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
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Item New Methodology for Predicting Ultimate Capacity of One-Sided Composite Patch Repaired Aluminum Plate(2019) Hart, Daniel C; Bruck, Hugh A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Composite patch repairs are an alternative to traditional weld repair methods to address cracking in aluminum plates. Analytical and numerical design methods use linear elastic fracture mechanics (LEFM) and do not account for elastic-plastic crack tip behavior demonstrated in static tests of one-sided patch repaired ductile panels. This research used digital image correlation (DIC) and three-dimensional finite element analysis (FEA) with first order elements to study crack tip effects due to the one-sided composite patch applied to center crack tension (CCT) specimens loaded monotonically to failure. The measurable effects on crack tip behavior due to the composite patch were ultimate tensile load increase of more than 100% and a total achieved crack opening displacement (COD) increase of 20% over the unpatched behavior. Crack tip fracture behavior was found to be an intrinsic property of the aluminum and directly related to the COD independent of the one-sided composite patch. Increased capacity was related to accumulation of large-strain free surface area and through thickness volume ahead of the crack tip. Test data and numerical predictions correlated with measured load, strain, displacement fields, and J-integral behavior. Correlation of displacement fields with HRR and K fields established a state of small scale yielding prior to failure. Data and predictions indicated critical COD occurs when unpatched and patched large strain area is equivalent, which occurs before crack tip behavior transitions from small scale to large scale yielding and crack growth. Identifying a critical COD for both small and large scale one-sided patch repaired cracked ductile panels results in a predicted failure closer to the ultimate tensile load and 80% greater than predicted with LEFM methods. Observations and predictions demonstrated in this research resulted in three scientific contributions: (1) development of criteria to determine crack growth in cracked ductile panels repaired with a one-sided composite patch using a critical COD, (2) development of a three-dimensional FEA to study development of the plastic zone and evolution of the large-strain region ahead of the crack tip, and (3) development of a numerical methodology to predict ultimate tensile load capacity of cracked ductile panels repaired with a one-sided composite patch.Item Development of a Fatigue Life Assessment Model for Pairing Fatigue-Damage Prognoses with Bridge Management Systems(2015) Saad, Timothy; Fu, Chung C; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fatigue damage is one of the primary safety concerns for steel bridges reaching the end of their design life. Currently, federal requirements mandate regular inspection of steel bridges for fatigue cracks with evaluative reporting to bridge management systems. The quality of the inspection is subjective and time delayed due to inspection cycles, which are scheduled for every two years. However, structural health monitoring (SHM) data collected between inspection-intervals can provide supplementary information on structural condition that ameliorates some drawbacks of current inspection methods. Through the use of SHM and finite element models, fatigue performance assessments can be utilized throughout the service life of fatigue sensitive bridge elements for mitigating fatigue damage and preventing sudden fatigue failure. These assessments will additionally be useful to inspectors when reporting bridge condition evaluations to bridge management systems. The main goal of this study is to develop a fatigue life assessment method used for determining the remaining useful life of steel bridges and to map these results to existing bridge management systems. In order to achieve this goal, the current practices and methodologies associated with fatigue life of bridge elements and the use of bridge management systems are investigated. For analyses of fatigue damage, the fatigue life is split into two different periods of analyses: a crack initiation period and crack growth period. In order to quantify the effects of fatigue damage, each period of the fatigue life is associated with a unique assessment method, an empirical correlation assessment and a fracture mechanics assessment. Structural health monitoring techniques are employed to monitor the behavior of the bridge components and bridge elements. These two assessment methods are combined to form a damage accumulation model to estimate the fatigue life. The proposed damage accumulation model uses the acquired data from structural health monitoring alongside finite element modeling to derive a damage prognosis of bridge elements. The damage prognosis attempts to forecast the structure's performance by measuring the cumulative fatigue damage, estimating future loads, and ultimately determining the remaining useful life of the bridge element. A technique for mapping the results of the damage prognosis into condition state classifications is proposed. The suitability and applicability of the proposed damage accumulation model is illustrated on an existing highway bridge. This bridge was selected as a good candidate for fatigue monitoring due to the average daily truck traffic and the identification of existing and active fatigue cracks. The application of the damage accumulation model is demonstrated and a damage prognosis is derived. Finally, the damage accumulation results are integrated with current condition state classifications used in bridge management systems.Item FRACTURE BEHAVIOR AND THERMAL CONDUCTIVITY OF POLYCRYSTALLINE GRAPHENE(2014) Fox, Andrew Oliver; Li, Teng; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation