The thermo-mechanical response of unidirectional P100 graphite fiber/6061 aluminum matrix composites (v_f = 0.47) was investigated at four temperatures: -150°F, +75°F, +250°F and +500°F, using test methods developed at Virginia Tech. Two types of tests, off-axis tension and Iosipescu shear, were used to obtain the desired properties. Good experimental-theoretical correlation was obtained for E_xx, v_xy and G₁₂. It is shown that E₁₁ is temperature independent, but E₂₂, v₁₂ and G₁₂ generally decrease with increasing temperature. Compared with rather high longitudinal strength, very low transverse strength was obtained for the graphite/aluminum. The poor transverse strength is believed to be due to the low interfacial bond strength in this material. The strength decreases significantly with increasing temperature. The tensile response at various temperatures is greatly affected by the residual stresses caused by the mismatch in the coefficients of thermal expansion of fibers and matrix. The degradation of the aluminum matrix properties at higher temperatures has a deleterious effect on composite properties. The composite has a very low coefficient of thermal expansion in the fiber direction. / M.S.

The objectives of this study were to characterize the effects of the space environment on the compressive behavior of T300/934 graphite/epoxy composite material and on the tensile properties of the neat (unfilled) epoxy matrix material. Both materials were tested in the baseline state and after exposure to electron radiation (total dose of 10,000 Mrads of 1 MeVelectrons at a dose rate of 50 Mrads/hr). Irradiation was conducted under vacuum and simulates 30 year, "worst case", exposure in geosynchronous earth orbit. A compressive test method was developed to characterize thin (8-ply) unidirectional coupons. Compression tests were conducted at cryogenic (-250°F; -157°C), room, and elevated (250°F; 121°C) temperatures. Elastic and strength properties were obtained in the principal material directions (E₁, E₂, v₁₂, v₂₁, X_c, Y_c). Tensile specimens of the neat Fiberite 934 epoxy resin were fabricated and tests were conducted at room and elevated (250°F; 121°C) temperatures. Elastic and strength properties (E, ν, δ_ult) were obtained. Irradiation and temperature were found to have a significant effect on composite and neat resin properties. Properties tended to improve at cryogenic temperature and degrade at elevated temperature. Irradiation degraded properties at all temperatures, with the degradation being most severe at elevated temperature. / M.S.

The 250 Kev Cockroft - Walton accelerator currently under construction at Virginia Polytechnic Institute is to be used as a pulsed neutron source. The time dependence of neutron flux, energy spectrum as a function of time, and various reactor parameters such as Fermi Age, slowing dotm time, and diffusion length in graphite will be measured using the pulsed source method. This thesis is a Monte Carlo calculation of the results to be expected in such an experiment. The neutron flux and energy spectrum was calculated at 1, 2, 5, and 10 microseconds after a burst of fast neutrons. The neutrons were “tracked" to thermal and the Fermi Age and slowing down time were determined. / Master of Science

