Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2007. / Source: Dissertation Abstracts International, Volume: 68-06, Section: B, page: 4099. Adviser: Nancy R. Sottos. Includes bibliographical references (leaves 118-121) Available on microfilm from Pro Quest Information and Learning.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2006. / Source: Dissertation Abstracts International, Volume: 67-11, Section: B, page: 6687. Advisers: Jonathan A. Dantzig; Nigel D. Goldenfeld. Includes bibliographical references (leaves 128-136) Available on microfilm from Pro Quest Information and Learning.

<p> Cardiovascular disease (CVD) is one of the largest epidemic in the world causing 800,000 annual deaths in the U.S alone and 15 million deaths worldwide. After a myocardial infarction, commonly known as a heart attack, the cells around the infarct area get deprived of oxygen and die resulting in scar tissue formation and subsequent arrhythmic beating of the heart. Due to the inability of cardiomyocytes to differentiate, the chances of recurrence of an infarction are tremendous. Research has shown that recurrence lead to death within 2 years in 10% of the cases and within 10 years in 50% of the cases. Therefore, an external structure is needed to support cardiomyocyte growth and bring the heart back to proper functioning. Current research shows that composite materials coupled with nanotechnology, a material where one of its dimension is less than or equal to 100nm, has very high potential in becoming a successful alternative treatment for end stage heart failure. The main goal of this research is to develop a composite material that will act as a scaffold to help externally cultured cardiomyocytes grow in the infarct area of the heart. The composite will consist of a poly-lactic co glycolic acid (PLGA) matrix, reinforced with carbon nanotubes. Prior research has been conducted with this same composite, however the significance of the composite developed in this research is that the nanotubes will be aligned with the help of an electro-magnetic field. This alignment is proposed to promote mechanical strength and significantly enhance proliferation and adhesion of the cardiomyocytes.</p><p>

<p> Nanomaterial composites hold improvement potential for many materials. Improvements arise through known material behaviors and unique nanoscale effects to improve performance in areas including elastic modulus and damping as well as various processes, and products. Review of research spurred development of a load-stage. The load stage could be used independently, or in conjunction with an AFM to investigate bulk and nanoscale material mechanics. </p><p> The effect of MWCNT content on structural damping, elastic modulus, toughness, loss modulus, and glass transition temperature was investigated using the load stage, AMF, and DMA. Initial investigation showed elastic modulus increased 23% with 1wt.% MWCNT versus pure epoxy and <i>in-situ</i> imaging observed micro/nanoscale deformation. </p><p> Dynamic capabilities of the load stage were investigated as a method to achieve higher stress than available through DMA. The system showed energy dissipation across all reinforce levels, with ~480% peak for the 1wt.% MWCNT material vs. the neat epoxy at 1Hz.</p><p>

<p> It has become apparent in recent years the advancement of manufacturing processes in the aerospace industry. Sealant nozzles are a critical device in the use of fuel tank applications for optimal bonds and for ground service support and repair. Sealants has always been a challenging area for optimizing and understanding the flow patterns. A parametric study was conducted to better understand geometric effects of sealant flow and to determine whether the sealant rheology can be numerically modeled. The Star-CCM+ software was used to successfully develop the parametric model, material model, physics continua, and simulate the fluid flow for the sealant nozzle. The simulation results of Semco sealant nozzles showed the geometric effects of fluid flow patterns and the influences from conical area reduction, tip length, inlet diameter, and tip angle parameters. A smaller outlet diameter induced maximum outlet velocity at the exit, and contributed to a high pressure drop. The conical area reduction, tip angle and inlet diameter contributed most to viscosity variation phenomenon. Developing and simulating 2 different flow models (Segregated Flow and Viscous Flow) proved that both can be used to obtain comparable velocity and pressure drop results, however; differences are seen visually in the non-uniformity of the velocity and viscosity fields for the Viscous Flow Model (VFM). A comprehensive simulation setup for sealant nozzles was developed so other analysts can utilize the data.</p><p>

