Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item THE DEI SIGNALING THRESHOLD: WHEN AND WHY MORE MESSAGING IS NOT ALWAYS BETTER(2024) Holmes, Tara; Derfler-Rozin, Rellie; Business and Management: Management & Organization; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)When it comes to messaging diversity, equity, and inclusion (DEI) efforts to employees, organizations take great care in considering the content of the signals they create. However, despite carefully designed communications, they continue to struggle to garner employee support and participation for these initiatives. Counter to the prevailing assumption that more DEI signaling is better (Roberson, 2006; Plaut et al., 2011; Nishii, 2013; Richard et al., 2013; Leslie, 2019; Hunt et al., 2020; Shuman et al., 2023), I argue that positive effects of organizational DEI signaling do not persist with increased exposure to DEI-related stimuli. Leveraging exposure effect research, I instead propose that employee attitudes shift from positive to negative as exposure to signaling increases, thereby decreasing their desire to engage with DEI at work. Specifically, I hypothesize that low and moderate levels of signaling are associated with employees feeling more engagement towards DEI, but at higher DEI fatigue and cynicism are more likely to develop, negatively impacting employees’ DEI effort. I further posit that because managers play a central role in shaping employee attitudes and behaviors, a manager’s consistency with organizational DEI signaling is the key to minimizing negative employee attitudes that emerge because of overexposure. I test these hypotheses in an experiment and a field study with implications for the literatures on DEI in organizations, issue fatigue, and behavioral integrity.Item THE EFFECTS OF TRAINING HABITS ON CUMULATIVE LOAD AND TIBIAL STRESS FRACTURE INJURY RISK IN RUNNERS(2020) Hunter, Jessica G.; Miller, Ross H.; Kinesiology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Running for exercise is beneficial for preventing chronic diseases, but the incidence and prevalence of running related injuries are high, creating a barrier to participation. Traditional research paradigms attribute high running injury rates to anatomical factors, training habits, and high peak loads resulting from gait mechanics. However, the specific mechanisms of tibial stress fracture injuries, a serious running-related injury, and why females are at such high risk for these injuries, are largely unknown. Runners often train at variable running speeds and durations that can affect the accumulation of potentially injurious loads, but until recently, studies on running injuries have mostly considered training habits and mechanical loads separately. Therefore, the purpose of this dissertation was to identify how training factors of running speed, volume, and duration contribute to the loads accumulated by the body in relation to tibial stress fracture injury risk. Specifically, this dissertation consists of three studies which determine i) the cumulative load of two proportions of running speed over a constant distance and average pace of running, ii) how fatigue-related gait adjustments affect the loads accumulated per-kilometer within a single prolonged run, and if there is a relationship between gait adjustments and physiological or cognitive fatigue outcomes; and iii) if fatigue-related changes in running gait affect the model-predicted cumulative damage and probability of tibial stress fracture. In study 1, a combination of slow and fast speeds led to greater estimated cumulative load compared to running at all normal speed. The greater cumulative load resulted from greater loading during slow running compared to fast running. In study 2, runners maintained gait mechanics and cumulative loads throughout an easy run to fatigue. In study 3, the model-predicted cumulative damage and probability of tibial stress fracture injury were similar between hypothetically maintained gait and measured fatigue-adjusted gait conditions. These results suggest running volume and average pace are not sufficient metrics for tracking cumulative load, and fatigue during running is not likely a major injury risk factor. Further, these results suggest that other training factors or individual factors may play a greater role in injury development than running speed, volume, or fatigue.Item Excitation-Contraction Coupling Disruption in a Mouse Model of Niemann-Pick Disease(2017) Li, Harry Zichen; Chin, Eva R; Kinesiology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Niemann-Pick disease (NPD) is a lysosomal storage disorder that results from deficient acid sphingomyelinase (ASM) activity. It was recently proposed that ASM and extracellular Ca2+ are required for membrane repair. Since plasma membrane integrity is an important component of excitation-contraction coupling (E-C) and skeletal muscle force production, we hypothesized that there would be E-C coupling defects in NPD related to intracellular calcium (Ca2+) dynamics. Our results demonstrate that ASM deficient (ASM-/-) fibers have a reduced ability to withstand repetitive contractions in comparison to wild-type (WT) fibers, and fibers from ASM-/- mice exhibited lower peak tetanic Ca2+ compared to WT. Lastly, no differences in peak tetanic Ca2+ were found between ASM-/- fibers and WT fibers deprived of Ca2+. Together, these results suggest that both ASM and extracellular Ca2+ are required for optimal E-C coupling in skeletal muscle and for the ability to respond to repetitive contractions that occurs with sustained activity.Item DEVELOPMENT OF AN OFF-LINE RAINFLOW COUNTING ALGORITHM WITH TEMPORAL PARAMETERS AND DATA REDUCTION(2016) Twomey, James Matthew; Pecht, Michael G; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Rainflow counting methods convert a complex load time history into a set of load reversals for use in fatigue damage modeling. Rainflow counting methods were originally developed to assess fatigue damage associated with mechanical cycling where creep of the material under load was not considered to be a significant contributor to failure. However, creep is a significant factor in some cyclic loading cases such as solder interconnects under temperature cycling. In this case, fatigue life models require the dwell time to account for stress relaxation and creep. This study develops a new version of the multi-parameter rainflow counting algorithm that provides a range-based dwell time estimation for use with time-dependent fatigue damage models. To show the applicability, the method is used to calculate the life of solder joints under a complex thermal cycling regime and is verified by experimental testing. An additional algorithm is developed in this study to provide data reduction in the results of the rainflow counting. This algorithm uses a damage model and a statistical test to determine which of the