Use of high intensity X-ray analysis as tool to create new, fundamental models for phase transformations and residual stress in ductile cast iron

Recent advances in high resolution X-Ray methods that involve use of synchrotron facilities have made it possible to do high resolution, in-situ experimental studies of phase transformations in engineering materials thus providing detailed and accurate information on the processes that take place during such phase transformations. This paper describes how such facilities can be applied to study solidification of cast iron and formation of residual stress after eutectoid transformation by resolving the processes in 3D and time

Thermomechanical modeling and experimental study of a multi-layer cast iron repair welding for weld-induced crack prediction

Large-scale components such as hubs in wind turbines are often made of cast iron to minimize the production costs. One of the common challenges in the casting process of such large-scale components is manufacturing defects. However, repair welding will induce residual stress which can initiate cracks in the repaired structure, especially since cast iron is not as tough as steel. The current study addresses developing a thermo-mechanical model of the cast iron repair weld validated with experiments to predict thermal and residual stresses and to identify critical locations for crack initiation. A thermo-mechanical weld model is developed, and the predicted temperature and residual stress distribution are validated against experimental data. Two repair weld experiments, one manual and one automated are carried out and are simulated using the developed thermo-mechanical model. The regions with maximum principal residual stresses are calculated by the thermo-mechanical model and the maximum principal stress method is used to predict the location and direction of the developed cracks in the repair weld. A comparison with the repair weld experiments shows good correlation with the observed cracks in the welded specimens. The outcome of this research provides a basis for repair weld optimization of large-scale cast iron components in order to reduce the carbon footprint caused by their reproduction

X-ray tomography, digital volume correlation and FE modelling: A synergistic combination to study the processing-structure-property relations in ductile iron

The mechanistic understanding of the processing-structure-property relations in ductile iron is still far from complete. One reason is that the impact on the mechanical properties of some of the microstructural features arising from the casting process can be hard or even impossible to investigate using experimental methods alone. The present work shows that a solution can be the synergistic combination of X-ray tomography, digital volume correlation (DVC) and finite element modelling, which are applied here to study the effect played by the Si micro-segregation and local residual stresses upon mechanical loading. First, miniaturized tensile and compact tension specimen are loaded incrementally while imaging with X-ray tomography. Then, the micro-scale displacement is reconstructed with DVC and used to prescribe the boundary conditions in high-fidelity 3D finite element models of the microstructure. Simulations are run considering or not the formation of the local residual stresses and build-up of micro-segregation during manufacturing. The numerical predictions are compared to the corresponding experimental data both at the macro-scale - applied load - and at the micro-scale - strain field reconstructed with DVC. This allows for a first assessment of the impact of the local residual stresses and Si micro-segregation on the mechanism of tensile deformation as well as of crack propagation of ductile iron

Corrosion surface morphology-based methodology for fatigue assessment of offshore welded structures

This work employed a novel corrosion-based fatigue model to determine the fatigue life of offshore welded structures to enable the fatigue assessment of welds under corrosive conditions. In addition to the material's ultimate strength, endurance limit, and stress ratio (mean stress effect), the model includes a corrosion factor concept to account for the impact of corrosion pits on the fatigue performance of welded S355 steel, which is the novel contribution in this paper. X-ray computed tomography scans of corroded S355 specimens in a salt spray chamber were characterized. Surface texture characterization was employed to obtain surface roughness, size, and aspect ratio of corrosion pits. The corrosion factor was determined based on notch and surface fatigue theories using the characterized pit size, aspect ratio, and surface roughness. Fatigue S-N curves were then predicted for critical pits and compared against the fatigue code DNVGL-RP-C203 and experimental data from the literature. The novel approach combining corrosion characterization method with corrosion-based fatigue model for the prediction of fatigue S-N curves provided a minor deviation of only 2.8% between predicted and measured data. This approach can potentially be integrated into predictive frameworks for the remaining life assessment of offshore structures