A hybrid data-physics driven reduced-order homogenization (dpROH) approach aimed at improving the accuracy of the physics-based reduced order homogenization (pROH), but retain its unique characteristics, such as interpretability and extrapolation, has been developed. The salient feature of the dpROH is that the data generated by a high-fidelity model based on the direct numerical simulations with periodic boundary conditions improve markedly the accuracy of the physic-based model reduction. The dpROH consist of the offline and online stages. In the offline stage, dpROH utilized surrogate-based Bayesian Inference to extract crucial information at the representative volume element (RVE) level. With the inferred data, online predictions are performed using a data-enhanced reduced order homogenization. The proposed method combines the benefits of physics-based reduced order homogenization and data-driven surrogate modeling, striking a balance between accuracy, computational efficiency, and physical interpretability. The dpROH method, as suggested, has the versatility to be utilized across different RVE geometries (including fibrous and woven structures) and various constitutive models, including elasto-plasticity and continuum damage models. Through numerical examples that involve comparisons between different variants of dpROH, pROH, and the reference solution, the method showcases enhanced accuracy and efficiency, validating its effectiveness for a wide range of applications. A novel pseudo-nonlocal eight-node fully integrated linear hexahedral element, PN3D8, has been developed to accelerate the computational efficiency of multiscale modeling for complex material systems. This element is specifically designed to facilitate finite element analysis of computationally demanding material models, enabling faster and more efficient simulations within the scope of multiscale modeling. The salient feature of the PN3D8 is that it employs reduced integration for stress updates but full integration for element matrices (residual and its consistent tangent stiffness). This is accomplished by defining pseudo-nonlocal and local stress measures. Only the pseudo-nonlocal stress is updated for a given value of mean strain or mean deformation measure for large deformation problems. The local stress is then post-processed at full integration points for evaluation of the internal force and consistent tangent stiffness matrices. The resulting tangent stiffness matrix has a symmetric canonical structure with an identical instantaneous constitutive matrix at all quadrature points of an element. For linear elasticity problems, the formulation of the PN3D8 finite element coincides with the classical eight-node fully integrated linear hexahedral element. The procedure is illustrated for small and large deformation two-scale quasistatic problems.

Composites, though used in a variety of applications from chairs and office supplies to structures of U.S. Navy ships and aircrafts, are not all designed to hold up to extreme heat flux and high temperature. Fiber-reinforced polymeric composites (FRPC) have been proven to provide the much needed physical and mechanical properties under fire exposure. FRPC notable features are its combination of high specific tensile strength, low weight, along with good corrosion and fatigue resistance. However FRPC are susceptible to thermal degradation and decomposition, which yields flammable gas, and are thus highly combustible. This property restricts polymeric material usage. This study developed a numerical model that simulated the degradation rate and temperature profiles of a fiber-reinforced polyester resin composite exposed to a constant heat flux and hydrocarbon fire in a cone calorimeter. A numerical model is an essential tool because it gives the composite designer the ability to predict results in a time and cost efficient manner. The goal of this thesis is to develop a numerical model to simulate a zonal-layer polyester resin and fiberglass mat composite and then validate the model with experimental results from a cone calorimeter. By inputting the thermal properties of the layered composite of alternating polymer and polymer-infused glass fiber mat layers, the numerical model is one step closer to representing the experimental data from the cone calorimeter test. The final results are achieved through adding a simulated heat flux from the pilot ignition of the degraded gas of the polyester resin. The results can be coupled into a mechanical model, which may be separately constructed for future study on the mechanical strength of composites under fire conditions.