investigates the effect of grain boundaries (GBs) in polycrystalline graphene on the tensile fracture behavior and thermal conductivity of the graphene sheets. Current techniques to fabricate large-scale graphene intrinsically introduce defects, e.g., GBs, resulting in polycrystalline graphene sheets. Though GBs in graphene are expected to affect the mechanical properties of graphene, mechanistic understanding and quantitative determination of such effects are far from mature. For example, existing studies on the effect of GBs on the tensile behavior of graphene only focus on a twin GB perpendicular to the tensile loading direction. However, GBs in a polycrystalline graphene sheet under uniaxial tension could be subject to tension in any arbitrary directions, depending on the GB and grain orientation in the graphene sheet. In this dissertation, we focus on the effect of GBs on the tensile and thermal response of polycrystalline graphene. The fracture process of polycrystalline graphene sheets under uniaxial tension was studied using molecular dynamics (MD) simulations to determine how GBs affects the ultimate strength and critical failure strain of the graphene. We also study the flow of heat through polycrystalline graphene to determine the effect of GBs on the thermal conductivity of graphene. A comprehensive study including 24 GB misorientation angles ranging from 2.1° to 54.3° and the whole range of loading angle (i.e., that between a GB and in-plane tensile loading direction, ranging from 0° to 90°) was carried out to quantitatively determine the effect of GBs. Stress-strain data were generated from the MD simulations and the failure strength and critical strain were analyzed. A theoretical model combining continuum mechanics theory and disclination dipole theory was introduced to predict the failure strength of the polycrystalline graphene sheets, which was shown to be in good agreement with the MD simulation results. Various failure modes of polycrystalline graphene under tension were also analyzed. The thermal conductivity of polycrystalline graphene as a function of GB misorientation angle and thermal loading angle was also quantitatively determined through systematic simulations. The quantitative findings from this dissertation could potentially bridge the knowledge gap toward a better understanding of defects and their effects on two-dimensional materials, and also shed light on possible defect control and engineering to achieve desirable properties of graphene in applications.Item Determination of Mixed Mode Energy Release Rates in Laminated Carbon Fiber Composite Structures Using Digital Image Correlation(2012) Puishys, Joseph Francis; Bruck, Hugh A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Carbon fiber composites have recently seen a large scale application in industry due to its high strength and low weight. Despite numerous beneficial attributes of composite materials, they are subject to several unique challenges; the most prevalent and troubling is delamination fracture. This research program is focused on developing an appropriate damage model capable of analyzing microscopic stress strain growth at the crack tip of laminated composites. This thesis focuses on capturing and identifying the varying stress and strain fields, as well as other microstructural details and phenomena unique to crack tip propagation in carbon fiber panels using a novel mechanical characterization technique known as Digital Image Correlation (DIC).Item Fracture of Brittle Layers Joined with High Elastic Modulus Composite(2007-04-25) Lee, James Jin-Wu; Lloyd, Isabel K; Lawn, Brian R; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Ceramic properties such as biocompatibility and inertness have secured their use in biomedical prosthetics. The brittle nature of ceramics governs their application in any design and fabrication technique. Current all-ceramic dental crowns have a reported failure rate of approximately 3% a year. An investigation into a possible improved design over current all-ceramic dental crowns is performed. Current methods of fabricating all-ceramic dental crowns involve laborious and time consuming application of porcelain veneer layers onto a core material. The proposed design is to join independently fabricated veneer and core layers together using a high elastic modulus composite. Fracture behavior of brittle layers joined by a high elastic modulus composite is studied in this dissertation. There are two dominant fracture mechanisms of concern for dental crowns when joining brittle layers with a more compliant interlayer; the formation of radial cracks in the veneer or core and the propagation of cracks between brittle layers. The occlusal loading on dental crowns can be simulated with the use of Hertzian contact testing on flat brittle laminates, which allow for the study of radial cracks in the veneer. It is shown for the first time that bottom-surface radial cracks in the veneer due to flexure can be suppressed using a high elastic modulus joining interlayer. The relationship between the critical loads for radial crack formation, Pcr and the interlayer modulus and thickness is elucidated. Furthermore, using the high modulus composite as an interlayer increases the long term cyclic loading lifetime over joins with similar moduli to currently available dental adhesives. The propagation of cracks between adjacent brittle layers is shown to be controlled by a reinitiation mechanism and not penetration through the adhesive. Cracks that traverse the layer of origin arrest at the join interface in brittle laminates. Reinitiation loads are dictated by strength of the adjacent brittle layer and modulus of adhesive. This study shows that it is possible to use a high modulus composite as a joining material in the fabrication of dental crowns, while suppressing the formation of radial cracks in the veneer and limiting the propagation of cracks between adjacent brittle layers.