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 HEALTH IMPACTS OF THERMAL RUNAWAY EVENTS IN OUTDOOR LITHIUM-ION BATTERY ENERGY STORAGE SYSTEM INSTALLATIONS(2024) Zhao, Zelda Qijing; McAllister, Jamie; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This study aimed to develop a methodology for characterizing health impacts of large-scale, outdoor, lithium-ion battery energy storage systems (BESS) thermal runaway events. A literature review was conducted to identify toxic gas yields produced during flaming and non-flaming thermal runaway, as well as mass loss rates, gas temperature, typical BESS unit capacity and dimensions, and event durations. Lithium-iron-phosphate and nickel-manganese-cobalt cell chemistries were assessed. The BESS unit thermal runaway events were modeled in Fire Dynamics Simulator with a bounding analysis for wind and ambient temperature. Concentrations were evaluated using Immediately Dangerous to Life or Health values for occupational exposure and the Protective Action Criteria for Chemicals hierarchy values (Acute Exposure Guideline Levels- Level 1, Emergency Response Planning Guidelines- Level 1, Temporary Emergency Exposure Limits- Level 1) for community exposure. Through application of the methodology, a relationship between exposure limit distance and wind speed, ambient temperature, event duration, cell chemistry, and toxic gas species can be assessed. Under the conditions modeled in this project, exposure limits were exceeded at longer distances in the non-flaming scenarios when compared to the flaming scenarios. Wind speed, ambient temperature, event duration, cell chemistry, and toxic gas species were the controlling factors for non-flaming exposure limit distances. Wind speed was the primary controlling factor for flaming exposure limit distances; however, event duration had some influence.Item FATIGUE DEGRADATION SENSING WITH SURFACE MOUNTED CONJUGATE-STRESS (CS) SENSOR(2024) Bascolo, Manuel; Dasgupta, Abhijit AD; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This study advances a unique dual-stiffness mechanical sensor concept in the literature (termed Conjugate-stress sensor), to evaluate and validate the effectiveness of surface-mounted versions of the Conjugate Stress (CS) sensor in detecting cyclic fatigue progression under both quasi-static axial cycling and dynamic flexural loading conditions. The CS sensor’s capability to recognize fatigue was examined by observing the correlation between its readings and the host material's stiffness. Low carbon steel dog bone coupons and welded cruciform specimens were subjected to quasi-static cyclic fatigue testing. Additionally, dynamic flexural tests were performed on low carbon steel cantilever beams and welded cruciform specimens, which underwent random vibration fatigue testing. The results demonstrated that CS sensors consistently track fatigue damage, offering a promising potential for in-situ structural health monitoring and for providing continuous, real-time estimations of the remaining useful life (RUL) of materialsItem TUBE-LOAD MODEL AS A DIGITAL TWIN FOR ABDOMINAL AORTIC ANEURYSM PATIENTS(2024) Kim, Donghyeon; Hahn, Jin-Oh; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Abdominal aortic aneurysm (AAA) is a life-threatening condition characterized by the abnormal dilation of the aorta, which can lead to vessel rupture and high mortality rates (>80%). Alarmingly, AAA is often asymptomatic and can remain undetected until it reaches a critical size or ruptures. Current methods for diagnosing and monitoring AAA, such as ultrasound, CT, and MRI, are effective but expensive for regular use and require specialized operators. These limitations hinder the widespread use of imaging-based techniques for regular AAA screening and surveillance. Therefore, creating a need for more accessible, affordable, and convenient tools to detect AAA in its early stages, monitor its progression, and assess treatment efficacy. This thesis explores the potential of tube-load (TL) model to non-invasively monitor AAA progression by analyzing arterial pressure waveforms, which change in response to aneurysm-induced alterations in aortic geometry and mechanical properties. These changes are captured and revealed by the parameters of the TL model. To evaluate the TL model’s capability to monitor AAA, we applied it to carotid and femoral artery tonometry waveforms collected from 79 subjects, including both controls and AAA subjects, as well as a subset of 35 AAA subjects before and after endovascular repair (EVAR) surgery. Our analysis showed that the TL model could fit the waveforms from pre-EVAR AAA subjects as accurately as those from controls and post-EVAR. Moreover, the TL model parameters exhibited physiologically explainable changes consistent with the structural changes of the aorta associated with AAA and its treatment. These findings suggest that the TL model has the potential as a digital twin to enable convenient and cost-effective personalized AAA monitoring.Item FROM PARTS TO WHOLE IN ACTION AND OBJECT UNDERSTANDING(2024) Devaraj, Chinmaya; Aloimonos, Yiannis; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The traditional paradigm of supervised learning in action or object recognition often relieson a top-down approach, ignoring explicit modeling of what activity or objects consist of. Recent approaches in generative AI research have shown us the ability to generate images and videos using text, indirectly indicating that we have control over the constituents of images and videos. In this dissertation, we explore ways to use the constituents of actions to develop methods to improve understanding of action. We devise different approaches to utilize the