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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Now showing 1 - 4 of 4
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    PROGRESSIVE MICROSCOPIC DAMAGE AND THE DEVELOPMENT OF MACROSCOPIC FRACTURE IN POROUS SANDSTONES
    (2011) Tamarkin, Thomas Francis; Zhu, Wenlu; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The precursory phenomena associated with dilatancy have been extensively studied as a potential means of earthquake prediction. It is known that microstructural damage induced dilatancy precedes macroscopic failure of a rock. However, the quantitative relationship between microstructural damage and fault development is not clearly understood. To better understand the mechanics of brittle faulting of rock and the association of precursory phenomena with faulting, a detailed microstructural study was conducted on porous sandstone deformed to different post-failure stages at different strain rates. A lateral relaxation loading configuration was adopted in which a cylindrical sample is deformed under decreasing radial stresses while the axial load remains constant. This loading path was proven to successfully map out the brittle failure envelope. Compared to conventional triaxial deformation testing, the relaxation loading configuration greatly increases the stability of fault growth. A suite of samples were deformed and subsequently unloaded at different post-failure stages, before macroscopic faulting occurred. Progressive microstructural damage was investigated via quantitative characterization of crack damage indices, crack density, and changes in porosity. Ultimately, this research will lead to an improved comprehension of the relationship between microscopic damage and macroscopic fracture development, providing a better insight into the brittle failure process.
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    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.
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    Microstructural Evolution in Friction Stir Welding of Ti-5111
    (2010) Wolk, Jennifer Nguyen; Salamanca-Riba, Lourdes; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Titanium and titanium alloys have shown excellent mechanical, physical, and corrosion properties. To address the needs of future naval combatants, this research examines an alternative joining technology, friction stir welding (FSW). Friction stir welding uses a non-consumable tool to generate frictional heat to plastically deform and mix metal to form a consolidated joint. This work focuses on FSW of Ti-5111 (Ti-5Al-1Sn-1Zr-1V-0.8Mo), a near alpha alloy. This study aims to gain a fundamental understanding of the relationship between processing parameters, microstructure, and mechanical properties of experimental 12.7mm and 6.35mm Ti-5111 friction stir welds. The resulting weld microstructure shows significant grain refinement within the weld compared to the base metal. The weld microstructures show a fully lamellar colony structure with peak welding temperatures exceeding beta transformation temperature. The friction stir weld shows material texture strengthening of the BCC F fiber component before transformation to D2 shear texture in the stir zone. Transmission electron microscopy results of the base metal and the stir zone show a lath colony-type structure with low dislocation density and no lath grain substructure. In situ TEM heating experiments of Ti-5111 friction stir welded material show transformation to the high temperature beta phase at significantly lower temperatures compared to the base metal. Thermal and deformation mechanisms within Ti-5111 were examined through the use of thermomechanical simulation. Isothermal constant strain rate tests show evidence of dynamic recrystallization and deformation above beta transus when compared with the FSW thermal profile without deformation. Subtransus deformation shows kinking and bending of the existing colony structure without recrystallization. Applying the friction stir thermal profile to constant strain rate deformation successfully reproduced the friction stir microstructure at a peak temperature of 1000ºC and a strain rate of 10/s. These results provide unique insight into the strain, strain rates, and temperatures regime within the process. Finally, the experimental thermal and deformation fields were compared using ISAIAH, a Eulerian based three-dimensional model of friction stir welding. These results are preliminary but show promise for the ability of the model to compute thermal fields for material flow, model damage prediction, and decouple texture evolution for specific thermomechanical histories in the friction stir process.