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This study aimed at understanding the adhesion between wood fibers and thermoplastics. Direct applications include the development of better wood fiber composites and better paper coatings. The objectives of the study were two-fold. First, to quantify the effects of surface treatments on the surface properties, and, second, to determine if adhesion can be described in terms of surface properties. A model consisting of amorphous cellulose spherical beads was used to eliminate the effects of morphology, composition, and fiber size and orientation; adhesion was studied only in terms of surface properties. The surface of the cellulose beads was modified by blending, by coating or by chemical surface reaction. Surface modification by blending was achieved by dissolving cellulose along with another polymer (cellulose propionate) and by beading the mixed solution. An alternative consisted of coating the beads with a thin layer of polymer, such as poly(4-vinyl-pyridine-co-styrene). Finally the surface was also modified by grafting functional groups or polymer segments onto the hydroxyl groups of cellulose. A thin layer of cellulose derivative, such as cellulose trifluoroethoxyacetate or cellulose laurate was produced on the bead surface. Polystyrene and polypropylene segments were grafted onto cellulose to create an interphase. The surface properties of the cellulose beads were then fully characterized. The surface composition was analyzed by X-ray Photoelectron Spectroscopy (XPS), and the morphology was investigated through Scanning Electron Microscopy. Inverse Gas Chromatography (IGC) was used to measure the surfaces' energetics. Two kinds of probes were used: alkanes to measure the dispersive component of adhesion, and acid/base probes to quantify the specific properties. Composites having variable bead content were made by injection molding. The adhesion between the cellulose beads and the thermoplastics were measured by tensile testing and by DMTA using the modified Nielsen Model with the damping factor (tan δ). The surface energy of cellulose was found to depend mostly on the presence and concentration of free hydroxyl groups on the surface. For low degrees of substitution (DS), how these OH groups are replaced by modification, whether by fatty acid type substituents or by fluorine-containing groups, is essentially irrelevant for surface characteristics. The dispersive component of the surface energy (γs^D) declined with DS, almost irrespective of substituent type. The acidic character of the cellulose surface is attributed mainly to the presence of hydroxy groups. It was furthermore established that, while wetting is a necessary condition, it is in itself insufficient for achieving good adhesion and adequate composite strength characteristics. The mechanical properties of polyethylene, polypropylene and polystyrene all decreased as cellulose beads were added in increasing amounts. It was found that improved cellulose fiber-reinforced composite performance requires the development of an interphase, such as by grafting polymer segments onto the cellulose surface which are capable of entanglement with the thermoplastic polymer in the melt. Maleic anhydride/polypropylene copolymers were found to be efficient coupling agents that better transmit stress when their molecular size increased. Adsorption of poly(4-vinylpyridine-co-styrene) [basic] by the cellulose beads [acidic] resulted in completely coated surfaces. However, strength differences between composites, with coated and uncoated beads, were insignificant probably owing to the large bead sizes used. / Ph. D.

A systematic study was conducted to examine the influence of fiber surface treatment and sizing on the formation of fiber-matrix interphase and its effects n the mechanical properties of composite laminates. Three material systems having the same Apollo graphite fibers and HC 9106-3 toughened epoxy matrix, but with different fiber surface treatments and sizings were used in this study. The fibers used in the 810A and 820 A systems received 100% and 200% industry standard surface treatments respectively and were sized with Bisphenol-A unreacted epoxy material. The 810 O system was manufactured with 100% surface treated fibers that were sized with pvp (polyvinylpyrrolidone), a thermoplastic material. The presence of different interphase in these materials was confirmed using a permanganic etching technique. Results indicate that the interphase is discontinuous and made of linear chain polymeric material in the 810 A system. The interphase in the 810 O system has a gradient morphology while the 820 A system does not possess a well defined interphase. Mechanical test results indicate that the 810 O system significantly greater longitudinal tensile strength and failure strain compared to the 810 A system. The 810 A and 820 A systems have similar longitudinal tensile properties. Transverse tensile test results indicate that the 820 A system has the highest strength while the 810 O system has the lowest strength. The (0,90₃), cross-ply laminates from the three material systems exhibit different damage mechanisms and failure modes under monotonic tensile loading. Fatigue test results indicate that the 810 O laminates have longer fatigue lives at higher load levels and shorter fatigue lives at lower load levels compared to the 810 A laminates. The 820 A laminates have longer life compared to the other two materials systems, at all three load levels. The 810 O material exhibits greater damage and stiffness reduction than the other two materials. The 810 A and 820 A systems exhibit a brittle stress concentration controlled failure, while the pvp sized 810 O system exhibits a global strain conuolled failure. A micromechanics model was developed to investigate the role of the interphase on the tensile strength of unidirectional laminates. A new parameter called the ‘efficiency of the interface’, is introduced in the model. A simple scheme that uses the experimentally determined tensile modulus of unidirectional laminates in a concentric cylinders model, is described to estimate the interfacial efficiency. The tensile fatigue performance of cross-ply laminates is predicted using this micromechanics model in a cumulative damage scheme. The predicted fatigue lives and failure modes agree well with experimental results. / Ph. D.