parts of actions, namely object motion, object state changes, and motion descriptions obtained by LLMs in various tasks like in the next active object segmentation, zero-shot action recognition, or video-text retrieval. We show promising benefits in action anticipation, zero-shot action recognition, and text-video retrieval tasks, demonstrating the practical applications of our methods. In the first part of the dissertation, we explore the idea of using the constituents of actions inGCNs for zero-shot human-object action recognition. The main idea is that semantically similar actions (of similar constituents) are closer in feature space. Thus, in our graph, we encode the edges connecting those actions with higher similarity. We introduce a method to visually ground the external knowledge graph using the concept of shared similarity between similar actions. We evaluate the method on the EPIC Kitchens dataset and the Charades dataset showing impressive results over baseline methods. We further show that visually grounding the knowledge graph enhances the performance of GCNs when an adversarial attack corrupts the input graph. In the second part of the thesis, we extend our ideas on human-object interactions in firstpersonvideos. Human actions involving hand manipulations are structured according to the making and breaking of hand-object contact, and human visual understanding of action relies on anticipation of contact, as demonstrated by pioneering work in cognitive science. Taking inspiration from this, we introduce representations and models centered on contact, which we then use in action prediction and anticipation. We train the Anticipation Module, a module producing Contact Anticipation Maps and Next Active Object Segmentations - novel low-level representations providing temporal and spatial characteristics of anticipated near future action. On top of the Anticipation Module, we apply Egocentric Object Manipulation Graphs (Ego- OMG), a framework for action anticipation and prediction. Using the Anticipation Module to aid Ego-OMG produces state-of-the-art results, achieving first and second places on the unseen and seen test sets of the EPIC Kitchens Action Anticipation Challenge and achieving state-of-the-art results on action anticipation and action prediction over EPIC Kitchens. In the same line of thinking of constituents of action, we next focus on investigatinghow motion understanding can be modeled in current video-text models. We introduce motion descriptions generated by GPT4 on three action datasets that capture fine-grained motion descriptions of activities. We evaluated several video-text models on the task of retrieval of motion descriptions and found them to need to catch up to the human expert performance. We introduce a method of improving motion understanding in video-text models by utilizing motion descriptions. This method is demonstrated on two action datasets for the motion description retrieval task. The results draw attention to the need for quality captions involving fine-grained motion information in existing datasets and demonstrate the effectiveness of the proposed pipeline in understanding fine-grained motion during video-text retrieval.Item Dynamics, Estimation, and Control for Stabilizing the Attitude and Shape of a Flexible Spacecraft(2024) Merrill, Curtis; Paley, Derek A; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Advances in technology have enabled the development of large spacecraft structures such as solar sails, expansive antennas, and large solar arrays. A critical design constraint for these structures is mass, necessitating lightweight construction which, in turn, increases structural flexibility. This flexibility poses significant challenges resulting from structural deformations and vibrations that complicate attitude control and can degrade the performance and lifespan of the spacecraft. The goal of this research is to develop estimation and control strategies to mitigate the effects of spacecraft flexibility.A flexible spacecraft model is derived using a hub and appendage framework. In this model one or more flexible appendages attach to a central rigid hub. The model represents the appendages as a discretized set of flexibly connected elements called panels. Stiff springs connect the panels, and the dynamic model of the system’s internal forces and moments uses coordinates in the hub’s reference frame. Reaction wheels on the hub perform attitude control, while distributed pairs of magnetic torque rods on the appendage influence its shape. Initially, the model restricts flexibility to one direction, resulting in a planar model. A Lyapunov-based control design provides a feedback law for the reaction wheel and torque rods in the planar model. Numerical simulations demonstrate that the proposed controller meets the control objectives and compares favorably to other controllers. An Extended Kalman Filter is applied to the system to perform state estimation and output feedback control, which performs at nearly the same level as state feedback control. The modeling framework and flexibility are extended to three dimensions. The development of a control law for the magnetic torque rods considers the attitude control of a single panel using two magnetic torque rods. Due to the system being underactuated, the attitude error is defined in terms of the reduced-attitude representation. Lyapunov analysis yields a control law that stabilizes the reduced attitude and angular velocity of a rigid panel using only two magnetic torque rods. Numerical simulations validate the control law’s performance for a single panel. This control law is then applied to the flexible appendage to stabilize its shape. Numerical