Mechanical Engineering Theses and Dissertations

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    Direct Laser Writing-Enabled Microstructures with Tailored Reflectivity for Optical Coherence Tomography Phantoms
    (2023) Fitzgerald, Declan Morgan; Sochol, Ryan D; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    As the continuous push to improve medical imaging techniques produces increasingly complex systems, so too must the phantoms critical to the accurate evaluation of these systems evolve. The inclusion of precise geometries is a well documented gap in optical coherence tomography (OCT) phantoms, a gap felt more severely as the technology improves. This thesis seeks to investigate the feasibility of utilizing new manufacturing techniques in the production of OCT phantoms with complex geometries while developing a phantom to determine the sensitivity of OCT systems. The new manufacturing methods include the replication of microstructures printed via direct laser writing into PMMA photoresist, the tailored smoothing of surface roughness inherent to direct laser writing, and the selective retention of surface roughness in certain regions. Each of these methods were implemented in the manufacture of an OCT sensitivity phantom and were found to be effective in each of their respective goals.The efficacy of the sensitivity phantom in evaluating the minimum reflectance still detectable by an OCT system shows promise. Effective reflectivity ranging from 0 to ~1 was accomplished within a single angled element and should provide a basis for determining the minimum reflectivity that results in a signal-to-noise ratio of 1. Further improvements must be made to the phantom footprint and manufacturing before the phantom’s reliability is certain.
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    (2023) Zhang, Xiguang; Duncan, James; Liu, Xinan; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The effects of temperature and surfactant on secondary droplets produced by the impact of raindrops on water surface were experimentally studied in a rain facility that consists of a rain generator and a deep water pool. The rain generator is a 0.9 m × 0.6 m rectangular tank with 360 hypodermic needles mounted on its bottom. A constant water height is maintained in the tank to obtain a constant dripping rate of raindrops from the needles. The rain generator is placed 2.2 meters above the water pool that is 1.22 m long by 1.22 m wide with a water depth of 0.31 m. A circular motion of the rain generator varies the impact locations of the raindrops on the water surface.Both the raindrops and secondary droplets are measured with an in-line holographic technique that employs a collimated laser beam and a high-speed camera. The diameters and two-dimensional positions of the raindrops and secondary droplets were first reconstructed in each holographic image using a GPU-based holographic reconstruction algorithm. Then an in-house particle tracking code was implemented to compute their diameters, trajectories and instantaneous velocities. The measurement data shown in this thesis was taken at 9.5 cm above the water surface of the pool. In this study, the effects of temperature and surface tension on the production of the secondary droplets were examined separately. When studying the temperature effect, the temperature of the water in the rain generator varied from 7 degrees Celsius to 20 degrees Celsius (room temperature) while the water temperature in the pool was maintained at room temperature. When studying the surface tension effect, certain amounts of soluble surfactant (Triton X-100) was added into the water pool to vary the surface tension from 40 mN/m to 73 mN/m, while the rain water is kept clean with a surface tension of 73 mN/m. It is found that both the rain temperature and the surface tension of the water pool have an impact on the production of secondary droplets. The temperature of the rain could change the viscosity by more than 40%, therefore resulting in a significant difference in the number and the size distribution of the production of secondary droplets. On the other hand, while the surface tension of the water pool does not heavily influence the number of secondary droplets, it does contribute to a difference in size distributions of these droplets at around R = 120 μm.
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    (2023) Sangepu, Lokesh; Das, Diganta; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Electronic part manufacturers often provide reliability values in metrics such as Mean Time Between Failures (MTBF) and its inverse, Failures in Time (FIT). These metrics assume a constant failure rate and do not account for damage accumulation or wear-out phenomena, making the part selection and management based on this information meaningless.This thesis will report on the challenges associated with manufacturers' avoidance of sharing critical part information and how insufficient information hampers decision-making for part selection. The thesis uses four die-level failure mechanisms (Electromigration, Time-Dependent Dielectric Breakdown, Hot Carrier Injection, and Negative Bias Temperature Instability) as demonstration cases. It investigates the extent to which industry-published documents can be used to obtain the data necessary to simulate these mechanisms. It will report on methods of selecting an appropriate failure model based on the part technology level and identifying the required parameters for estimating the part's time to failure. Various scattered part information sources, literature, and industry-published documents may include the input parameters of failure models. The thesis provides insights into the complexity of understanding these information sources and various methods to obtain the required parameters to estimate the time to failure distributions. The methodology considers the susceptibility of parts to die-level failure mechanisms and compares components for reliability. A simulation template that facilitates practical implementation by enabling designers, engineers, and procurement teams to make informed decisions while selecting electronic parts for specific applications is introduced. The research findings and methodology presented provide valuable insights for users to improve the reliability and performance of electronic systems through effective part selection.
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    (2023) Kamrah, Eesh; Fuge, Mark D; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Design researchers have struggled to produce quantitative predictions for exactly why andwhen diversity might help or hinder design search efforts. This thesis addresses that problem by studying one ubiquitously used search strategy—Bayesian Optimization (BO)—on different ND test problems with modifiable convexity and difficulty. Specifically, we test how providing diverse versus non-diverse initial samples to BO affects its performance during search and introduce a fast ranked-DPP method for computing diverse sets, which we need to detect sets of highly diverse or non-diverse initial samples. We initially found, to our surprise, that diversity did not appear to affect BO, neither helping nor hurting the optimizer’s convergence. However, follow-on experiments illuminated a key trade-off. Non-diverse initial samples hastened posterior convergence for the underlying model hyper-parameters—a Model Building advantage. In contrast, diverse initial samples accelerated exploring the function itself—a Space Exploration advantage. Both advantages help BO, but in different ways, and the initial sample diversity directly modulates how BO trades those advantages. Indeed, we show that fixing the BO hyper-parameters removes the Model Building advantage, causing diverse initial samples to always outperform models trained with non-diverse samples. These findings shed light on why, at least for BO-type optimizers, the use of diversity has mixed effects and cautions against the ubiquitous use of space-filling initializations in BO. To the extent that humans use explore-exploit search strategies similar to BO, our results provide a testable conjecture for why and when diversity may affect human-subject or design team experiments. The thesis is organized as follows: Chapter 2 provides an overview of existing studies that explore the impact of different initial stimuli. In Chapter 3, we explain the methodology used in the subsequent experiments. Chapter 4 presents the results of our initial study on the diverse initialization of BO (Bayesian Optimization) applied to the wildcat wells function. In this chapter we also investigate the conditions under which less diverse initial examples perform better and expand on these findings in Chapter 5 by considering additional ND continuous functions. The final chapter discusses the limitations of our findings and proposes potential areas for future research.
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    In-situ investigation of lithium dendrite growth and its interactions with a polymer separator in a lithium metal cell
    (2023) Kong, Lingxi; Pecht, Michael; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Lithium dendrites are metallic structures that initiate and grow inside a lithium battery duringcharging. Lithium dendrite growth can negatively affect battery cycle life and safety. Observing the dendrite growth process and revealing its interaction with other components is necessary to improve battery safety. This study uses a transparent optical cell to directly observe the dendrite growth process, explore the lithium dendrite growth modes under various current densities, evaluate the interactions between the dendrite and separator, and explore the effect of electrolyte additives on dendrite growth behavior. The dendrite growth under different current densities showed the transition of dendrite morphologies from a dense structure to a porous structure. The examination of the dendrite-separator interaction regions showed that dendrites can deform and penetrate the separator. We show that additives can enhance the uniformity of lithium dendrite distribution compared with the dendrite formed in the electrolyte without additives.