Part I (Efficient Endochronic Finite Element Analysis: an Example of a Cyclically Loaded Boron/Aluminum Composite): A convenient and efficient algorithmic tangent matrix approach has been developed for 3-D finite element (FE) analysis using the endochronic theory without a yield surface. The underlying algorithm for integrating the endochronic constitutive equation was derived by piecewise linearization of the plastic strain path. The approach was employed to solve a micromechanics boundary value problem of a cyclically loaded unidirectional boron/6061 aluminum composite. All the FE results consistently demonstrate superior numerical stability and efficiency of the proposed method. Extensions of the method to endochronic plasticity with a yield surface and to the plane stress case are also presented. Part II (Simple and Unified Finite Element Formulation for Doubly Periodic Models: Applications to Boron/Aluminum Composites): A simple and unified weak formulation and its convenient FE implementation have been proposed. The weak formulation is valid for any doubly periodic model under uniform 3-D macro-stress, and serves as a common rational foundation of different FE approaches. The algorithmic tangent matrix approach for the endochronic theory has been incorporated into the FE formulation to model isothermal, rate-independent plastic macro-deformation of unidirectional fibrous composites with idealized two-phase micro-structure and backed-out inelastic matrix properties. Methods for determining inelastic material parameters of the matrix have been established. Numerical results for a B/6061 AI composite subjected to on-axis and off-axis monotonic tensile loadings are in good agreement with experimental data. The micromechanics model also shows the potential for quantitative characterization of complicated cyclic behavior. Finally, some effects of model geometry on overall plastic response of the B/6061 AI composite are discussed from the viewpoint of theoretical-experimental correlation. / Ph. D.

The adhesion of carbon fibers to several high performance polymers, including a phosphorus-containing bismaleimide, a cyanate ester resin, and a pyridine-containing thermoplastic, was evaluated using the microbond single fiber pull-out test. The objective was to determine the chemical and mechanical properties of the fiber and the polymer which affect the fiber/polymer adhesion in a given composite system. Fiber/matrix adhesion is of interest since the degree of adhesion and the nature of the fiber/matrix interphase has a major influence on the mechanical properties of a composite. The surface chemical composition, topography, tensile strength, and surface energy of untreated AU-4 and commercially surface treated AS-4 carbon fibers were evaluated using x-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), single fiber tensile tests, and dynamic contact angle analysis. The commercial surface treatment which converted the AU-4 to the AS-4 fiber oxidized the carbon fiber surface. The surface of the AS-4 carbon fiber was further modified using air, oxygen, ammonia, and ethylene plasmas. The AS-4 fiber tow was also characterized following exposure to the aqueous poly(amic acid) solution used to disperse the matrix powder during aqueous suspension prepregging of thermoplastic matrix composites. The air and oxygen plasma treatments significantly oxidized and roughened the surface of the AS-4 carbon fibers. In addition, the air and oxygen plasma increased the the polar component of the AS-4 fiber surface energy. The ammonia plasma increased the concentration of nitrogen on the fiber surface, without significantly altering the fiber topography (at a nlagnification of 50,000X). The atomic oxygen present in the air and oxygen plasma treatments is capable of reacting with both the edge and basal planes in the carbon fiber structure. As a result, the oxygen-containing plasmas progressively ablated the organic material in the carbon fiber surface. Energetic species in the ammonia plasma cleaned the fiber surface and reacted with the carbon fiber surface, increasing the concentration of amine groups in the fiber surface. The ethylene plasma deposited a layer of plasma polymerized polymer on the carbon fiber surface. The AS-4 carbon fibers were coated with poly(amic acid) when the tow was wet with the aqueous suspension prepregging solution. The carbon fiber adhesion of bis(3-maleimido phenoxy) triphenylphosphine oxide was compared to that of Ciba-Geigy's Matrimid 5292 A/B bismaleimide system. With both bismaleimides, the carbon fiber adhesion increased significantly when the fiber received an oxidative commercial surface treatment or was exposed to an air or ammonia plasma prior to bonding. In contrast, the poly(pyridine-bis A) microbond pull-out test results showed that the carbon fiber adhesion of poly(pyridine-bis A) was not affected by the fiber surface chemical composition, fiber surface energy, or topography. / Ph. D.