simulations show that this implementation of shape control significantly reduces structural deformations and dampens structural oscillations compared to scenarios without shape control. To perform state estimation of the high-dimensional flexible spacecraft model, dynamic mode decomposition generates a reduced order model that is linear with respect to the evolution of the resulting modes. A Kalman filter estimates the mode amplitudes of the reduced order model from a limited set of measurements, enabling the reconstruction of the entire system state. The optimization of the number and placement of sensors maximizes the observability of the observer. Numerical simulations demonstrate that this framework yields accurate state estimates with reduced computational cost.Item DEVELOPMENT AND VALIDATION OF A PYROLYSIS MODEL FOR FLEXIBLE POLYURETHANE FOAM(2024) Kamma, Siriwipa; Stoliarov, Stanislav I; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Flexible polyurethane foam (FPUF) is a common material contained in household goods such as upholstered furniture and mattresses, which are known to significantly contribute to fire growth. An accurate prediction of fire development on FPUF containing items requires knowledge of FPUF pyrolysis and combustion properties. These properties include reaction kinetic parameters, thermodynamic parameters, and thermal transport properties. While many past studies focused on the thermal decomposing mechanism and thermodynamic properties of the reactions, the thermal transport properties have not been determined. In this study, a complete pyrolysis model of FPUF was developed by extending the thermal decomposition model from a previous study. The thermal transport properties were obtained using inverse modeling of the Controlled Atmosphere Pyrolysis Apparatus II experimental data. The complete model was validated against cone calorimetry data and found to perform in an adequate manner.Item Experimental Characterization of the Thermal Response of Firefighter Protective Ensembles Under Non-Flaming Convective Exposure(2024) DiPietro, Thomas Phillip; Raffan-Montoya, Fernando; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Thermal burns are one of the most serious injuries a firefighter can sustain while operating in a structure fire despite being fully covered in gear designed to protect them from thermal exposure. Extensive experimentation has been conducted into the performance of a firefighter’s protective ensemble when caught in a high radiative heat flux environment to ensure the wearer has enough time to escape to safety. High heat flux tests are beneficial in estimating safe operating times, but firefighters are also getting burned in fire environments that are thought to be routine exposures. The current study explored the thermal response of three-layer firefighter protective ensembles exposed to a majority convective, low-level heat flux in an oven. Through experimentation, the temperature of a copper calorimeter simulating skin beneath two different protective ensembles were measured while exposed to temperatures of 100°C, 150°C, 200°C, 250°C, and 300°C. The time for the copper calorimeter to reach a temperature of 55°C (the temperature a second-degree burn has the potential to occur to human skin) was recorded and compared to currently accepted thermal operating time limits for firefighters. Results show that once exposure reached above 100°C the time for a potential burn injury to occur fell below the predicted safe operational time for firefighters of 15–20 minutes when the PPE was in contact with the copper disk. The time to potential burn injury and test temperature exhibited an exponentially decaying relationship which is expected to continue as temperatures increase beyond those tested in the current study. Although consisting of different layers of material, both types of protective ensembles tested responded similarly and demonstrated no significant differences in time to potential burn injury at every temperature. Additional tests were conducted in the oven with an air gap placed below the protective ensemble as well as using the original test set up with a mostly radiative heat source to compare results and evaluate different exposures and conditions for future experimentation.Item MOLD PROCESS INDUCED RESIDUAL STRESS PREDICTION USING CURE EXTENT DEPENDENT VISCOELASTIC BEHAVIOR(2024) Phansalkar, Sukrut Prashant; Han, Bongtae; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Epoxy molding compounds (EMC) are widely used in encapsulation of semiconductor packages. Encapsulation protects the package from physical damage or corrosion due to harsh environments. Molding processes produce residual stresses in encapsulated components. They are combined with the stresses caused by the coefficient of thermal expansion (CTE) mismatch to dictate the final warpage at room and reflow temperatures, which must be controlled for fabrication of redistribution layer (RDL) as well as yield during assembly. During molding process, EMC is continuously curing and the mechanical properties continue to evolve; more specifically, the equilibrium modulus and the relaxation modulus. The former defines behavior after complete relaxation while the latter describes the transient behavior. It is thus necessary to measure cure-dependent viscoelastic properties of EMC to be able to determine mold induced residual stresses accurately. This is the motivation for this thesis. In this thesis, a set of novel methodologies are developed and implemented to quantify a complete set of cure-dependent viscoelastic properties, including the fully cured properties. Firstly, an advanced numerical scheme has been developed to quantify cure