Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2008. / Source: Dissertation Abstracts International, Volume: 69-11, Section: B, page: 7110. Adviser: Daniel A. Tortorelli. Includes bibliographical references (leaves 93-96) Available on microfilm from Pro Quest Information and Learning.

Thesis (Ph. D.)--University of Illinois at Urbana-Champaign, 2008. / Source: Dissertation Abstracts International, Volume: 69-11, Section: B, page: 7104. Advisers: Armand Beaudoin; Iwona Jasiuk. Includes bibliographical references (leaves 150-159). Available on microfilm from Pro Quest Information and Learning.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2007. / Source: Dissertation Abstracts International, Volume: 69-02, Section: B, page: 1293. Adviser: Armand J. Beaudoin. Includes bibliographical references (leaves 84-88) Available on microfilm from Pro Quest Information and Learning.

<p> Acceptance of Carbon Fiber Reinforced Polymer (CFRP) structures requires a robust surface preparation method with improved process controls capable of ensuring high bond quality. Surface preparation in a production clean room environment prior to applying adhesive for bonding would minimize risk of contamination and reduce cost. Plasma treatment is a robust surface preparation process capable of being applied in a production clean room environment with process parameters that are easily controlled and documented. Repeatable and consistent processing is enabled through the development of a process parameter window utilizing techniques such as Design of Experiments (DOE) tailored to specific adhesive and substrate bonding applications. Insight from respective plasma treatment Original Equipment Manufacturers (OEMs) and screening tests determined critical process factors from non-factors and set the associated factor levels prior to execution of the DOE. Results from mode I Double Cantilever Beam (DCB) testing per ASTM D 5528 [1] standard and DOE statistical analysis software are used to produce a regression model and determine appropriate optimum settings for each factor.</p>

<p> This thesis presents a mathematical modeling process for characterizing a hyperelastic material with viscous response under dynamic loading conditions. The model is designed with the advantage of performing only one compressive dynamic test in order to provide the requisite parameters to fully determine the hyper-viscoelastic response. This is achieved in both deformations and contact forces, using digital image correlation and force sensors. Experiments performed at strain rates ranging from 10^&ndash;3&ndash;10² s^&ndash;1 correlate with computational simulations at the same loading rates up to 80% compression. The validity of the fit and prediction is assessed using MATLAB along with ABAQUS finite element software. </p><p> The results provided by this novel methodology, i.e. the mathematical model using non-homogeneous deformations and the subsequent dynamic experimental techniques, proves that this approach is a more effective alternative to the current standards used to characterize the mechanical response of hyperelastic, viscoelastic, and hyper-viscoelastic materials.</p><p>

<p> An adaptive arbitrary-order curvilinear progressive 2D crack growth algorithm is presented. The method uses the ZFEM hypercomplex finite element program to compute arbitrary order derivatives of strain energy with respect to self-similar or perpendicular crack extensions, and then constructs a family of Taylor series functions of strain energy versus crack growth direction. An adaptive algorithm automatically selects the best high-degree polynomial to extrapolate a curvilinear crack path, and adjusts the length of the crack growth increment added during each simulation step to maintain the crack path and model energy within desired tolerances. The method is automated such that the full crack path from inception to failure is computed with multiple FE analyses. Numerical examples up to fifth order are presented and compared against experiments. </p><p>

