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The ability to predict potential strength degradation of a filament wound sphere was developed using an incremental finite element model of the composite during fabrication. The sphere was modeled taking into account the winding/loading pattern and the resulting internal layer boundaries. The thickness profile of the sphere's layers were computed using a thickness profile/pattern simulation program. This thickness profile was used by the mesh generating program to ensure that the elements generated did not cross , layer boundaries. The elements used were four noded isoparametric quadrilateral elements and these were collapsed to triangular elements for transitions. The input to the finite element program was prepared by an interface program which combines the mesh generator output with the loading and option control data. The main feature of the finite element program was the incremental construction and loading of the model. Strength degradation definitely occurs when the average fiber layer strain is negative. The negative strain means that all the winding tension has been lost from the layer and the fibers in uncured resin will buckle when they try to support compressive loading. Then when the resin cures the buckled region of fibers are degraded in strength. This model gives a layer-by-layer analysis of the potential strength loss of the composite. / Ph. D.

The effect of surface pretreatment on the adhesive bonding and bond durability of carbon fiber/epoxy and carbon fiber/bismaleimide matrix composites was studied. Methyl ethyl ketone (MEK) wipe, peel ply, grit blast and gas plasma treatments were the pretreatments of interest. Chemical and physical changes which occurred in the cured composite surfaces following pretreatment were characterized with x-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy (ISS), contact angle analysis, diffuse reflection infrared spectroscopy (DRIFT), profilometry and scanning electron microscopy (SEM). Double lap shear and Boeing wedge configurations were used to evaluate the strength as well as the durability of the composites bonded with an epoxy film adhesive. Fluoropolymer residues which were found on the composite surfaces were fully removed by grit blasting and oxygen plasma treatments, but not by an MEK wipe. The use of a peel ply prevented fabrication contamination from depositing on the bonding surfaces. In addition to its cleaning effect, oxygen plasma was also capable of incorporating additional polar functionality into the composite surface. The presence of the fluoropolymer contamination on the MEK-wiped surface resulted in low surface energy and wettability, whereas peel ply, grit blast and oxygen plasma improved both the surface energy and the wettability of the composite surfaces. The grit blasted and peel ply surfaces were observed to have a significant degree of roughness, as measured by profilometry and seen by SEM. A rubber-toughened epoxy film adhesive was used for the bonding studies. Lap shear strengths were evaluated under ambient conditions as well as at 82°C, both dry and following a 30 day/71°C water exposure. Wedge durability testing was carried out in a dry 75°C oven, 75°C water, 100°C water and aircraft de-icing fluid. Relative to the MEK-wiped controls, lap shear strength as well as hot/wet durability was improved by the peel ply and oxygen plasma treatments for both epoxy and bismaleimide composites. Grit blasting was seen to have some utility for the epoxy composites at room temperature, but was generally observed to be detrimental to strength and durability, particularly in the case of the bismaleimide composites. In order to separate the effect of surface chemistry from the effect of surface roughness on composite bond strength, a study was carried out in which surface functionality was varied while the topography remained constant. For this purpose, peel ply surfaces, which have a consistent and reproducible degree of roughness, were treated with fluoropolymer compounds and gas plasmas, as well as left untreated. It was found that the removal of fluoropolymer contamination was the main contributor to the observed bond strength improvement following plasma treatment; however, highly functionalized oxygen plasma-treated surfaces showed evidence of improved durability in a hot aqueous environment. The effect of elapsed time following oxygen plasma treatment of epoxy composites was also studied. XPS atomic concentration, wettability by water and a liquid epoxy resin, and lap shear strengths were plotted as a function of time following removal from the plasma reactor. Changes which occurred in the chemistry and wettability of an oxygen plasma-treated surface had a subsequent negative effect on the lap shear strengths of the bonded specimens. A study was carried out using model epoxy and bismaleimide compounds in thin film form, for the purpose of studying surface chemistry and interfacial reactions following an oxygen plasma treatment. XFS and infrared reflection-absorption spectroscopy (IR-RAS) were used to probe the reactions which occurred. Close correspondence was found between the XPS and IR-RAS analysis of functional groups incorporated into the surface of the films by the plasma treatment. IR-RAS analysis of the model surfaces following exposure to a neat, liquid epoxy resin revealed that, while adsorption of the liquid epoxy occurred on both plasma-treated and nonplasma-treated surfaces, the oxygen plasma treated surface alone was capable of initiating ring-opening reactions in the epoxy. However, this effect was not observed unless immediate contact was made between the plasma-treated surface and the liquid epoxy resin. / Ph. D.

A one-dimensional cylindrical cure model was developed to describe the curing process of an axisymmetric filament wound composite structure. For a specified cure cycle, the cure model can be used to calculate the temperature distribution, the degree of cure of the resin, and the resin viscosity inside the composite case. Solutions to the cylindrical cure model were obtained numerically using the finite element technique. The cylindrical cure model was verified by measuring the temperature distribution in a small 5.75 inch graphite - epoxy test bottle. The data were compared with the results calculated with the computer code for conditions employed in the tests. Good agreement was found between the data and the results of the computer code. The error between the experimental data and the results of the computer code was less than 10 %. A cure cycle optimization problem is formulated for the curing process using a calculus of variations approach. The optimum cure cycle should tailor the temperature in the composite such that a uniform temperature and degree of cure distribution is achieved in the composite while minimizing the reaction exotherms and thermal lag. Cure simulations of an one inch thick graphite - epoxy composite case predict a minimization of the reaction exotherms and the thermal lag. The the final process time needed to achieve uniform degree of cure and uniform temperature distribution in the composite is also predicted. The resultant cure cycle appears to approach the boundary temperatures specified as limits on the cure cycle temperature. / Master of Science

