by smoothing stress intensity factors and the shape of the crack front. Moreover, inaccurate contour integral results were excluded from analysis through extrapolation. A Python code was developed for updating the crack front for a given number of loading cycles integrated in Paris' law. The investigated crack was located at the rib-to-crossbeam joint of an OSD. Here, crack propagation occurs along the lower weld toe and in the longitudinal direction of the rib. Validation of results was obtained through experimental studies conducted at the TU Delft laboratory. The weld geometry was incorporated in the model from measurements in order to identify local stresses. Finally, results of strain gauges were used to validate the model. Measurements of the experimental crack growth were obtained at the surface of the crack only. Consequently, assumptions were made of both the depth and shape of the crack in the thickness direction of the rib. A semi-elliptical crack shape was selected with an initial crack depth of 1 mm. Initially, Paris law parameters C and m were selected as recommended by BS7910. The numerical results of crack path complied with those of experimental results, with deviations of no more than 1 mm. Both data sets show that the propagation rate remained somewhat constant. On the other hand, fatigue life assessment results showed stronger deviation. A parametric study was performed to investigate the effect of the initial crack depth. Three different cases were examined for which the initial crack depths were selected as 1, 2 and 3 mm. Varying the initial crack depth did not lead to a significant change in crack path at the surface of the rib, while the depth in the thickness direction was affected. Regarding the fatigue life assessment, the results with respect to an initial crack depth of 1 mm, complied best with experimental results. Given the strong correlation between experimental and numerical results, the methodology followed in this thesis has proven valuable regarding fatigue crack modelling. As the methodology is based on fracture mechanics, an initial crack is required prior to evaluation. Consequently, the application in engineering practices lies in re-evaluation of structures. Moreover, as the parameters in crack propagation relations have a significant influence on the fatigue life assessment, these should be carefully chosen. Therefore, more insights must be obtained regarding the selection of such parameters.

Orthotropic bridges are being built for more than half a century. These bridges are prone to high cycle fatigue due to traffic loading. Early onset of fatigue cracking is a serious issue in both old and recently built bridges. To prevent this a number of different cross section have been suggested. Sandwich Panel is one of the suggested alternative that has become more feasible to produce due to advancement in Hybrid Laser Arc Welding (HLAW) technology. One type of sandwich panel with corrugated core was investigated in this thesis. Ease of manufacturing, stronger welds, reduced complexity of welding and improved transverse shear stiffness are some advantages of sandwich panel over conventional orthotropic decks. The aim of this thesis is to apply J-Integral based local approach for fatigue assessment of sandwich panels subjected to lateral loads. J-Integral represents a way to calculate the strain energy release rate and is a measure of crack tip elastic-plastic field. It has already been used successfully to quantify fatigue performance of web core sandwich panels and is therefore used in this thesis. A second aim of this thesis is to justify the use of foam filling in sandwich panels by comparing fatigue performance of empty and foam filled sandwich panels. For validation, bending behaviour of panel was first investigated using Finite Element Analysis (FEA). The model developed for dual stake laser welded panel accurately estimated response parameters (compared to experimental results of Tan et al. [33]) like maximum deflection (2.2% difference), critical stresses in transverse direction (9% difference) for top and bottom face plates etc. For verification of Contour Integral technique of Abaqus/CAE, J-Integral at notch tip of ASTM CT specimen was calculated. Compared with empirical solution the FE estimate showed 16% difference with FE estimate being more conservative. Seven different types of foam were considered as filling in sandwich panels. The foams were of polymeric type - H45, H60, H80, H100, H130, H180, H200, and H250 (Unit weight of foams - 48 to 250 kg/mኽ and Young’s Modulus - 45 to 250 MPa). Based on two stage 3D FEA on empty and foam filled sandwich panels, it was observed that J-Integral value at critical tensile notch decreased consistently as stronger foam fillings were used in the panel (83.8% reduction observed between empty panel and H250 foam filled panel). Cycles to failure for panels were calculated using regression equation derived from existing research on web-core sandwich panel. Between empty and H100 foam filled panel an increase of 171 times was observed in cycles to failure. This increase justifies the use of foam filling.