Engineering Doctoral Work

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    Cement Changes and their Effects on Shrinkage and Durability of Concrete
    (North Dakota State University, 2018) Sharma, Achintyamugdha Surendra
    In this work, the effects of different cementitious combinations on drying shrinkage and durability of concrete are investigated. Shrinkage is caused due to loss of water from concrete due to evaporation. Cement paste is the most vulnerable part of concrete to shrinkage. As concrete consists of cement paste and aggregates, the latter pose as restraints to the shrinking paste. As a result, tensile stresses are formed thereby leading to the formation of cracks. Cracks in concrete are responsible for the ingress of deleterious chemical ions such as, Chloride and Sulfate ions, which may form expansive compounds. Chloride ions also induce corrosion of steel reinforcement bars. Prior literature has indicated that coarser cementitious systems lead to less shrinkage in concrete. Additionally, tri-calcium silicate (C3S) and tri-calcium aluminate contents in cement also influence the extent of shrinkage in concrete. However, the extensive use of such cement in concrete as per current industry standards will be challenging. Therefore, in this study investigations are made with Type IL (10) portland limestone with or without supplementary cementitious materials (SCMs), such as fly ash, and engineered nanomaterials such as nanosilica, to develop concrete with similar shrinkage performance as compared to coarse ground portland cement, without adversely affecting other engineering properties of concrete. Key engineering properties include workability, degree of hydration, compressive and splitting tensile strengths, electrical resistivity, setting time, and bleeding. Portland limestone cement used in this study has a partial replacement level of 10% of portland cement with limestone. Although, ASTM C595 allows up to 15% replacement with limestone, current industry use is limited to ~5%. Micro-cracking tendency of selected concrete mixtures from this study is also investigated by using fluorescence microscopy and micro computed tomography (µ-CT). In this study, life cycle assessment (LCA) studies are conducted on select cementitious combinations to quantify the benefits of such replacement of portland cement. Additionally, life cycle cost analysis (LCCA) is also performed to determine feasibility of cost. Results show that Type IL portland limestone cement can be an effective alternative for reduced shrinkage strain and enhanced durability properties.
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    An Investigation of Mechanics of Collagen and Fibril in Bone and Interactions in Swelling Clays: A Molecular and Multiscale Modeling Study
    (North Dakota State University, 2012) Pradhan, Shashindra Man
    A fundamental study of the mechanics at the molecular scale and bridging it to the continuum level through multiscale modeling is the focus of this work. This work investigates how the material properties of nanoscale systems are influenced by the nonbonded interactions and molecular conformations. The molecular model is then bridged with the finite element model to link mechanics at nanoscale with the continuum scale. This work provides an unprecedented insight into how the interactions at the molecular scale influence mechanical properties at higher scales. Two materials are considered for the molecular modeling study: bone and Na-montmorillonite swelling clay. Bone is composed of composed of collagen molecules and hydroxyapatite in the molecular scale, which are organized into collagen fibril. The molecular dynamics study is carried out to study the nature of collagen-hydroxyapatite interface and the mechanics of collagen in bone. Furthermore, the molecular model of full-length collagen is built for the first time to show the differences in its conformation and deformation mechanism during pulling as compared to the short molecules, upon which the current understanding of is based. The mechanics of collagen is explained with the help of three-tier helical hierarchy not seen in short molecules. Two mechanisms of deformation and conformational stability of collagen are proposed: (i) interlocking gear analogy, and (ii) interplay between level-1 and level-2 hierarchies, the hydrogen bonds acting as an intermediary. The multiscale model of collagen fibril is developed by bridging nanomechanical molecular properties of collagen into the finite element model. This model shows that the molecular interactions between collagen and mineral significantly affect the mechanical response of collagen fibril. The deformation mechanism of collagen fibril and the effect of collagen crosslinks are also elucidated in this study. In recent years Na-montmorillonite has been proposed for bone regenerative medicine, besides other existing engineering applications. The molecular dynamics study of Na-montmorillonite at different levels of hydration is carried out to understand the role played by molecular interactions in the swelling behavior of Na-montmorillonite. This study greatly adds to our understanding of clay swelling, and provides important insights for modeling exfoliation and particle breakdown in clay.
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    Characterizing the Aging-Driven Degradation Rate of Electrical Contact Resistance and Mechanical Integrity of Plastic-Encapsulated Au/Al Wire Bonds
    (North Dakota State University, 2019) Ahmad, Syed Sajid
    Gold-aluminum interconnect is an integral part of conventional chip packaging. The gold-aluminum interface deteriorates during the operation of a device due to the formation of gold-aluminum intermetallic compounds. Spatial changes during intermetallic formation cause voids, which separate the interface, resulting in the catastrophic failure of the bond, and hence the device. This phenomenon is driven by temperature and time. With increasing device densities and overall package miniaturization, device heat dissipation densities are increasing, necessitating adequate understanding of the phenomena to assure appropriate device life for the intended application. Intermetallic formation is governed by diffusion rules, but the observed failure rates reported in the literature many times diverge from Fickian pattern. This conflict is resolved in this dissertation. Various failure rates are considered to show that a digression from Fickian pattern is possible. An effort is undertaken to improve the understanding of failure modeling. Failure mechanism is analyzed and a solution is presented.