Search

The aim of this PhD project is to investigate the fatigue crack propagation behaviour in an additive manufactured high strength titanium alloy, Ti-6Al-4V. Test specimens were made by the Wire+Arc Additive Manufacture (WAAM) process. The research focus is the fatigue crack behaviour (crack growth rate and trajectory) near interface between WAAM and substrate materials. The challenges are the understanding and assessment of the effects of residual stress, microstructure change and anisotropic material properties as the result of rapid heating and cooling, and rapid solidification cycles in additive manufacturing. This PhD project has focused on numerical modelling and simulation of fatigue crack behaviour. Specimen fabrication and experimental tests were conducted by our collaborators in linked projects. Finite element method (FEM) was employed to evaluate the influence of anisotropic Young's modulus and yield strength properties on the crack tip stress intensity factor and crack tip plasticity. Residual stress distribution in the compact tension, C(T), specimens were obtained by FEM based on experimentally measured residual stress in a much larger WAAM-substrate wall, from which the C(T) specimens were extracted. Residual stress profile was also established by an analytical approach for another WAAM-substrate wall, from which the fatigue crack growth rates (FCGR) for pure WAAM material were measured. Based on these calculations, fatigue crack growth rate and life were predicted by empirical methods from the Linear Elastic Fracture Mechanics, namely the modified Paris law and the Harter T-method. Residual stress effect is accounted for by the superposition method, via the effective R ratio parameter. Key findings and main conclusions are: (1) the difference in the stress intensity factor is less than 1% when considering the anisotropic material properties. Therefore, the influence of anisotropic material properties on the crack growth driving force can be neglected. (2) After extracting a C(T) specimen from a larger wall sample, retained residual stress in the C(T) specimen is much reduced; consequently the stress intensity factor due to the residual stress is also small. (3) Residual stress free assumption is not valid for C(T) specimens which provide FCGR data for pure WAAM material. (4) The Harter T-method is better when predicting FCGRs in WAAM material and residual stress effect should not be ignored. (5) Predicted fatigue crack growth life for the specimens containing WAAM-substrate interface have a difference about 25% compared to experiments. Predicted fatigue crack deviation angles for various crack locations and orientations are consistently larger than the experimental measurement, as the crack closure has reduced the effect of mode II stress intensity factor.

Polymer thin films are of increasing interest in many industrial and technological applications. Superhydrophobic, self-cleaning surfaces have attracted a lot of attention for their application in self-cleaning, anti-sticking coatings, stain resistance, or anti-contamination surfaces in diverse technologies, including medical, transportation, textiles, electronics and paints. This thesis focuses on the preparation of nanometer to micrometer-size particle textured surfaces which are desirable for super water repellency. Textured surfaces consisting of nanometer to micrometer-sized lightly sulfonated polystyrene ionomer (SPS) particles were prepared by rapid evaporation of the solvent from a dilute polymer solution cast onto silica. The effect of the solvent used to spin coat the film, the molecular weight of the ionomer, and the rate of solvent evaporation were investigated. The nano-particle or micron-particle textured ionomer surfaces were prepared by either spin coating or solution casting ionomer solutions at controlled evaporation rates. The surface morphologies were consistent with a spinodal decomposition mechanism where the surface first existed as a percolated-like structure and then ripened into droplets if molecular mobility was retained for sufficient time. The SPS particles or particle aggregates were robust and resisted deformation even after annealing at 120¿¿¿¿C for one week. The water contact angles on as-prepared surfaces were relatively low, ~ 90¿¿¿¿ since the polar groups in ionomer reduce the surface hydrophobicity. After chemical vapor deposition of 1H,1H,2H,2H-perfluorooctyltrichlorosilane, the surface contact angles increased to ~ 109¿¿¿¿ on smooth surfaces and ~140¿¿¿¿ on the textured surfaces. Water droplets stuck to these surfaces even when tilted 90 degrees.Superhydrophobic surfaces were prepared by spraying coating ionomer solutions and Chemical Vapor Deposition (CVD) of 1H,1H,2H,2H-perfluorooctyltrichlorosilane onto textured surfaces. The surfaces after CVD of silane exhibited water contact angle of 152¿¿¿¿ and the water droplet stuck to the surfaces without falling even when tilted upside down. This kind of sticky superhydrophobic surface would have potential applications in no-loss transport of liquid, and cleaning robots.