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<p dir="ltr">Defects in composite manufacturing often lead to compromised structural integrity and reduced performance of the final product. A robust constitutive modeling framework is needed to efficiently and accurately predict the deformation responses of dry fabrics and pre-impregnated fibers, paving the way for defect simulation. This thesis presents a comprehensive study on the development and application of a novel constitutive model of fabric preforms and pre-impregnated fibers during composite manufacturing processes.</p><p dir="ltr">This work proposes an integrated constitutive study for textile fabrics in the aspects of mesoscale tow and macroscale fabric behavior. First, a textile architecture-based discrete modeling approach was developed to predict and visualize fiber tow and fabric deformation. The fabrics consist of interlacing virtual fiber tows which are represented by Timoshenko beams joined by translational and rotational springs and rotary dashpots, which are used to capture the energy dissipation during in-plane shear deformation. Second, an anisotropic hyper-viscoelastic model was developed using the strain energy density function of a homogenized unit cell to predict the fabric deformation as a continuous field. A Maxwell model consisting of one Maxwell element and an additional spring is used to consider the nonequilibrium stresses generated during in-plane shear, transverse shear, and through-thickness compaction deformations. Both approaches were experimentally characterized and applied to a hemisphere draping model in the commercial Finite Element Analysis (FEA) software, Abaqus, to demonstrate the predictive capabilities.</p><p dir="ltr">Then, the robust hyper-viscoelastic model is extended to predict prepreg compaction and bending behavior. In the compaction aspect, a coupling term of energy that captures the effect of squeezing flow and a highly nonlinear transverse compression energy are proposed to predict the compaction response of prepreg with liquid and rubbery resin. The viscoelastic parameters were characterized by a Computational Fluid Dynamics (CFD) model for liquid resin and a discrete micromechanics model for rubbery resin. The method was applied to stepwise compaction simulation at different temperatures in Abaqus and compared to experiments for validation. In the bending aspect, the effective shear modulus is expressed as a function of the second-order gradient of deformation. Modeling parameters were characterized by an analytical model that captures the underlying fiber and matrix deformation mechanism. Parametric study was conducted to illustrate the influence of each parameter and the capability to enhance the accuracy of bending prediction.</p>

<p>  </p> <p>Thermal shrinkage in advanced composite manufacturing causes residual stress in a cylindrical anisotropic segment. The residual stress later induces a spring-in angle when  the temperature change is negative. The superposition method in the finite element method (FEM) by ABAQUS©  proves that only the residual stress in the circumferential direction controls the spring-in angle and induces the radial residual stress. To predict the angle change, the residual stress is firstly determined by using the closed-loop geometry in FEM and then implemented into the cylindrical cross-ply symmetric laminate segment. Consequently, the geometry creates the spring-in angle under the traction-free surface. The angle change is in good agreement with the Radford equation and is found to depend on the coefficient of thermal expansion (CTE) in the circumferential and radial directions rather than other material properties and geometry dimensions. </p> <p>The study found a new limitation of the Radford equation, in that it is accurate when the part is anisotropic symmetric laminate, but not when it is unsymmetric. The accuracy of the Radford equation is further explored with the double curve geometry. Using the superposition method, the circumferential residual stress along the major curve is found to have an influence on the angle change not only of the major curve, but also of the minor curve. The negative temperature change produces the spring-in angle on the major curve, and both spring-in and -off angles on the minor curve, which rely on the radius ratio. In addition, the spring-in angle on the major curve is coincident with the Radford equation. In sum, knowing the spring-in angle is very helpful in designing a tool in advanced composite manufacturing, and the superposition method and the Radford equation are applicable to predict the spring-in angle.</p>

<p>In an AP-HTPB propellant microstructure, the local strain rate depends on the AP crystal size and the material, while the local temperature rate depends on the impact velocity, AP crystal size, and the material.  Larger AP crystals lead to higher local strain rates and higher local temperature rates, which means hot spots are more likely to occur in AP-HTPB propellants with more large AP crystals.</p>