Carbon nanotubes (CNTs) and graphite nanoplatelets (GNPs) are carrying great promise as two important constituents of future multifunctional materials. Originating from their minimal defect confined nanostructure, exceptional thermal and electrical properties have been reported for these two allotropic forms of carbon. However, a brief survey of the literature reveals the fact that the incorporation of these species into a polymer matrix enhances its effective properties usually not to the degree predicted by the composite\\textquoteright s upper bound rule. To exploit their full potential, a proper understanding of the physical laws characterizing their behavior is an essential step. With emphasis on the electrical and thermal properties, the following study is an attempt to provide more realistic physical and computational models for studying the transport properties of these nanomaterials. Originated from quantum confinement effects, electron tunneling is believed to be an important phenomenon in determining the electrical properties of nanocomposites comprising CNTs and GNPs. To assess its importance, in this dissertation this phenomenon is incorporated into simulations by utilizing tools from statistical physics. A qualitative parametric study was carried out to demonstrate its dominating importance. Furthermore, a model is adopted from the literature and extended to quantify the electrical conductivity of these nanocomposite. To establish its validity, the model predictions were compared with relevant published findings in the literature. The applicability of the proposed model is confirmed for both CNTs and GNPs. To predict the thermal properties, a statistical continuum based model, originally developed for two-phase composites, is adopted and extended to describe multiphase nanocomposites with high contrast between the transport properties of the constituents. The adopted model is a third order strong-contrast expansion which directly links the thermal properties of the composite to the thermal properties of its constituents by considering the microstructural effects. In this approach, a specimen of the composite is assumed to be confined into a reference medium with known properties subjected to a temperature field in the infinity to predict its effective thermal properties. It was noticed that such approach is highly sensitive to the properties of the reference medium. To overcome this shortcoming, a technique to properly select the reference medium properties was developed. For verification purpose the proposed model predictions were compared with the corresponding finite element calculations for nanocomposites comprising cylindrical and disk-shaped nanoparticles. To shed more light on some conflicting reports about the performance of the hybrid CNT/GNP/polymer nanocomposites, an experimental study was conducted to study a hybrid ternary system. CNT/polymer, GNP/polymer and CNT/GNP/polymer nanocomposite specimens were processed and tested to evaluate their thermal and electrical conductivities. It was observed that the hybrid CNT/GNP/polymer composites outperform polymer composites loaded solely with CNTs or GNPs. Finally, the experimental findings were utilized to serve as basis to validate the models developed in this dissertation. The experimental study was utilized to reduce the modeling uncertainties and the computational predictions of the proposed models were compared with the experimental measurements. Acceptable agreements between the model predictions and experimental data were observed and explained in light of the experimental observations. The work proposed herein will enable significant advancement in understanding the physical phenomena behind the enhanced electrical and thermal conductivities of polymer nanocomposites specifically CNT/GNP/polymer nanocomposites. The dissertation results offer means to tune-up the electrical and thermal properties of the polymer nanocomposite materials to further enhance their performance. / Ph. D.

The wave propagation mechanism of mode transitions was studied in unidirectional composite materials. Theoretical calculations were compared with experimental measurements. Mode transitions occur when the wave vector orientation is varied in unidirectional samples of T300/5208 graphite/epoxy composite with a 0.6 fiber volume fraction. The mode transition occurs at 51.85° wave vector orientation with respect to the fiber direction. Energy flux deviation and particle displacement directions and amplitudes were also compared with theory. To show this mode transition, an attenuation study was performed. The attenuation coefficient, measured in units of reciprocal time, does not appear to depend on the wave vector orientation and the wave type (quasi-transverse and quasi-longitudinal waves) at 5 MHz frequency. But the attenuation coefficient, expressed in units of reciprocal length, does depend on the wave type and the wave vector orientation because the wave velocity is included in the calculation of this coefficient. / Master of Science

Energy absorption during complete penetration of thin graphite composites is experimentally shown to be significantly improved by low volume fractions of embedded superelastic shape memory alloy (SMA) fibers. Graphite/Bismaleimide laminates were embedded with 3% and 6% volume fractions of superelastic nitinol fibers. Quasi-static tests were performed on wide clamped-clamped beams to identify progressive damage mechanisms. Low velocity (13.9 ft/s) impact tests, at an impact energy of 31.5 ft-lbs, resulting in complete penetration were also performed on wide clamped-clamped beams. These tests show that only after peak load is there a contribution made by the SMA to the load deflection behavior of the composite. Owing to the SMA's high strength and high strain to failure it remains undamaged after failure of the base composite. The interaction between the base composite and the SMA creates an increase in absorbed energy over the base composite of as much as 41 % in a Graphite/Bismaleimide laminate embedded with a 6% volume fraction of nitinol fibers. C-scans of the hybrids embedded with bi-directional nitinol fibers show a 22% larger delamination areas compared to plain graphite epoxy. The larger delaminations are a result of the nitinol fibers distributing the impact energy to a larger area of the base composite. This interaction between the nitinol and the graphite is one of the reasons for the increases in absorbed energy. Fiber pull-out and strain energy of the nitinol fibers also adds to the increase in absorbed energy. Although damage initiation and peak loads do not seem to be affected by the embedded nitinol fibers, the energy absorption after peak loads is greatly improved. This improvement is a result of increased energy distribution through the SMA to the graphite. The large improvements in energy absorbing capabilities offered by SMA fibers give SMA hybrid material systems promise in applications where penetration resistance is imperative. / Master of Science

