Numeriek en experimenteel onderzoek naar het vermoeiingsgedrag van met wire arc additive manufacturing vervaardigde componenten

Wire arc additive manufacturing (WAAM) has emerged as a promising technology for producing large-scale components because of its high deposition rate, efficient material utilisation, design flexibility, and comparatively low equipment cost. These advantages make WAAM particularly attractive for structural applications in which conventional manufacturing routes may involve substantial material waste, long machining times, or limited geometric freedom. However, the wider adoption of WAAM in load-bearing structures remains constrained by uncertainties in structural performance, particularly fatigue resistance. Fatigue behaviour is strongly influenced by process-induced features, including thermal history, microstructural heterogeneity, residual stress, geometric irregularities, and surface roughness. Existing fatigue design methods were primarily developed for conventional wrought, machined, or welded components, and their direct application to WAAM components is not straightforward. This thesis therefore aims to establish a rational experimental-numerical basis for fatigue assessment and fatigue-oriented design of WAAM low-carbon steel components. The research first investigates the influence of key process parameters on the performance of S355 low-carbon steel flat components fabricated using cold metal transfer (CMT)-based WAAM. The parameters considered include deposition trajectory, substrate preheating temperature, deposition power, travel speed, wire feed speed, shielding gas composition, and heat source configuration. Their effects on thermal behaviour, geometric stability, microstructural evolution, and mechanical properties are systematically examined. Particular attention is given to the comparison between sinuous (SI) and raster (RT) deposition trajectories. The results show that deposition trajectory strongly affects heat accumulation, surface morphology, distortion, and mechanical performance. These findings provide practical guidance for selecting WAAM process parameters according to specific structural and manufacturing requirements. To further understand the thermo-mechanical behaviour of WAAM components, a fully coupled three-dimensional thermo-mechanical finite element (FE) modelling framework is developed and validated. The model is validated against experimentally measured thermal histories, stress evolution, and post-unclamping deformation. The validated FE model enables detailed analysis of temperature evolution, stress development, deformation, and residual stress formation during deposition, cooling, unclamping, and substrate removal. The results demonstrate that residual stress distributions are governed primarily by the number of deposited layers and by the interaction between structural stiffness and constrained thermal deformation. Boundary conditions, platform stiffness, and bolt pretension force significantly affect stress development during deposition and cooling, although their influence within the deposited component is largely reduced after substrate removal. Sensitivity analyses further highlight the importance of appropriate heat source calibration, thermal boundary conditions, element activation strategy, and numerical modelling choices for accurate residual stress prediction. The fatigue behaviour of WAAM components with machined surfaces is then investigated experimentally. The geometric and mechanical properties of the specimens are characterised using static tensile tests, hardness measurements, and microstructural observations. Fatigue tests show that WAAM specimens with machined surfaces exhibit exceptionally high fatigue resistance, exceeding the highest fatigue classes commonly specified in structural design standards such as the Eurocodes and IIW recommendations. This superior fatigue performance is attributed to the combined effects of fine-grained microstructures, low porosity, limited position-dependent mechanical variation, and low tensile residual stresses, particularly for specimens manufactured using the SI deposition trajectory. Reliability analyses based on Weibull statistics further confirm that the high fatigue categories obtained for machined WAAM specimens are robust and reproducible. These results demonstrate the strong potential of WAAM low-carbon steel components for fatigue-critical structural applications when surface irregularities are removed by machining. The thesis also addresses the more challenging case of as-deposited WAAM components with rough surfaces. Surface waviness and roughness, inherent to the layer-by-layer deposition process, act as local stress raisers and significantly reduce fatigue resistance. Conventional nominal stress, hot spot stress, and effective notch stress methods are evaluated for rough WAAM specimens, but their applicability is limited by the highly irregular and continuously varying surface geometry. To overcome this limitation, a generalised notch stress (GNS) method is proposed and implemented within a materially nonlinear FE framework. The GNS method captures the local stress state at fatigue-critical surface features and enables the fatigue behaviour of both machined and as-deposited WAAM components to be described within a unified assessment framework. In addition, several geometric descriptors are evaluated, among which the ten-point roughness height, Rz, is identified as the most influential parameter controlling fatigue resistance. A predictive relationship between Rz and fatigue resistance is therefore formulated, providing a practical and physically meaningful basis for estimating the fatigue performance of WAAM components with different surface conditions. In conclusion, this thesis establishes an integrated experimental-numerical framework for understanding and assessing the fatigue behaviour of CMT-based WAAM low-carbon steel components. The work links process parameters, structural features, material properties, and fatigue performance within a coherent process-structure-property-performance framework. The findings contribute to the development of fatigue-oriented design rules for WAAM steel components and help bridge the gap between additive manufacturing research and structural engineering practice.