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Two optical techniques to measure the thermal expansion of fiber-reinforced composites are discussed. These techniques are moire interferometry and Fizeau interferometry. The pertinent features of each setup are presented and the preparation of the composite specimens discussed. Thermal expansion measurements in the range 116 to 422 K for 0 deg, 90 deg, and quasi-isotropic graphite-epoxy composite laminates are presented.
Short beam testing was used to determine the shear properties of laminates consisting of T-300 and Celion 3000 and 6000 graphite fibers, in epoxy, hot melt and solvent bismaleimide, polyimide and polystyrylpyridine (PSP). Epoxy, composites showed the highest interlaminar shear strength, with values for all other resins being substantially lower. The dependence of interlaminar shear properties on the fiber-resin interfacial bond and on resin wetting characteristics and mechanical properties is investigated, and it is determined that the lower shear strength of the tested composites, by comparison with epoxy resin matrix composites, is due to their correspondingly lower interfacial bond strengths. An investigation of the effect of the wettability of carbon fiber tow on shear strength shows wetting variations among resins that are too small to account for the large shear strength property differences observed.
The effects of galvanic coupling between D6AC steel, 6061-T6 aluminum, Inconel 718, and graphite-epoxy composite material (G/E) in 3.5% NaCl were studied. Measurements of corrosion potentials, galvanic currents and corrosion rates of the bare metals using weight-loss methods served to establish the need for corrosion protection in cases where D6AC steel and 6061-T6 aluminum are galvanically coupled to G/E in salt water while Inconel 718 was shown to be compatible with G/E. Six tests were made to study corrosion protective methods for eliminating galvanic corrosion in the cases of D6AC steel and 6061-T6 aluminum coupled to G/E. These results indicate that, when the G/E is completely coated with paint or a paint/polyurethane resin combination, satisfactory protection of the D6AC steel is achieved with either a coat of zinc-rich primer or a primer/topcoat combination. Likewise, satisfactory corrosion protection of the aluminum is achieved by coating it with an epoxy coating system.
Traditional composite lamination theory was used to predict composite graphite-epoxy laminate stiffnesses for comparison to quasi-experimental stiffnesses developed from cylindrical bottle pressure testing. Stiffness sensitivities were examined for variations in constituent materials and geometric properties. The material component interactions examined were the fiber longitudinal and transverse Young's modulus, the fiber shear modulus and primary Poisson's ratio, the resin Young's modulus, shear modulus, and Poisson's ratio. The geometric variation of the helical winding angle was also examined. Two computer programs were written to generate the data used to demonstrate the stiffness variations.
The effects of high energy radiation on mechanical properties and on the molecular and structural properties of graphite fiber reinforced composites are assessed so that durability in space applications can be predicted. A listing of composite systems irradiated along with the maximum radiation dose applied and type of mechanical tests performed is shown. These samples were exposed to 1/2 MeV electrons.
Experimental results are presented for the elastic and strength properties of T300/5208 graphite-epoxy at room temperature, 116K (-250 F), and 394K (+250 F). Results are presented for unidirectional 0, 90, and 45 degree laminates, and + or - 30, + or - 45, and + or - 60 degree angle-ply laminates. The stress-strain behavior of the 0 and 90 degree laminates is essentially linear for all three temperatures and that the stress-strain behavior of all other laminates is linear at 116K. A second-order curve provides the best fit for the temperature is linear at 116K. A second-order curve provides the best fit for the temperature dependence of the elastic modulus of all laminates and for the principal shear modulus. Poisson's ratio appears to vary linearly with temperature. all moduli decrease with increasing temperature except for E (sub 1) which exhibits a small increase. The strength temperature dependence is also quadratic for all laminates except the 0 degree - laminate which exhibits linear temperature dependence. In many cases the temperature dependence of properties is nearly linear.
Radiation effects on engineering properties, dimensional stability, and chemistry on state of the art composite systems were characterized. T300/934 graphite-epoxy composite was subjected to 1.0 MeV electron radiation for a total dose of 1.0 x 10(10) rads at a rate of 5.0 x 10(7) rads/hour. This simulates a worst case exposure equivalent to 30 years in space. Mechanical testing was performed on he 4-ply unidirectional laminates over the temperature range of -250 F (116K) to +250 F (394K). A complete set of in-plane tensile elastic and strength properties were obtained (E sub 1, E sub 2, nu sub 12, G sub 12, X sub T, Y sub T, and S). In addition electron microscopy was used to study and analyze the fracture surfaces of all specimens tested. Results indicate that little difference in properties is noted at room temperature, but significant differences are observed at both low and elevated temperatures.
A specially textured surface of pyrolytic graphite exhibits extremely low yields of secondary electrons and reduced numbers of reflected primary electrons after impingement of high energy primary electrons. Electrode plates of this material are used in multistage depressed collectors. An ion flux having an energy between 500 iV and 1000 iV and a current density between 1.0 mA/sq cm and 6.0 mA/sq cm produces surface roughening or texturing which is in the form of needles or spires. Such textured surfaces are especially useful as anode collector plates in high tube devices.
The imparting of flame retardancy to graphite-reinforced composites without incurring mechanical property deterioration is investigated for the case of an experimental, trifunctional epoxy resin incorporating brominated polymeric additives (BPAs) of the diglycidyl type. Such mechanical properties as flexural strength and modulus, and short beam shear strength, were measured in dry and in hot/wet conditions, and the glass transition temperature, flammability, and water absorption were measured and compared with nonbromilated systems. Another comparison was made with a tetrafunctional epoxy system. The results obtained are explained in terms of differences in the polymeric backbone length of the bromine carrier polymer. BPAs are found to be a reliable bromine source for fire inhibition in carbon-reinforced composites without compromise of mechanical properties.