<p> Aerosol jet printing (AJP)&mdash;a direct-write, additive manufacturing technique&mdash;has emerged as the process of choice particularly for the fabrication of flexible and hybrid electronics. AJP has paved the way for high-resolution device fabrication with high placement accuracy, edge definition, and adhesion. In addition, AJP accommodates a broad range of ink viscosity, and allows for printing on non-planer surfaces. Despite the unique advantages and host of strategic applications, AJP is a highly unstable and complex process, prone to gradual drifts in machine behavior and deposited material. Hence, real-time monitoring and control of AJP process is a burgeoning need. In pursuit of this goal, the objectives of the work are, as follows: (i) <i>In situ </i> image acquisition from the traces/lines of printed electronic devices right after deposition. To realize this objective, the AJP experimental setup was instrumented with a high-resolution charge-coupled device (CCD) camera, mounted on a variable-magnification lens (in addition to the standard imaging system, already installed on the AJ printer). (ii) <i>In situ </i> image processing and quantification of the trace morphology. In this regard, several customized image processing algorithms were devised to quantify/extract various aspects of the trace morphology from online images. In addition, based on the concept of shape-from-shading (SfS), several other algorithms were introduced, allowing for not only reconstruction of the 3D profile of the AJ-printed electronic traces, but also quantification of 3D morphology traits, such as thickness, cross-sectional area, and surface roughness, among others. (iii) Development of a supervised multiple-input, single-output (MISO) machine learning model&mdash;based on sparse representation for classification (SRC)&mdash;with the aim to estimate the device functional properties (e.g., resistance) in near real-time with an accuracy of &ge; 90%. (iv) Forwarding a computational fluid dynamics (CFD) model to explain the underlying aerodynamic phenomena behind aerosol transport and deposition in AJP process, observed experimentally. </p><p> Overall, this doctoral dissertation paves the way for: (i) implementation of physics-based real-time monitoring and control of AJP process toward conformal material deposition and device fabrication; and (ii) optimal design of direct-write components, such as nozzles, deposition heads, virtual impactors, atomizers, etc.</p><p>

<p> In the past few decades, composite materials especially carbon fiber reinforced polymers (CFRP) have been widely used as structural materials for its high strength to weight ratio, tailorable properties, and excellent corrosion properties. Applications that require better understanding of the electrical properties of CFRP laminates include carbon fiber assisted heating during composites manufacturing, self-sensing of damage of composite structures, integrated electromagnetic shielding, and lightning strike protection. Accurate predictive model describing the electrical conduction behavior of CFRP laminates is the key for them to be used for such applications.</p><p> Different approaches have been explored to model the electrical conduction of CFRP under various current conditions. A comprehensive literature review revealed that most methods used to model electrical conduction of CFRP fail to capture the impact of micro-structure of CFRP, especially the fiber-fiber contact, and resin-rich layer between plies, which can drastically change the conduction pattern.</p><p> The aim of this dissertation work is to develop a model that capture key electrical conduction mechanisms of CFRP, which address the impact of the micro-structure and geometrical parameters. The model is constructed in a modular fashion by validating the model with experimental validation after the addition of each key mechanism module. First, the model constructs a resistor network framework for describing electrical conduction behavior of UD laminas and fiber tows subjected to low DC currents. The model is validated with reported experimental results, and by characterization of resistivity of dry carbon fiber tows.</p><p> The next module investigates the specific features of a multi-ply laminate such as: varying ply orientation, existence of resin-rich layer, and dependence on geometric parameters that influence the local resistivity. A meso-scale fiber bundle model is proposed to strike a balance between the level of details modeled and the computational cost. Influence of the resin-rich layer is described with an inter-ply connectivity term. Expressions for estimating contact resistance from multiple sources including direct fiber-fiber contact and tunneling resistance across thin resin layer are introduced. The refined model is compared against experimental results and finite element model. A parametric study is conducted to investigate the impact of geometrical parameters.</p><p> Finally, the dissertation work investigates the impact of high current density both numerically and experimentally. Simplified analytical model examining the impact of localized Joule heating revealed that current concentrations due to microstructure constraints can introduce excessive Joule heating at contact spots. Thus, it is vital not to under-estimate the temperature rise at contact points, even at seemingly small overall applied currents. Based on these analysis, the model is further refined with the implementation of the module that introduces Joule heating. Both reversible change in resistivity such as temperature dependent resistivity and irreversible change such as thermal and electric degradation of resin matrix is considered.</p><p> Electrical characterization under high current density is carried out for dry fiber tows and cured composites experimentally. The contributions of reversible and irreversible resistivity change are identified with carefully designed repetitive current tests. It is found that for dry fiber tows with sizing and for cured composites, thermal breakdown of the thin resin/sizing layer contributes significantly to the nonlinear conduction behavior under high current density. The developed model captures important characteristics of the electrical conduction behavior when compared with experimental results. Possible explanations are offered for cases and regions where the model shows discrepancies with experimental results. This model should prove useful to address and design and fabricate composite components in which electric and thermal conductivity play a key role in defining their functional properties. </p><p>