Composite materials are considered here as structural materials of highway bridge superstructures. Bridge deck designs can be done according to <i>AASHTO</i>¹ specification and elastic design concepts. In order to evaluate the feasibility of composites as structural materials of highway bridge superstructures, composite materials are compared not only to composite materials themselves but also to the most popular bridge structural materials, which are reinforced concrete and structural steel. The <i>AASHTO</i>¹ HS2O-44 truck load is selected as the standard loading condition of all designs. Loads other than dead load and live load are not considered. Configurations of the bridges are different. Appropriate cross-section of girders are selected according to the material types. For fiber reinforced composite materials, box girder is used, for reinforced concrete, T-beam is selected; in addition, steel concrete composite section is another case. Design methods are different from material to material. Reinforced concrete T-beam design is based on the 'Ultimate Strength Design' method. Steel concrete composite sections are designed according to the 'Load & Resistance Factor Design'. For composite materials, 'Elastic Design' is selected. The results derived are as expected. Substantial weight saving is achieved by simply replacing concrete or steel with composite materials. This also results in many other advantages such as construction time, cost, foundation settlement and support requirements. / Master of Science

The influence of crenulated noncircular fibers on the micromechanical stress states due to a transverse strain and to a temperature change in carbon/carbon composites is examined using the finite element method. Stresses at the interface of both fully bonded and fully disbonded fibers having two crenulation amplitudes and with two fiber volume fractions are presented. In each case, these interface stresses are compared to stresses at the interface of circular fibers which have the same degree of disbond and fiber volume fraction and are under the same loading conditions. For the disbonded cases, deformed meshes showing locations of fiber/matrix contact are also included. In addition to the interface stress states, selected composite properties are also computed and compared in each case examined. Interest in studying noncircular fibers stems from a desire to increase the transverse properties of carbon/carbon by introducing a mechanical interlocking between the fiber and the matrix. Results presented here indicate that this interlocking does in fact occur. Evidence from the interface stress data suggests, however, that any possible advantage of this interlocking may be outweighed by the disadvantage of stress concentrations which arise at the interface due to the crenulated geometry of the fibers / Master of Science

A time-dependent characterization study was performed on T300/934 graphite/epoxy in a simulated space environment. Creep tests on irradiated and non-irradiated graphite/epoxy and bulk resin specimens were carried out at temperatures of 72 °F and 250 °F. Irradiated specimens were exposed to dosages of penetrating electron radiation equal to 30 years 'exposure at GEO-synchronous orbit. Radiation was shown to have little effect on the creep response of both the composite and bulk resin specimens at 72 °F while radiation had a significant effect at 250 °F. A healing process was shown to be present in the irradiated specimens where broken bonds in the epoxy due to radiation recombined over time to form cross-links in the 934 resin structure. An analytical, micromechanical model was also developed to predict the viscoelastic response of fiber reinforced composite materials. The model was shown to correlate well with experimental results for linearly viscoelastic materials with relatively small creep strains. / 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

The plane in-plane compression response for a symmetrically laminated composite panel with a spatially varying fiber orientation has been analyzed for four different boundary conditions. Variation of the fiber angle along the length of a composite laminate results in stiffness properties that change as a function of location. The laminates are therefore termed variable stiffness panels. This work presents an analysis of the stiffness variation and its effect on the in-plane and buckling response of the panel. The fiber orientation is assumed to vary only in one spatial direction, although the analysis can be extended to fibers that vary in two spatial directions. A system of coupled elliptic partial differential equations that govern the in-plane behavior of these panels has been derived. Solving these equations yields the displacement fields, from which the strains, stresses, and stress resultants can be subsequently calculated. A numerical solution has been obtained using an iterative collocation technique. Corresponding closed form solutions are presented for the in-plane problem for four different sets of boundary conditions. Three of the cases presented have exact solutions, and therefore serve to validate the numerical model. The Ritz Method has been used to find the buckling loads and buckling modes for the variable stiffness panels. Improvements in the buckling load of up to 80% over straight fiber configurations were found. Results for three different panel aspect ratios are presented. / Master of Science

Due to the rapid expansion of applications of composite materials, investigations of their properties have greatly increased. Since theoretical and numerical methods have many limitations for anisotropic materials, experimental methods are sometimes the only way to answer the questions. It has been proved that moire interferometry is a powerful technique in the study of composite materials. The high sensitivity and resolution of a measurement technique is the key to determining the properties of a material which has a fine and complicated structure such as fiber reinforced composite laminates. In this paper, a carrier fringe method is introduced to increase the resolution of the fringe gradient in the moire technique. The ability of measurement is extended to the micromechanics region. High strain concentrations and the dramatic displacement variations can be determined by measuring the slopes of carrier fringes. Strain distributions across the plies (with the thickness of 125 μm) in graphite/epoxy composites and strain concentrations in the resin-rich zones (with the thickness of 10 μm) between neighboring plies are revealed by the carrier fringe technique. Three experiments are presented to show the effectiveness of the application of carrier fringes to resolve fringe gradients and obtain strains. The current moire technique is limited to the optical laboratory because it is extremely sensitive to the disturbance of the environment. A vibration with magnitude of 0.2 μm can completely wash out the contrast of a moire fringe pattern. The study has been done in moving moire interferometry off the optical table. Vibration insensitive moire systems are investigated to extend the moire technique to the tests of large structures and using testing machines for loading. Vibration problems are discussed and the new ideas for eliminating vibration effects are presented. Six representative schemes are analyzed and three of these systems are built to perform experiments in rough environments such as on a hydraulic testing machine. The results show the great success of these new systems. / Ph. D.