The feasibility of using diffuse reflectance in combination with Fourier transform infrared spectroscopy to obtain information on cured graphite fiber reinforced polymeric matrix resin composites was investigated. Several graphite/epoxy, polysulfone, and polyimide composites exposed to thermal or radiation environments were examined. An experimental polyimide-sulfone adhesive tape was also studied during processing. In each case, significant changes in resin molecular structure was observed due to environmental exposure. These changes in molecular structure were correlated with previously observed changes in material properties providing new insights into material behavior.
This work discusses the behavior of eight-ply, quasi-isotropic, graphite epoxy laminates subjected to low velocity transverse impact loading. Large deflection theory of plates was used to predict the load-deflection characteristics during the impact event. The impact model considered that the indentation, flexural and shear stiffnesses could be represented by three-equivalent springs in series. The analysis of static and dynamic impact loading test-data concluded that the membrane parameter, beta, used in flexural stiffness relation was proportional to the square of the coefficient of restitution, e, of the impactor. Use of beta=0.443 e-squared was made in load-deflection relations to consider the drop in plate stiffness due to delamination. The results of impact tests up to impact velocity 5.3 m/sec. were found in close agreement with the predictions.
This report contains the study of Low Velocity Transverse Impact Damage of graphite-epoxy T300/5208 composite laminates. The specimen, 100 mm diameter clamped plates, were impact damaged by a cantilever-type instrumented 1-inch diameter steel ball. Study was limited to impact velocity 6 m/sec. Rectangular strips, 50 mm x 125 mm, were cut from the impact-damage specimens so that the impact damage zone was in the center of the strips. These strips were tested in tension to obtain their residual strength. An energy dissipation model was developed to predict the residual strength from fracture mechanics concepts. Net energy absorbed I(a) was evaluated from coefficient of restitution concepts based on shear dominated theory of fiber-reinforced materials, with the modification that during loading and unloading the shear deformation are respectively elastic-plastic and elastic. Delamination energy I(d) was predicted by assuming that the stiffness of the laminate dropped due to debonding. Fiber-breakage energy, assumed to be equal to the difference of I(a) and I(d), was used to determine the residual strength. Predictions were compared with test results.
The thermooxidative stability in air at 316 C of unidirectional, uncoated and aluminum-coated Celion 6000 graphite fiber/PMR 15 polyimide composites has been determined for exposure times up to nominally 2000 h. Comparison of the weight loss data and microstructural integrity reveals that a thin aluminum-foil coating can provide significant protection from oxidation. A quantitative description of the average depth of the reaction zone and the maximum length of cracks produced during oxidation as a function of time in uncoated Celion 6000/PMR 15 are given.
Failure characteristics of compressively loaded graphite-epoxy components are described. Experimental results for both strength-critical laminates and structural components with postbuckling strength are presented. Effects of low speed impact damage and circular holes on compressive strength are discussed. Delamination and shear crippling failure mechanisms that limit the performance of strength-critical laminates are described. Transverse shear and skin-stiffener separation failure mechanisms that limit the performance of components with postbuckling strength are also described. The influence of matrix properties on compressive strength improvements for impact damaged laminates is discussed. Experimental data and results from a failure analysis for strength-critical laminates with cutouts are discussed and compared with impact damage results. Typical postbuckling test results are compared with analytical predictions. Previously announced in STAR as N83-12174
A constrained-displacement finite-element approach for studying the influence of transverse cracks and delaminations on the thermal response of laminated composites is presented. Typical results are given in the form of percent-retention curves for the coefficient of thermal expansion as a function of crack density. Cross-ply and quasi-isotropic T300/5208 graphite-epoxy laminates are considered. It is shown that transverse cracks can have a significant influence on the coefficient of thermal expansion, but delaminations located symmetrically about the laminate midplane have no influence on thermal expansion.
The chemical characterization of cured graphite fiber reinforced polymer matrix composites is complicated by the fact that the resins are insoluble and the composites are apaque. Standard analyses which depend either on the ability to dissolve the sample or to detect transmitted radiation are impossible. As a result, data reported on environmentally exposed composites primarily concern macroscopic information such as weight loss or changes in selected mechanical properties. Diffuse reflectance in combination with fourier transform infrared spectroscopy was developed to gain a basic chemical understanding of composite and adhesive behavior. Several composite and adhesive materials were characterized before and after environmental exposure. In each case significant changes in resin molecular structure were observed and correlated with changes in mechanical properties, providing new insights into material performance.
The (13)C NMR chemical shifts of graphite intercalation compounds were calculated. For acceptor types, the shifts come mainly from the paramagnetic (Ramsey) intra-atomic terms. They are related to the gross features of the two-dimensional band structures. The calculated anisotropy is about -140 ppm and is independent of the finer details such as charge transfer. For donor types, the carbon 2p pi orbitals are spin-polarized because of mixing with metal conduction electrons, thus there is an additional dipolar contribution which may be correlated with the electronic specific heat. The general agreement with experimental data is satisfactory.
The effects of high energy radiation on the mechanical properties and on the molecular and structural properties of graphite fiber reinforced composites were assessed so that the durability of such composites in space applications could be predicted. Investigative techniques including ESR and infrared spectroscopy, ESCA, contact angle measurements, and dynamic and static mechanical testing (3-point bending and interlaminar shear) were employed. The results using these different techniques are individually described, and the implications of the data are discussed. The proposed plan of work for the next fiscal year is outlined.