resultant cycles are statistically insignificant to a given confidence level. This makes the resulting data file to be smaller, and for a simplified load history to be reconstructed.Item Damage Detection in Fiber Reinforced Concrete with Ultrasonic Pulse Velocity Testing(2012) Hong, Rongjin; Goulias, Dimitrios G; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In concrete, fatigue and freeze-thaw are associated with the progressive growth of internal microcracks. The Ultrasonic Pulse Velocity (UPV) technique, one of the most widely used Nondestructive Testing (NDT) methods, is promising in evaluating internal microcracks and eventually detecting damage. The primary objective of this research was to determine the effectiveness of using UPV to detect damage development in polypropylene fiber reinforced concrete under fatigue and freeze-thaw conditions. In order to realize this, i) several experiments were conducted on control samples to assess the response and limitations of UPV, and ii) fatigue and freeze-thaw samples were tested with UPV to evaluate its ability to detect crack development. In terms of modeling, three alternative models were examined and presented relating UPV with porosity and damage.Item EVOLUTION OF THE MICROSTRUCTURE AND VISCOPLASTIC BEHAVIOR OF MICROSCALE SAC305 SOLDER JOINTS AS A FUNCTION OF MECHANICAL FATIGUE DAMAGE(2009) Cuddalorepatta, Gayatri; Dasgupta, Abhijit; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The effect of mechanical cycling fatigue damage and isothermal aging histories on the evolution of the constitutive and fatigue responses, and microstructure of microscale SAC305 solder joints is investigated. In particular, the study examines if joint dependent behavior should be expected from as-fabricated and cycled microscale SAC305 joints that exhibit an initial non-homogenous coarse-grained Sn microstructure. In addition, the ability of traditionally used macroscale constitutive models based on continuum mechanics to represent the viscoplastic constitutive behavior of the non-homogenous as-fabricated microscale SAC305 specimens is explored. Insights into the effect of key microstructural features and dominant creep mechanisms influencing the measured viscoplastic behavior of SAC305 are provided using a multi-scale mechanistic modeling framework. Modified lap-shear microscale SAC305 specimens are characterized using the thermomechanical microscale test setup (TMM). Microscale SAC305 solder specimens show significant piece-to-piece variability in the viscoplastic constitutive properties under identical loading histories in the as-fabricated state. The mechanical response is strongly influenced by the grain microstructure across the entire joint, which is non-repeatable and comprises of very few highly anisotropic Sn grains. The statistical non-homogeneity in the microstructure and the associated variability in the mechanical properties in the microscale SAC305 test specimen are far more significant than in similar Sn37Pb specimens, and are consistent with those reported for functional microelectronics solder interconnects. In spite of the scatter, as-fabricated SAC305 specimens exhibit superior creep-resistance (and lower stress relaxation) than Sn37Pb. Macroscale creep model constants represent the non-homogeneous behavior of microscale joints in an average sense. Macroscale modeling results show that the range of scatter measured from macroscale creep model constants is within the range of scatter obtained from the stress relaxation predictions. Stress relaxation predictions are strongly sensitive to the inclusion or exclusion of primary creep models. The proposed multiscale framework effectively captures the dominant creep deformation mechanisms and the influence of key microstructural features on the measured secondary creep response of microscale as-fabricated SAC305 solder specimens. The multiscale model predictions for the effect of alloy composition on SAC solders provide good agreement with test measurements. The multiscale model can be extended to understand the effects of other parameters such as aging and manufacturing profiles, thereby aiding in the effective design and optimization of the viscoplastic behavior of SAC alloys. Accumulated fatigue damage and isothermal aging are found to degrade the constitutive and mechanical fatigue properties of the solder. The scatter gradually decreases with an increasing state of solder damage. Compared to the elastic-plastic and creep measurements, the variability in the fatigue life of these non-homogenous solder joints under mechanical fatigue tests is negligible. Recrystallization is evident under creep and mechanical fatigue loads. Gradual homogenization of the Sn grain microstructure with damage is a possible reason for the observed evolution of scatter in the isothermal mechanical fatigue curves. The yield stress measurements suggest that SAC305 obeys a hardening rule different from that of isotropic or kinematic hardening. The measured degradation in elastic, plastic and yield properties is captured reasonably well with a continuum damage mechanics model from the literature.Item Reliability-Based Design Of Piping: Internal Pressure, Gravity, Earthquake, and Thermal Expansion(2007-08-09) Avrithi, Kleio; Ayyub, Bilal M.; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Although reliability theory has offered the means for reasonably accounting for the design uncertainties of structural components, limited effort has been made to estimate and control the probability of failure for mechanical components, such as piping. The ASME B&PV Code, Section III, used today for the design of safety piping in nuclear plants is based on the traditional Allowable Stress Design (ASD) method. This dissertation can be considered as a primary step towards the reliability-based design of nuclear safety piping. Design equations are developed according to the Load and Resistance Factor Design (LRFD) method. The loads addressed are the sustained weight, internal pressure, and dynamic loading (e.g., earthquake). The dissertation provides load combinations, and a database of statistical information on basic variables (strength of steel, geometry, and loads). Uncertainties associated with selected ultimate strength prediction models -burst or yielding due to internal pressure and the ultimate bending moment capacity- are quantified for piping. The procedure is based on evaluation of experimental results cited in literature. Partial load and resistance factors are computed for the load combinations and for selected values of the target reliability index, β. Moreover, design examples demonstrate the procedure of the computations. A probabilistic-based method especially for Class 2 and 3 piping is proposed by considering only cycling moment loading (e.g., thermal expansion). Conclusions of the study and provided suggestions can be used for future research.