A new method for stress-rupture experiments in bending has been developed and used to characterize unidirectional polymer matrix composites. The method. which makes use of very simple fixtures, led to coherent results. These results have been modeled using the large deflection of buckled bars theory (elastica) and it is possible to predict with good accuracy the strain at each point of the specimen if the end-to-end distance is known. The failure process has been experimentally characterized. The formation and propagation of microbuckles leads to a compressive failure. Based on the elastica and the classical lamination theory, a model for the distribution of the Young's modulus along the length of the specimen has been established. Three different micromechanical models have been applied to analyze the time-to-failure versus strain behavior at two temperatures - one below and one above the glass transition. The first micromechanical model considers the nucleation of the microbuckles as the main cause of failure. In addition, the stiffness and stress distributions at any time before failure are calculated based upon the rotation of the fibers in the damaged region. The second and last models, respectively based upon a Paris Law and energy considerations relate the time-to-failure to the propagation of the main microbuckle. For this last model, a good correlation between experimental and theoretical data has been obtained. Finally the influence of the temperature on these models has been studied. / Master of Science

Recent interest in utilizing hydrogen fuel cell technology for automotive applications has lead to concerns regarding the durability of fiber reinforced polymer (FRP) composite materials. Automotive fuel cell power train systems must prove themselves as a reliable alternative to the combustion engines and automatic transmissions. The use of polymer composites in fuel cells to serve as manifolds is promising because of their high strength to weight ratio, and they do not corrode like metals manifolds. Composite materials designed for use in Polymer Electrolyte Membrane (PEM) Fuel Cells are exposed to very high humidity environment and operated at elevated temperatures (~85°C). The susceptibility of fiber reinforced polymers to reduction in modulus, strength, and life in chemical environments has been well documented in the literature, especially at elevated temperatures. A chopped carbon fiber epoxy composite (Material A) and a chopped glass fiber epoxy composite (Material B) were exposed at 85°C to air, water, and a 50/50 water/antifreeze mixture, and periodically examined for tensile, compression, and flexural strengths at various temperatures. Following 2000 hours (83 days) of exposure, Materials A & B did not reach full saturation. Fatigue tests were conducted at various load levels and temperatures to determine their effect on cycles to failure, and carpet plots were generated. Blister formation in aged composites led to reductions in material properties as great as 25% to 75%. A mechanistic explanation was developed for the formation of blisters in the epoxy composite. Recommendations for material improvement and feasibility of material use for fuel cell manifolds and pressure plates were made. / Master of Science

Two comprehensive composite fabrication simulation computer codes have been written. These codes when coupled together have the capability to model the filament winding and curing processes for composite structures. The "Filament Winding Cure" (FWCURE) code is a thermo-kinetic model. FWCURE models the resin viscosity, percent of cure, temperature, resin flow, and layer location. As these characteristics change, they have an effect on the fiber tension within the composite. The "Winding and Curing Stress Analysis Finite Element" (WACSAFE) code models the filament winding process and predicts manufacturing stresses and strains based on material properties, lay-down tension and wind angle. The permeability model in FWCURE requires constants that are found experimentally. The WACSAFE code requires an input tension that is equivalent to the initial spool tension minus the instantaneous tension losses. The permeability constants and the instantaneous tension losses were found experimentally. The codes were then used to predict fiber tension, tension losses and mandrel strains for experimental test cylinders. The predictions were compared to test data. / Master of Science

Damage initiation and growth under compression-compression fatigue loading were investigated for a stitched uniweave material system with an underlying AS4/3501-6 quasi-isotropic layup. Performance of unnotched specimens having stitch rows at either 0° or 90° to the loading direction was compared. Special attention was given to the effects of stitching-related manufacturing defects. Damage evaluation techniques included edge replication, stiffness monitoring, X-ray radiography, residual compressive strength, and laminate sectioning. It was found that the manufacturing defect of inclined stitches had the greatest adverse effect on material performance. 0° and 90° specimen performances were generally the same. While the stitches were the source of damage initiation, they also slowed damage propagation both along the length and across the width and affected through the thickness damage growth. A pinched layer zone formed by the stitches particularly affected damage initiation and growth. The compression failure mode was transverse shear for all specimens, both in static compression and fatigue cycling tests. Specimens without stitches were not available for comparison. / Master of Science

