Atomistic modeling of crack tip behavior in ductile materials
This dissertation is composed of three papers and related unpublished work that laid the foundation for the succeeding scientific discoveries. The first two papers detail work intended to illuminate the atomic-scale mechanisms governing the near threshold fatigue crack growth phenomenon. First, by harnessing a concurrent multiscale approach and contemporary computational resources, fatigue crack simulations with cycle counts well beyond those analyzed previously have been performed. The validity of long-hypothesized material separation mechanisms thought to control near threshold fatigue crack growth in vacuum is assessed. Results show that fatigue crack growth arrests after an initial transient period, reconciling reports of crack growth in atomistic simulations at loading amplitudes below experimental crack growth thresholds. It is also observed that sustained crack growth in vacuum only occurs when emitted dislocations return to the crack tip on a slip plane behind its original one, which is resulted from slip trace intersection in the 2D simulated system and is expected to occur in 3D crystals by other additional mechanisms. Second, building upon the concurrent atomistic-continuum multiscale modeling framework, the isolated effect of material dissolution on crack growth is investigated. A series of dissolution simulations are carried out with different loading conditions and dissolution rates. Simulation results and subsequent quantitative analysis suggest that while dissolution is capable of freeing arrested fatigue cracks, the crack tip is always blunted under both static and cyclic loading, implying that dissolution has an overall crack arresting effect, and the dissolution-induced-blunting is found to be independent of the mechanical loading magnitude. Finally, from a standpoint of how state-of-the-art engineering and technology can be applied to other applications in an attempt to directly better the human condition, the third paper details a field work about the assessment of additive manufacturing for increasing sustainability and productivity of smallholder agriculture. This study analyzes and compares different manufacturing approaches from the perspectives of structural performance and cost efficiency and seeks to provide solutions to the mechanical obstacles encountered by smallholder farmers in the field. The acquired data suggests that the material extrusion 3D printing technology is able to provide functional parts more rapidly, accelerating the design cycle, and lowering cost relative to local fabrication routes, while traditional means is proved to be more economical in the case where mechanical performance of the part is the most critical.
Warner, Derek H.
Miller, Matthew Peter; Earls, Christopher J.
Civil and Environmental Engineering
Ph. D., Civil and Environmental Engineering
Doctor of Philosophy
dissertation or thesis