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    MICROSTRUCTURAL CHANGES UNDER ISOTHERMAL AGING AND THEIR INFLUENCE ON THERMAL FATIGUE RELIABILITY FOR TIN-LEAD AND LEAD-FREE SOLDER JOINTS, INCLUDING MICROSTRUCTURAL CHANGES UNDER ISOTHERMAL AGING IN MIXED SOLDER JOINTS
    (2007-11-26) choubey, anupam; Pecht, Michael; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Most electronics companies have transitioned to lead-free processes, both to comply with government legislation and to avoid issues related to mixing of tin-lead and lead-free metallurgies. However, exemptions from lead-free legislation have been granted for certain products, especially those intended for high-reliability applications. One major concern with these exempt products is that, during assembly or rework, lead-free components will have to be used due to the unavailability of tin-lead components. This will result in the mixing of tin-lead and lead-free metallurgies. The mixing of metallurgies can induce new reliability concerns. This study is focused on mixed solder joints formed by attaching lead-free components with tin-lead paste. Solder interconnect reliability is influenced by the environmental imposed load, solder material properties and the microstructure formed between the solder and the metal surfaces to which the solder is bonded. Several lead-free metallurgies are being used for component terminals, board pad plating and solder materials. These metallurgies react to form the microstructure of a solder joint. Microstructure of a solder joint continuously evolves and affects solder joint properties. A fundamental understanding on the microstructure is required to analyze the changes occurring in a solder joint with time and temperature and make predictions on solder joint reliability under thermal loading conditions. This dissertation determines key microstructural features present in SnPb, lead-free and mixed solder joints. Changes in the microstructural features were determined for SnPb, lead-free and mixed solder joints exposed to isothermal aging conditions. The effect of microstructural changes on reliability was determined by conducting thermal fatigue reliability tests for SnPb and lead-free solder joints. Whereas, for mixed solder joints, hypotheses has been determined based on microstructural analysis on their thermal fatigue performance compared to SnPb joints. This dissertation doesn't include the effect of microstructural changes on the reliability of mixed solder joints. This dissertation doesn't include the reliability tests for mixed solder joints. Two microstructural features namely, intermetallic compounds (IMC) and Pb phase were characterized for SnPb, lead-free and mixed solder joints. IMCs are formed at the solder to pad metallization interface and in the bulk solder. It was determined that reaction between Sn3.0Ag0.5Cu solder and Ni/Au component side metallization result in interfacial IMCs consisting of Ni3Sn4 IMC in the as-reflowed stage and IMCs such as (NiCu)3Sn4, (Cu,Ni)6Sn5 and (Au,Ni)Sn4 after thermal aging of 350 hours at 125ºC. With pad metallization of ImAg, ImSn and OSP, IMCs such as Cu6Sn5 are formed after reflow followed by formation of a new Cu3Sn IMC phase after thermal aging of 350 hours at 125ºC. Cu6Sn5 and Ag3Sn IMC were found distributed in bulk solder joints in the as-reflowed and aged (125ºC for 100, 350 and 1000 hrs) solder joint. This dissertation demonstrated that under thermal cycling, intergranular crack propagates between Sn grains in the bulk solder and Cu6Sn5 IMCs present at Sn grain boundaries in the bulk solder influence crack propagation. It was demonstrated that isothermal aging for 350 hrs at 125ºC causes coarsening of Cu6Sn5 IMC particles in the bulk solder which results in a 50% reduction in number of Cu6Sn5 IMC particles in the bulk solder, thus promoting the crack to propagate faster along the grain boundary. This dissertation determined that isothermal aging for 350 hrs at 125ºC would cause a 25% reduction in characteristic life for lead-free solder joints due to the changes associated with Cu6Sn5 IMC particles. In conventional SnPb solder joints Pb phase present in the bulk solder coarsens as a function of time and temperature and influences thermal fatigue reliability. Due to the presence of Pb in mixed solder joint, this dissertation determined the extent of coarsening in mixed solder joints compared with SnPb joints. It was determined that mixed solder joints are not prone to Pb phase coarsening under aging for 350 hrs at 125ºC as opposed to SnPb solder joints and therefore would have better thermal fatigue performance compared to SnPb joint under these conditions. This dissertation demonstrated that the presence of Pb in mixed solder results in a 30 to 40% lower IMC thickness compared to Pb-free and SnPb solder joints by being present at the interface as a diffusion barrier between Ni and Sn for IMC formation. Presence of Pb has been known to act as diffusion barrier for SnPb solder joints.