<p> The objective of this research is to understand the fundamental relationships necessary to develop a method to optimize both the topology and the internal gradient material distribution of a single object while meeting constraints and conflicting objectives. Functionally gradient material (FGM) objects possess continuous varying material properties throughout the object, and they allow an engineer to tailor individual regions of an object to have specific mechanical properties by locally modifying the internal material composition. A variety of techniques exists for topology optimization, and several methods exist for FGM optimization, but combining the two together is difficult. Understanding the relationship between topology and material gradient optimization enables the selection of an appropriate model and the development of algorithms, which allow engineers to design high-performance parts that better meet design objectives than optimized homogeneous material objects.</p><p> For this research effort, topology optimization means finding the optimal connected structure with an optimal shape. FGM optimization means finding the optimal macroscopic material properties within an object. Tailoring the material constitutive matrix as a function of position results in gradient properties. Once, the target macroscopic properties are known, a mesostructure or a particular material nanostructure can be found which gives the target material properties at each macroscopic point.</p><p> This research demonstrates that topology and gradient materials can both be optimized together for a single part. The algorithms use a discretized model of the domain and gradient based optimization algorithms. In addition, when considering two conflicting objectives the algorithms in this research generate clear &lsquo;features&rsquo; within a single part. This tailoring of material properties within different areas of a single part (automated design of &lsquo;features&rsquo;) using computational design tools is a novel benefit of gradient material designs.</p><p> A macroscopic gradient can be achieved by varying the microstructure or the mesostructures of an object. The mesostructure interpretation allows for more design freedom since the mesostructures can be tuned to have non-isotropic material properties. A new algorithm called Bi-level Optimization of Topology using Targets (BOTT) seeks to find the best distribution of mesostructure designs throughout a single object in order to minimize an objective value. On the macro level, the BOTT algorithm optimizes the macro topology and gradient material properties within the object. The BOTT algorithm optimizes the material gradient by finding the best constitutive matrix at each location with the object. In order to enhance the likelihood that a mesostructure can be generated with the same equivalent constitutive matrix, the variability of the constitutive matrix is constrained to be an orthotropic material. The stiffness in the X and Y directions (of the base coordinate system) can change in addition to rotating the orthotropic material to align with the loading at each region. </p><p> Second, the BOTT algorithm designs mesostructures with macroscopic properties equal to the target properties found in step one while at the same time the algorithm seeks to minimize material usage in each mesostructure. The mesostructure algorithm maximizes the strain energy of the mesostructures unit cell when a pseudo strain is applied to the cell. A set of experiments reveals the fundamental relationship between target cell density and the strain (or pseudo strain) applied to a unit cell and the output effective properties of the mesostructure. At low density, a few mesostructure unit cell design are possible, while at higher density the mesostructure unit cell designs have many possibilities. Therefore, at low densities the effective properties of the mesostructure are a step function of the applied pseudo strain. At high densities, the effective properties of the mesostructure are continuous function of the applied pseudo strain.</p><p> Finally, the macro and mesostructure designs are coordinated so that the macro and meso levels agree on the material properties at each macro region. In addition, a coordination effort seeks to coordinate the boundaries of adjacent mesostructure designs so that the macro load path is transmitted from one mesostructure design to its neighbors.</p><p> The BOTT algorithm has several advantages over existing algorithms within the literature. First, the BOTT algorithm significantly reduces the computational power required to run the algorithm. Second, the BOTT algorithm indirectly enforces a minimum mesostructure density constraint which increases the manufacturability of the final design. Third, the BOTT algorithm seeks to transfer the load from one mesostructure to its neighbors by coordinating the boundaries of adjacent mesostructure designs. However, the BOTT algorithm can still be improved since it may have difficulty converging due to the step function nature of the mesostructure design problem at low density.</p><p>