kinetics of thermosets with both single and dual-reaction systems. Secondly, a unique methodology is proposed to measure the viscoelastic bulk modulus -K(t,T) of EMC using hydrostatic testing. The methodology is implemented with a unique test setup based on inert gas. The results of viscoelastic testing along with the shear modulus (G) and bulk modulus (K) master curves and temperature-dependent shift factors (a(T)) of fully-cured EMC are presented. Thirdly, a novel test methodology utilizing monotonic testing has been developed to measure two sets of equilibrium moduli of EMC as a function of cure extent (p). The main challenge for the measurements is that the equilibrium moduli could only be measured accurately only when the EMC has fully relaxed. The temperatures for complete relaxation typically occur above the glass transition temperature, Tg (p), where the curing rate is also high. A special measurement procedure is developed, through which the EMC moduli above Tg can be determined quickly at a near isocure state. Viscoelastic testing of partially-cured EMC is followed to determine the cure-dependent shift factors of EMC. The test durations have to be very long (several hours) and it is conducted below Tg (p) of the EMC where the reaction is slow (under diffusion-control) Finally, a numerical scheme that can utilize the measured cure-dependent viscoelastic properties is developed. It is implemented to predict the residual stress evolution of molded packages during and after molding processes.Item INTERFACES IN THIN-FILM SOLID-STATE BATTERIES(2024) Castagna Ferrari, Victoria; Rubloff, Gary GWR; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The lack of a diagnostics approach to monitor interface kinetics in solid-state batteries (SSBs) results in an incomplete knowledge of the mechanisms affecting device performance. In this study, a new protocol for process control of SSB interface formation and their evolution during operation is presented. Thin-film SSBs and diagnostic test devices that are composed by a permutation of isolated layers were simultaneously fabricated using sequential sputtering deposition and in-situ patterning using shadow masks. Physics-based electric circuit models were designed for deconvolution of impedance profiles, which enabled an evaluation of bulk properties and space-charge layers at interfaces individually and during operation under different states-of-charge. Relative permittivity values of fundamental battery components (cathode, electrolyte and anode) were calculated as a function of the frequency and the applied voltage. Interfacial impedances, as well as space-charge layers formed at heterojunctions during charge and discharge processes, were successfully deconvoluted using the diagnostic test devices and electric circuit modeling. The cathode-electrolyte interphase was kinetically stable under a voltage window of 0 – 3.6 V vs Cu, and it had an estimated ionic conductivity of the order of 10-9 S/cm, hence it is a localized limiting factor for Li+ transfer. The anode-electrolyte interphase was thermodynamically stable upon completion of the fabrication process, but it became kinetically unstable during charge and discharge cycles. Hence, the proposed diagnostics protocol enlightened the necessity of implementing interfacial engineering on these interphases in the future for improvement of cyclability and stability of SSBs and ionic devices.Item VOLUMETRIC SOLAR ABSORBING FLUIDS AND THEIR APPLICATIONS IN TWO-PHASE THERMOSYPHON(2024) Zhou, Jian; Yang, Bao; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A two-phase thermosyphon is a passive system utilizing gravity to transfer working fluids. The working fluids of a two-phase thermosyphon must undergo vaporization and condensation in the same system. Two-phase thermosyphons can also be used as solar collectors. Traditional solar collectors utilize surface absorbers to convert incident solar radiation into thermal energy, but those systems feature a large temperature difference between the surface absorbers and heat transfer fluids, resulting in a reduction in the overall thermal efficiency. Volumetric solar absorbing fluids serve both as solar absorbers and heat transfer fluids, therefore significantly improving the overall efficiency of solar collectors. Comparing to pure fluids, nanofluids possess both enhanced thermal conductivity and solar absorption capacity as volumetric absorbing fluids. Nanofluids, when serving as volumetric solar absorbing fluids, are so far reported to work only at relatively low temperatures and in a single-phase heat transfer regime due to stability issue. This research investigates the possibility of using nanofluids, especially graphene oxide (GO) nanofluids, as volumetric solar absorbing fluids in two-phase thermosyphons. Despite their reputation as both stable and solar absorptive among nanofluids, graphene oxide nanofluids still deteriorate quickly under boiling-condensation processes (~100 °C). The solar transmittance of the GO nanofluids declines from 38 to 4%, during the first 24 h of testing. Further investigation shows that the stability deterioration is caused by the thermal reduction of GO nanoparticles, which mainly featured with de-carboxylation and de-hydroxylation. A commercial dye named acid black 52, when dissolved in water, exhibits great broadband solar absorption properties and excellent stability. It remains stable for over 199 days in two-phase thermosyphon, and their transmittance in solar spectral region varies less than 9%. The stability of acid black 52 aqueous solution is further confirmed with the 191-day enhanced radiation test, as it shows less than 5% transmittance change in solar spectral region.