Polymeric diphenylmethane diisocyanate, pMDI, is a wood adhesive that provides excellent composite board properties. Much is unknown about the specific mechanism of pMDI/wood adhesion under conditions that are typical of wood gluing operations. The present research describes the use of ¹⁵N cross-polarization, magic-angle spinning (CP/MAS) NMR as a technique for probing the cure chemistry and bondline morphology of pMDI-bonded wood composites. A 99% ¹⁵N-enriched pMDI resin with desirable adhesive properties was synthesized. A series of model cellulose/¹⁵N-pMDI composites, cured as a function of cellulose precure moisture content, were tested prior to solid wood composites in order to test the feasibility of this technique. Solid wood/¹⁵N-pMDI composites were then cured as a function of wood precure moisture content, cure temperature, and cure time. The ¹⁵N CP/MAS NMR spectra clearly show the dominance of the isocyanate/water reaction on the cure chemistry of all composites tested, both cellulose and solid wood. Four prominent resonances are observed in each spectrum: residual isocyanate, polyurea, and the amide and imide nitrogens of biuret type structures. Different trends in the relative intensities of these resonances are observed as a function of the press variables. Significant amounts of urethane formation are not detected; however, low amounts could be obscured by signal overlap. Relaxation studies using variable contact time experiments were complicated by excessively long cross-polarization rates for nonprotonated nitrogens. However, experiments using variable spin lock times prior to fixed contact periods indicate that the cured resin in these composites is probably a homogeneous continuum. The utility of ¹⁵N CP/MAS NMR for elucidating fine structural and morphological information from complex isocyanate-cured wood composites is clearly demonstrated. / Master of Science

Aqueous-polymer two-phase systems consist of various combinations of water, polymer(s), low molecular weight component(s), and salts. These aqueous-polymer systems are comprised of two phases, each of which contains about 90 percent (by weight) water. Due to some very unique properties, these systems have been applied to separations involving biological molecules for at least a quarter of a century. In particular, these systems are inexpensive, efficient, and provide a mild (aqueous) and possibly stabilizing environment for fragile biologically-active molecules. These systems may also be designed for a high degree of selectivity. Although much effort has been expended in the area of polymer solution theory, the theory of why these systems exhibit this extraordinary two-phase behavior that characterizes them as viable liquid-liquid extraction systems for use with biologically-active molecules is not completely understood. A thermodynamic model which could accurately represent the phase equilibria exhibited by these systems would be useful for the design of systems for use in many different applications. A potpourri of thermodynamic models and their underlying theoretical structure have been critically studied for their particular application to predicting the phase behavior of aqueouspolymer two-phase systems. In particular, the Flory-Huggins model is reviewed (with discussion of its inadequacies and subsequent modifications); the theory of Ogston; the model by Heil; several local composition models (NRTL, Wilson, and UNIQUAC); and two group-contribution models (ASOG and UNIFAC) are all discussed. The development of a solvent-electrolyte model (Chen's model) based on local composition theory (in particular the NRTL model) is reviewed, and the subsequent possible modification of this theory for solvent-polymer-electrolyte systems is discussed. The pros and cons of each model are discussed and qualitative results are given. Quantitative comparisons with experimental data are made with several of these models when appropriate data are available. The main conclusions of this work are: 1. A major limitation to the modeling of these aqueous-polymer two-phase systems is the lack of experimental data. Sufficient, accurate data is needed for the reduction of meaningful thermodynamic parameters by which thermodynamic models can be tested for their applicability. There exists a definite need for the generation of accurate, meaningful thermodynamic data from well characterized systems. 2. The most promising model identified in this work is the theory of Ogston. First, the model is based on the virial expansion and is thus quite suitable for dilute solutions. The Ogston model is the simplest theoretically-relevant dilute-solution model. Second, it appears to be easily extended to solvent-polymer-electrolyte solutions. 3. The Flory equation of state approach appears to be promising for representing polymer solutions. The free volume dissimilarity effect on which it is based is extremely important for solvent-polymer solutions. The most important aspect of this theory is its ability to predict lower critical solution temperature (LCST) behavior -- for which the Flory-Huggins theory is totally inadequate. / M.S.

Carbon fiber reinforced polymer (CFRP) composites are known for their high stiffness-to-weight and high strength-to-weight ratios and hence are of great interest in several engineering fields such as aerospace, automotive and defense. However, despite their light weight, high stiffness and high strength, their application in these fields is limited due to their poor energy dissipation and vibration damping capabilities. This thesis presents a two-phase microlattice design to overcome this problem. To realize this design, a novel tape casting integrated multi-material stereolithography system is developed and mechanical properties of samples fabricated using this system are evaluated. The design incorporating a stiff phase (CFRP) and a high loss phase, exhibiting high stiffness as well as high damping, is studied via analytical and experimental approaches. To investigate its damping performance, mechanical properties at small-strain and large-strain regimes are measured through dynamic material analysis (DMA) and quasi-static cyclic compression tests respectively. It is seen that both intrinsic (small-strain) and structural (large-strain) damping in terms of a figure of merit (FOM), E1/3tanδ/ρ, can be enhanced by a small addition of a high loss phase in Reuss configuration. Moreover, it is seen that structural damping is improved at low relative densities due to the presence of elastic buckling during deformation. For design usefulness, tunability maps, displaying FOM in terms of design parameters, are developed by curve fitting of experimental measurements. The microlattice design is also evaluated quantitatively by comparing it with existing families of materials in a stiffness-loss map, which shows that the design is as stiff as commercial CFRP composites and as dissipative as elastomers. / Master of Science / Carbon fiber reinforced polymer (CFRP) composites are known for their lightweight, high stiffness and high strength and hence are of great interest in several engineering fields such as aerospace, automotive and defense. However, despite these advantages, their application in these fields is limited due to their poor energy dissipation and vibration damping capabilities. This thesis presents a novel cellular lattice design to overcome this problem. Recent growth in stereolithography (SLA) has enabled the fabrication of complex structures with high resolution. Using this capability of SLA additive manufacturing, a cellular design is developed to improve both the stiffness and damping performance of CFRP composites while reducing weight. Experiments are conducted to determine the stiffness and damping properties and small and large deformations. It is seen that the stiffness and damping properties can be increased through a two-material hybrid design, comprising of a high stiffness phase and a high damping phase, arranged in a specific pattern. The microlattice design is evaluated quantitatively by comparing it with the existing families of materials using an Ashby chart. The design shows a two order-of-magnitude increase in the stiffness-damping performance when compared to commercially available CFRP.

