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.

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.

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.

An Orthotropic Steel Deck (OSD) is suspectable to fatigue and this forms an important design criterium. In this research the rib-to-crossbeam connection with a cope hole is studied using Finite Element Methods (FEM). The fatigue assessment is performed with the hot spot stress method making use of surface stress extrapolation. Two fatigue cracks are investigated: the crack initiating from lower weld end in the rib, and the crack which appears at the weld toe of the rib-to-crossbeam connection and propagating in the crossbeam at some distance from the soft toe of the cut-out. Engineering firms widely use shell elements to model the structure, instead of solid elements. The modelling and computational time is limited when compared to the solid element. When a comparison is made between the results from the solid and shell FE models, a scatter in results is often seen. This thesis studies the difference of shell and solid element modelling in the hot spot stress calculation of OSDs. Improvement of the shell element modelling is given based on the FE analysis using various shell element modelling techniques and compared with solid element models which represent as a reference. Three different weld modelling techniques are applied to shell FE model: IIW, Eriksson and a combination of the first two approaches. The weld modelling techniques for the shell FE model are first applied on a simple single sided fillet welded transverse stiffener connection. In this study both the deformations and hot spot stress are compared. A small parametric analysis is performed to investigate the difference in hot spot stress for pure in-plane and out-of-plane rotation of the crossbeam. In this study the different weld modelling techniques for the shell FE models are included. In the extensive parametric analysis two different studies are performed: one based on the influence of the loading positions, the second study is based on different thicknesses of the parts of the OSD. Based on the loading positions study the critical loading positions are further investigated. From both the fillet welded transverse stiffener specimen and the full OSD specimen it is concluded that the shell FE model in which use is made of the combined weld modelling technique, gives the largest improvement. The ratio between shell and solid FE models of the obtained hot spot stress is reduced from a factor 1.25 to a factor 1.05 for the lower weld toe crack in the rib. For the crack in the trough, the mean value of the hot spot stress ratios is equal to 1.04 for both the old and new OSD variant and the coefficient of variation (CV) is equal to 0.8% and 2.2% respectively. For the crack in the crossbeam, the mean values are equal to 0.80 and 0.76 for the old and new OSD variant respectively. The CV is equal to 1.9% and 2.0% for the old and new OSD variant respectively. Furthermore, the stress gradient and the stress at 40mm from the weld toe show similar results when compared with the solid FE models. The coefficient of variation for both investigated cracks is low, i.e. below 5%, thus the ratio between obtained hot spot stress of the shell with combined weld and solid FE models is consistent.

The phenomenon of fatigue in orthotropic steel deck (OSD) bridges is a predominant problem because of complexity of the prediction methods. In the past, many researchers have studied the fatigue behaviour of various details in OSDs via both experiments and Finite Element Modelling (FEM). In the present research, the connection of open stiffener to crossbeam at the location of cope hole in OSDs has been be studied. Structural hot-spot stress method using surface stress extrapolation has been used to investigate the cracks in stiffener in the longitudinal direction and cracks in crossbeam. FEM is extensively used for analysing OSDs. In engineering applications, 2D shell elements are widely used instead of 3D solid elements for analysis due to less computational cost. The welds are generally not modelled with shell elements for fatigue assessment of welded structures. In this study, large difference of SHSS is obtained by shell and solid elements for both simple and complex fillet welded details and also for the OSD. This difference in structural hot-spot stress (SHSS) is reduced by the application of three weld modelling techniques with shell elements: (i) the IIW approach, (ii) the Eriksson’s approach and (iii) a combination of IIW and Eriksson’s approaches. All the three methods are based on increasing the thickness of shell elements at the weld region which are easy to be applied in practice. The dependence of SHSS on mesh size and element type is also investigated in this thesis. A parametric study is performed first on simple and then on complex fillet welded details to check whether the weld modelling technique can be applied to different geometries, loading and boundary conditions. The solid element model of the complex detail is first validated with experimental strain measurements. Then, SHSS values from other numerical models are compared with the solid model. Representative load cases are investigated initially followed by load combinations. The weld modelling method with shell elements gives good consistency in the ratio of hot-spot stress compared to the solid element model for these details. The deformations are also investigated for all load cases and load combinations. The combined weld modelling technique (iii) with shell elements replicated the weld stiffness of the solid model for both in-plane and out-of-plane load cases. As a final step in checking the consistency of SHSS ratios between shell elements with welds and solid elements in the application of OSD, a parametric investigation is performed. This study involved two geometric variants of OSD with different load positions. These two variants were based on the design of existing bridges in The Netherlands with relatively thin plate and newly designed ones with thicker plates. The parametric study is divided into two parts. The first part is based on representative load cases. The second part is based on SHSS influence lines for determination of critical loading positions having maximum and minimum hot-spot stress. For both these studies, the weld modelling approaches with shell elements gave a good match of SHSS compared to the solid models. The SHSS results from the shell model with weld are more consistent compared to the regular shell model without weld. From the preliminary parametric study on OSD, it is found that after weld modelling with shell elements using the combined approach of IIW and Eriksson, less scatter is observed in the SHSS ratios. The coefficient of variation (CV) in SHSS ratio for crossbeam is 6.8% and that for stiffener is around 5.1% which is low. The SHSS values are computed based on the stress perpendicular to weld toe. From the detailed parametric study, the mean value of SHSS ratio is 1.07 (range: 0.99-1.15) for the crossbeam and 1.02 (range: 0.98-1.10) for the stiffener. The CV of SHSS ratio is 5.4% for the crossbeam and 4% for the stiffener. The stress profiles are also investigated at the critical locations of OSD. The shell model with the combined weld modelling approach is in good agreement with not only SHSS but also with the stress at a distance far away from the stress concentration when compared to the solid model. The deformations are also very similar for both the numerical models. Thus, it is concluded that the combined weld modelling technique using the IIW and the Eriksson’s approach with shell elements could be used for accurate fatigue life assessment using hot-spot stress method where the measure of accuracy is with respect to the solid element model.