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<p>Polymer based multifunctional material systems (MFMS) have gained increasing attention in the past two decades. The addition of nanofillers and nanoparticles allows for modification of physical properties as well as the discovery of new features. Multifunctionalization of composites allows us to “do more with less”. For example, electrically conductive additives can eliminate the need for sensors through self-sensing principles, shape morphing matrices can reduce the need for actuators, and the inclusion of fire-resistant constituents can reduce flammability in stringent fire protection measures. With added capabilities, the applications of multifunctional composites extends beyond the aerospace and automotive industries to healthcare, infrastructure, electronics, among others, and optics.</p> <p><br></p> <p>The current state of the art is largely focused on single-filler composites or multifiller composites with complementary attributes. For example, carbon nanotubes (CNTs) when mixed with graphene produces higher conductivity than can be achieved via modification with either CNTs or graphene alone. The majority of investigations conducted in this domain have fillers selected with the aim of imparting a singular property. Much less has been done in the area of multifiller and multifunctional polymer matrix composites (PMCs) which can exhibit multiple properties. Consequently, this work seeks to contribute towards the field of synergistic functional composites. That is, a multifiller composite material system comprised of differently functional fillers. This approach has potential to yield smart material systems that outperform single-filler or single-functionality materials through the discovery of novel synergistic coupling between the differently functional phases.</p> <p><br></p> <p>In light of the preceding motivation, this work presents the results on the experimental electromagnetic and mechanical characterization of multi-walled carbon nanotubes (MWCNTs) + carbon-coated iron nanoparticle (CCFeNP)-modified polymers. Carbon nanotubes with their electrical properties and iron nanoparticles with their magnetic attributes present potential for synergistic electromagnetic interactions in a well-percolated network. We report on the electro-magnetic properties of MWCNT + CCFeNP/epoxy composites including DC and AC conductivity, dielectric permittivity, magnetic permeability, and piezoresistance as a function of varying relative MWCNT and CCFeNP concentrations. The results are in a large part linked to the manufacturing process described herein. This work seeks to establish the foundations of synergistic functional filler combinations that could lead to new multifunctional capabilities in the future.</p>

<p> Demand ever increases for clean-burning, high-efficiency, and power-dense jet engines. This demand raises the thermal requirements and stresses on fuel systems for every new generation of gas turbine engine. Fuel is used to cool subsystems such as engine oil, pumps, electronics, valves, etc. resulting in elevated fuel temperatures upstream of combustor nozzles. Carbonaceous deposits or fouling occurs if the wetted wall temperature is elevated sufficiently, especially at fuel nozzle tips where temperatures are maximized. Fouling within fuel nozzles diminish atomization performance producing incomplete combustion, instability, and polluting byproducts. Therefore, the industry seeks strategies to mitigate carbon deposition without reducing the thermal requirements placed on the fuel. Existing carbon mitigation techniques rely on coating the fuel-wetted surfaces in an inert layer via anodic oxidation, chemical vapor deposition, etc. In this proposal, we aim to investigate a novel approach: inducing the lotus effect (heterogenous wetting) along the walls of fuel passageways. The lotus effect minimizes wetting area along a liquid-solid interface using a highly ordered set of micro or nano features with weak interfacial energy resulting in the liquid only wetting the peaks of said features. We hypothesized that the combination of a chemically inert surface with reduced wetting area diminishes the opportunity for deposit to form. The mitigating effect can be enhanced by the thermal insulation provided by the vapor or gas pockets trapped between the liquid-solid interface, passively reducing the thermal loading of the fuel. As a preliminary step, we produced the lotus effect on multiple aerospace alloys such as Inconel 718, stainless steel 304, and pure titanium via electrochemical etching and surface modification. We then exposed treated tubes to fuel under fouling-favorable conditions to compare their relative deposition rates. Our results indicate that the lotus effect loses stability at pressures well below those used in practical applications. However, the electrochemical etch we developed consistently produced negligible deposit where it would typically be maximized. Depending on if the surface is etched, FAS17 (a perfluoroalkyl silane used to generate superphobicity) can act to encourage or discourage carbon deposition. We determined that the electrochemical etch or FAS17 alone may be a method to mitigate carbon deposition regardless of the wetting behavior </p>

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