With the large number and diversity of materials available today, the ability of the manufacturer to control properties is critical for the success of a product in the market. Although we have little or no control over the engineering properties of solid wood, the potential for the design of material properties in composites is great. Large strides are presently being made in the design of non-veneer structural panels by using material science principles. However, a large gap in our knowledge of the composite system is in the understanding of how raw material properties and processing variables interact to influence the internal geometry and material properties of the components in situ. The ability to use production variables to control material properties of the composite is an extremely valuable tool. The goal of this research is to provide an understanding of how the heat and mass transfer inside a flakeboard during pressing, interacts with the viscoelastic behavior of individual flakes to influence density gradient formation and in situ flake properties. The specific objectives: l. To use observed changes in the temperature and gas pressure of the internal environment of panels during the pressing cycle to describe the composition of the gas phase. 2. To use the calculated composition of the gas phase and measured temperature for the internal environment as boundary conditions for a fundamental heat and mass transfer model to access changes in the temperature and moisture content of the wood component during pressing. 3. To use the temperature and moisture content relations above to qualitatively relate press conditions to the formation of density gradients through changes in the glass transition temperature of the amorphous polymers in wood. 4. To utilize micromechanical models of cellular materials in conjunction with linear viscoelasticity of polymers to develop a nonlinear viscoelasticity model for wood in transverse compression. The approach couples the viscoelastic behavior of the amorphous polymers in wood with the structure imposed by anatomy. These theories, if applicable to wood, can greatly simplify the study of many similar systems combining environmental conditions and mechanical properties. / Ph. D.

"lnterlayering" - the incorporation of low-modulus film adhesive between the plies of composite Iaminates - has proved to be a successful technique for reducing debilitating out-of-pIane stresses. This work seeks to determine the effect interlayering has on a composite Iaminate's in-plane performance. Two Iaminate systems, an unnotched, 16-ply, quasi-isotropic, AS4/C985 and a centernotched, 32-ply, quasi-isotropic, AS4/C1808, were furnished in an interlayered and baseline (non-interlayered) configuration. The interlayers, 0.0005 in. each in thickness, appeared between each ply in every Iaminate tested. Both configurations of these two material systems were subjected to a regimen of in-plane loading tests. These tests included monotonic tension and compression, fully-reversed (R=-1), tension-compression fatigue cycling, and long-term tensile loading. A new test method, called the Incremental Strain Test (IST), was developed in an attempt to isolate and distinguish the long-term, tensile response of the interlayered and baseline Iaminates. This technique and its utility are described herein. The interlayered Iaminates exhibited superior performance during monotonic and IST loading. Distinctly higher ultimate loads and strains were achieved by the interlayered laminates. The notched fatigue performance of the interlayered Iaminates was sub-standard in comparison to the baseline results at the load level tested. The residual tensile strength of the fatigued interlayered Iaminates fell sharply at an early fraction of the laminates’ total life. The presence of the interlayers did not degrade the laminates’ IST performance. Several non-destructive techniques were used to monitor the damage mechanisms. These results, when combined with the experimental findings, helped explicate the disparity found between the interlayered and baseline Iaminate response. / Master of Science

This investigation concerns the validity and feasibility of using moduli reduction to monitor the effect of fatigue damage in graphite epoxy composites. Five laminate orientations were considered, [O]₄, [90]₄, [±45]_s, [0,90]_s, [0,90,±45]_s, and four inplane-stiffness properties were monitored for each. The stiffness parameters were E_xx, E_yy, G_xy, and v_xy, and were measured using a longitudinal tension test, a rail shear test and a transverse bend test. Nondestructive testing techniques such as C-scan and edge replication were also performed to aid in the observation of damage development. Results describe the response of each laminate orientation in tension-tension fatigue, including a record of changes in the stiffness properties at intervals during fatigue. Longitudinal stiffness (E_xx) and shear stiffness (G_xy) were shown to significantly decrease for the [0,90,±45]_s, laminate following fatigue loading. The inplane stiffness properties for the other four laminates remain essentially unchanged following fatigue loading. Matrix cracking and delamination appears to contribute to the stiffness reductions that occur in the [0,90,±45]_s laminate. / Master of Science