An experimental and analytical study of the tribological behavior of polymer composites is presented. Glass, aramid, and carbon fiber-filled polyamide (Nylon 6,6) composites serve as models for understanding friction and wear processes encountered when polymer composites are used in tribological applications. Experimental results not only include measurements of friction and wear, but surface temperatures produced by frictional processes during oscillating contact experiments. Since an optically flat, transparent sapphire disk is used as the oscillating countersurface, surface temperatures can be measured directly at the interface using an infrared microscope. Experimental results show that the presence of fibers in the polyamide matrix lowers wear, friction, and surface temperature when compared with the unfilled polymer. Rationale for this improved tribological behavior is presented and discussed. Fiber-type is shown to have a direct influence on the tribological behavior of the polymer composite, and the chemical behavior at and near the interface is shown to be significant by examining worn and transferred material through surface analytical techniques. In particular, evidence is presented for the tribochemical degradation of intramolecular bonds in the polyamide macromolecule. Measurements of surface temperatures are compared with theoretical predictions using models for the real area(s) of contact, and results from “scanning” experiments are also presented in which the infrared microscope is used to measure surface temperatures at possible real areas of contact within the apparent contact region. Instantaneous measurements of surface temperature and friction over a single cycle of motion are also presented which allows for the performance of a frequency domain analysis. This technique not only shows the frequency content of the friction and surface temperature signals, but it also shows correlations between these two parameters. The role of intermolecular attractions in frictional processes is addressed, and evidence for relatively strong intermolecular attractions between the polyamide surface and sapphire disk is discussed. / Ph. D.

A continuous physical cake made up of porous fibrous media can be formed by using air to draw the fibers to a moving screen. A numerical model of the formation of this cake has been formulated and solved. The numerical model is based on solving Darcy’s law, the Bernoulli equation, and two-material related experimental correlations at discrete points along the screen. A permeability measurement test apparatus was designed and built, and experiments were run to determine the experimental relations for two different materials. A computer code was then written to solve the system of equations at each point on the screen and give a density distribution of the resulting cake. Tests were then run to see the effects of various density anomalies in the material at different points along the screen. The results of the experiments show that the first material was more permeable and more compressible than the second material. This lead to distinct differences in the cake that the two formed in the numerical model. The first material formed a fairly constant density cake that was not greatly affected by the density anomalies. The second material had a large variation in density across the final cake height and was affected more by the different density anomalies. / Master of Science

A comprehensive stress model of the filament winding fabrication process, previously implemented in the finite element program, WACSAFE, was implemented using the ABAQUS finite element software package. This new implementation, referred to as the ABWACSAFE procedure, consists of the ABAQUS software and a pre/postprocessing routine that was developed to prepare necessary ABAQUS input files and process ABAQUS displacement results for stress and strain computation. The ABWACSAFE procedure has a structure and general flow pattern similar to the WACSAFE program. ABAQUS now performs the fundamental finite element procedures needed in the fabrication stress model. The ABWACSAFE pre/postprocessing routine utilizes many subroutines from the WACSAFE program. Some subroutines are used in their original form while many were significantly modified. New subroutines have been written as well. / Master of Science