A technological basis for efficient ceramic machining requires a fundamental understanding of the prevailing grinding mechanisms. Most past research on grinding mechanisms for ceramics has followed either the "indentation fracture mechanics" approach or the "machining" approach. The indentation fracture mechanics approach likens abrasive workpiece interactions to idealized small-scale indentations. The machining approach typically involves measurement of cutting forces together with microscopic observations of grinding debris and surfaces produced. Both approaches provide important insights into the grinding mechanisms for ceramic materials. However, up to now, no physical model has been presented which can quantitatively account for the energy associated with grinding of ceramics. The present research was undertaken to investigate grinding mechanisms for ceramics and to account for the energy expended. SEM observations of grinding debris for various ceramics and a glass over a wide range of conditions indicate material removal mainly by brittle fracture associated with lateral cracking and crushing. However, the ground surfaces reveal extensive ductile flow with characteristic scratches along the grinding direction and smearing. Ductile flow typically extends to a depth of 1-5 $\mu$m below the ground surface. For silicon nitride, etching with hydrofluoric acid removed the smeared layer, which would indicate that it consists of a glassy phase probably formed by oxidation at elevated grinding temperatures. Although material removal appears to occur mainly by brittle fracture, most of the grinding energy is apparently associated with ductile flow. An order of magnitude analysis indicates that the energy expended by brittle fracture constitutes a negligible portion of the total grinding energy. An upper bound plowing analysis is presented which can account the specific energy in terms of the geometry of the plowed groove. A new model has been developed which relates the grinding power to the rate of plowed surface area generated by the diamond cutting points on the wheel surface interacting with the workpiece. Over a wide range of grinding conditions, the power increases approximately proportionally with the rate of surface area generated, which suggests a nearly constant energy per unit area of plowed surface. Values obtained for energy per area for plowing are much bigger than the corresponding fracture surface energies, which further indicates that most of the grinding energy is associated with ductile flow.

A new model of solidification of liquid amorphous polymers between cooled parallel plates predicts the measured in-plane and through-thickness shrinkage of injection-molded plaques. The model uses a reference strain field to account for the melt that is forced or removed from the cavity during the packing phase to fill the space created by material contraction during solidification. The model can also account for material freeze-off effects in which the cavity pressure is controlled by the solidification process and the through-thickness shrinkage is significantly bigger than the in-plane shrinkage. The model predicts that packing pressure significantly effects part shrinkage, warpage and distribution of residual stresses. This reference-strain approach to modeling solidification can also be used to predict local surface defects such as sink marks and global dimensions of injection-molded plastic parts. By accounting for sprue freeze-off, and by using the measured cavity pressure as a time-dependent boundary condition, the model correctly predicted the measured plaque shrinkages in a controlled test.

Percolation concepts successfully correlate the effect of crystallinity on the mechanical properties of the poly(butylene terephthalate) (PBT)/BT copolymer blends (miscible and immiscible); the degree of miscibility depends on the hard block length of the BT copolymer. The dependence of the modulus (E) and the yield stress ($\sigma\sb{\rm y}$) on crystallinity can be modelled as the deformation of a percolative crystalline network. It is proposed that tie molecules between the crystals resist the deformation. Yielding and drawing of semi-crystalline polymers is shown to be compatible with a stress induced melting (decrystallization) model. In addition, the pressure dependence of the yield stress for several semi-crystalline polymers agrees with predictions from the Clausius-Clapeyron equation for the effect of pressure on a phase transformation. The stress induced melting model supports and incorporates the proposed percolation model. A new test procedure, the "zero ligament" method, to measure the toughness (J$\sb{\rm c}$) of ductile polymers was developed that does not require identification of the initiation event nor measurement of the extent of crack growth. J$\sb{\rm c}$ obtained by this technique was in good agreement with the toughness (J$\sb{0.2}$) measured according to the proposed ASTM J-integral protocol for polymers. The toughness of the blends, determined by the "zero ligament" method, was shown to be primarily from the energy dissipated by the $\alpha\to\beta$ transformation for the miscible blends, and from copolymer enhanced shear yielding for the immiscible blends, in which the energy dissipation mechanisms was identified with the aid of percolation concepts. Also, fractal analysis of the fracture surface showed that the fractal dimension (D) depended on the fracture mechanism only, and the scaling factor (K) correlated with fracture toughness and crack propagation resistance.

<p> Bonding composite structures using adhesives offers several advantages over mechanical fastening such as better flow stress, weight saving, improved fatigue resistance and the ability to join dissimilar structures. The hesitation to adopt adhesively bonded composite joints stems from the lack of knowledge regarding damage initiation and propagation mechanisms within the joint. A means of overcoming this hesitation is to continuously monitor damage in the joint. This study proposes a methodology to conduct structural health monitoring (SHM) of an adhesively bonded composite lap joint using acoustic, guided Lamb waves by detecting, locating and predicting the size of damage. Finite element modeling of a joint in both 2D and 3D is used to test the feasibility of the proposed damage triangulation technique. Experimental validation of the methodology is conducted by detecting the presence, location and size of inflicted damage with the use of tuned guided Lamb waves.</p>