The effect of the cylindrically orthotropic morphology known to exist in graphite fibers on the effective properties of a composite material was studied using the composite cylinder assemblage model. The cylindrical orthotropy of the fibers was found to have no effect on the properties of a composite with purely orthotropic fibers. For fibers with a transversely isotropic core both the size of the core and the morphology of the sheath were found to have an effect on the composite properties. The stress states resulting in the composite cylinder for axial, radial, axial shear, and thermal loads were examined. Singular stresses were observed to occur at r=0 in some fibers in some load conditions. The presence of a transversely isotropic core, which must exist in a real fiber, removed this singularity. The strength of the composite cylinder was found to depend on uÌ ber morphology. The size of the transversely isotropic core within the uÌ ber also affected the strength. The strength of the uÌ ber increased with increasing transversely isotropic core size in some instances. In general, for axial loading failure is expected to be caused by fiber breakage. For radial, axial shear, and thermal loading the failure mode is uÌ ber splitting. / 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

Space simulated thermally induced deformations and stresses in continuous fiber-reinforced composites were investigated with a micromechanics analysis. The investigation focused on two primary areas. First, available explicit expressions for predicting the effective coefficients of thermal expansion (CTE's) for a composite were compared with each other and with a finite element (FE) analysis, developed specifically for this study. Analytical comparisons were made for a wide range of fiber/matrix systems, and predicted values were compared with experimental data. All of the analyses predicted nearly identical values of the axial CTE, α₁, for a given material system, and all of the predictions were in good agreement with the experimental data. Results from the FE analysis, and those from the solution of a generalized plane strain boundary value problem, were in excellent agreement with each other and with the experimental data for the transverse CTE, α₂. Less rigorous formulations were in poor agreement with the experimental data. The second area of investigation focused on the determination of thermally induced stress fields in the individual constituents. Stresses predicted from the FE analysis were compared to those predicted from a closed-from solution to the composite cylinder (CC) model, for two carbon fiber/epoxy composites. A global-local formulation, combining laminated plate theory and FE analysis, was used to determine the stresses in multidirectional laminates. Thermally-induced damage initiation predictions were also made. The type of analysis (i.e. CC or FE) was shown to significantly affect the distributions and magnitudes of the predicted stresses. Thermally-induced matrix stresses increased in absolute value with increasing fiber volume fraction but were not a strong function of fiber properties. Multidirectional [0₂/±θ]s laminates had larger predicted thermally induced matrix stresses than unidirectional ([0]) laminates, and these stresses increased with increasing lamination angle θ. Thermally-induced matrix failure predictions, using a maximum stress failure criterion based on the normal interfacial stress component and the measured transverse lamina strength, were in excellent agreement with experimental data. / Ph. D.

The objective of this investigation was to characterize two cyanate ester resins and a eight harness satin (8HS) graphite fabric for use in resin film infusion (RFI) processing. Two cyanate ester resin systems were characterized to determine their cure-kinetics, and viscosities during cure. A 8HS graphite fabric was tested in compaction and through the thickness permeability. A one-dimensional, through the thickness, flow and cure computer simulation was run. The resin cure-kinetics models predicted the curing behavior of the resins as functions of time, temperature, and degree of cure. The proposed viscosity models determined the resin viscosity as a function of temperature and degree of cure. The 8HS graphite fabric was tested in compaction and through the thickness permeability to determine the effect of compaction pressure on fiber volume fraction and in turn on through the thickness permeability. The one-dimensional RFI flow and cure simulation combined the cure-kinetics and viscosity models of the resins with the characteristics of the graphite fabric and determined resin infiltration and cure times. The proposed cure-kinetics and viscosity models were more than adequate in modeling the cure and flow behavior of the cyanate ester resin systems. Power law curve fits accurately represented the compaction and through the thickness permeability of the 8HS graphite fabric. Finally, the one-dimensional RFI flow and cure simulation showed that resin viscosity was the major influence on the infiltration times. / Master of Science