<p> The application of sophisticated geometric structures within future host materials for increasing energy absorption and compression strength, while being fabricated from crack-healing materials, is of high interest for many functions. Raw feedstock extrusion and three-dimensional printing (3DP) technology were used to develop precise honeycomb structures through intricate deposition of polycaprolactone (PCL) filament. For standardization purposes during 3D model slicing and print quality consistency, constant wall thickness was used for honeycomb structure fabrication, manipulating only the cellular width to obtain variation of cell size to wall thickness ratios. </p><p> The honeycomb structures&rsquo; compression behaviors were studied through use of in-plane quasi-static uniaxial compression testing. Multiple cycles of compression loading were applied to the specimens in both transverse and ribbon directions at temperatures of 5 &deg;C, room temperature (i.e. 22 &deg;C), and 40 &deg;C at a speed of 1.27 mm/min (0.05 in/min) per ASTM D6641. The energy absorption efficiencies of the honeycomb structure were calculated based on the compression strengths and behaviors displayed, which were then used to obtain the stepping upward stress theoretically. Using the specified stepping upward stresses, the energy absorption capabilities were found in both the transverse and ribbon directions at different temperatures per unit volume. The ability for &ldquo;shape recovery&rdquo; of the structures after each loading cycle was also calculated. </p><p> Outcomes from this research displayed exceptional recovery of PCL honeycomb structures after repeated compression loading cycles. Samples with relative density of 0.20 absorbed energies of up to 0.99 J/cm³. Upon removing compression loads, samples were capable of shape recovery up to 80% after the first deformation and up to 72% after the fifth deformation. When PCL honeycomb structures are used to reinforce host materials, they increase energy absorption capabilities while being capable of crack-healing functions with remarkable compressive strength. These properties make PCL advantageous for many industries.</p><p>

<p> Aluminum particles are widely used as a metal fuel in solid propellants. However, poor combustion efficiencies and two-phase flow losses result due in part to particle agglomeration. Recently, engineered composite particles of aluminum (Al) with inclusions of polytetrafluoroethylene (PTFE) or low-density polyethylene (LDPE) have been shown to improve ignition and yield smaller agglomerates in solid propellants. Reductions in agglomeration were attributed to internal pressurization and fragmentation (microexplosions) of the composite particles at the propellant surface. </p><p> Here, we explore the mechanisms responsible for microexplosions in order to better understand the combustion characteristics of composite fuel particles. Single composite particles of Al/PTFE and Al/LDPE with diameters between 100&ndash;1200 &mu;m are ignited on a substrate to mimic a burning propellant surface in a controlled environment using a CO₂ laser in the irradiance range of 78&ndash;7700 W/cm². The effects of particle size, milling time, and inclusion content on the resulting ignition delay, product particle size distributions, and microexplosion tendencies are reported. For example, particles with higher PTFE content (30 wt.%) had laser flux ignition thresholds as low as 77 W/cm², exhibiting more burning particle dispersion due to microexplosions compared to the other materials considered. Composite Al/LDPE particles exhibit relatively high ignition thresholds compared to Al/PTFE particles, and microexplosions were observed only with laser fluxes above 5500 W/cm² due to low LDPE reactivity with Al resulting in negligible particle self-heating. However, results show that microexplosions can occur for Al containing both low and high reactivity inclusions (LDPE and PTFE, respectively) and that polymer inclusions can be used to tailor the ignition threshold. This class of modified metal particles shows significant promise for application in many different energetic materials that use metal fuel.</p><p>