Alfred C. Loos

Curing of Epoxy Matrix Composites

Models were developed which describe the curing process of composites constructed from continuous fiber-reinforced, thermosetting resin matrix prepreg materials. On the basis of the models, a computer code was developed, which for flat-plate composites cured by a specified cure cycle, provides the temperature distribution, the degree of cure of the resin, the resin viscosity inside the composite, the void sizes, the temperatures and pressures inside voids, and the residual stress distribution after the cure. In addition, the computer code can be used to determine the amount of resin flow out of the composite and the resin content of the composite and the bleeder. Tests were performed measuring the temperature distribution in and the resin flow out of composites constructed from Hercules AS/3501-6 graphite epoxy prepreg tape. The data were compared with results calculated with the computer code for the conditions employed in the tests and good agreement was found between the data and the results of the computer code. A parametric study was also performed to illustrate how the model and the associated computer code can be used to determine the appropriate cure cycle for a given application, which results in a composite that is cured uniformly, has a low void content, and is cured in the shortest amount of time.

Heat of Reaction, Degree of Cure, and Viscosity of Hercules 3501-6 Resin

The heat of reaction, degree of cure, and viscosity of Hercules 3501-6 resin were measured using a differential scanning calorimeter and a disc and plate type viscometer. Expression were developed for correlating the rate of degree of cure and the viscosity with the degree of cure. Viscosities calculated by these ex pressions were compared with data reported previously, and good agreement was found between the present results and the previous data.

MOISTURE ABSORPTION OF GRAPHITE-EPOXY COMPOSITES IMMERSED IN LIQUIDS AND IN HUMID AIR.

Moisture absorption of graphite-epoxy composites immersed in liquids and in himid air were investigated. The moisture content as a function of time and temperature was measured for three materials: Fiberite T300/1034, Hercules AS/3501-5 and Narmco T300/5208. Tests were per formed a) with the materials immersed in No. 2 diesel fuel, in jet A fuel, in aviation oil, in saturated salt water, and in distilled water (in the range of 300 to 322 K) and b)with the materials exposed to humid air (in the range 322 to 366 K). The results obtained were compared to available composite and neat resin data.

MOISTURE ABSORPTION OF POLYESTER-E GLASS COMPOSITES.

Moisture absorption of polyester-E glass composites immersed in liquids and in humid air were investigated. The weights of the composites as a function of exposure time and temperature were measured for three differ ent types of materials. Tests were performed a) with the materials im mersed in distilled water, in saturated salt water, in No. 2 diesel fuel, in jet A fuel, in synthetic aviation lubricant, in gasoline, and b) with the materi als exposed to humid air. The apparent maximum moisture contents and the apparent diffusivities were deduced from the data.

Structure–property relationships of void-free phenolic–epoxy matrix materials

Abstract The structure–property relationships of phenolic–epoxy networks have been investigated by several methods. Network densities have been explored by measuring the moduli in the rubbery regions and these experimental values were compared with those predicted from stoichiometry. The T g s decreased, and toughness increased, as the phenolic Novolac content in the network was increased. Both results could be correlated to the decrease in network densities along this series. Analysis of the cooperativity of the networks suggests a crossover in properties from two competing factors, network density and intermolecular forces (hydrogen bonding). Measured fracture toughness values exceed those of typical untoughened epoxy networks and far exceed existing commercial phenolic networks. In addition, an increase in Novolac content improves the flame retardance rather dramatically. Thus, by controlling the Novolac content to reach an appropriate phenol to epoxy ratio, a void-free system with both favorable mechanical properties and flame retardance can be achieved.

The effects of fluid type and viscosity on the steady-state and advancing front permeability behavior of textile preforms

The effects of fluid type and viscosity on the permeability of both saturated and dry preforms were investigated. Fluids used were water, corn oil, and Epon 815, an epoxy resin. Preforms tested included style 162 E-glass, a plain weave E-glass fabric, and IM7/8HS, an eight harness satin carbon fabric. Two methods were used to measure the permeability of the textile preforms. The first, known as the steady-state method, measures the permeability of a saturated preform under constant flow rate conditions. The second, denoted the advancing front method, measures the permeability of a dry preform to an advancing fluid. Results from the two methods showed that fluid viscosity had no significant influence on the permeabilities of the two fabrics. Steady-state and advancing front permeabilities for the warp direction of the two fabrics were similar. In addition, advancing front permeability values were found to be similar for different fluids over a wide range of values for the capillary number. Contact angle measurements indicated that Epon 815 wets both fibers better than the corn oil. In addition, E-glass has lower contact angles with both fluids.

A process simulation model for the manufacture of a blade-stiffened panel by the resin film infusion process

An analytical model has been developed which can be used to simulate non-isothermal infiltration of a hot-melt resin into a textile preform of complex shape. In this study, the model was used to simulate the resin film infusion process of a single blade-stiffened panel. The development and application of the two-dimensional model are discussed. Results of the stiffened panel simulation were then used to study the effects of compaction pressure on the total resin infiltration time. Also described is the use of the model in the development of an extended processing cycle which ensures the complete infiltration of epoxy resin into the stitched graphite preform. Model predictions of temperature, resin viscosity, and extent of cure during infiltration and cure are presented and discussed.

Effects of Thermal Spiking on Graphite-Epoxy Composites

Tests were performed evaluating the effects of thermal spikes on the moisture absorption characteristics, the ultimate tensile strength, and the buckling modulus of Thornel 300/Fiberite 1034 composites. Measurements were made on unidirectional and π/4 laminates, using different types of thermal spikes. A survey was also made of the existing data. This survey, together with the present data indicate how thermal spikes affect the mois ture absorption and the mechanical properties of different graphite-epoxy composites.

A Plane-Strain Finite Element Model for Process-Induced Residual Stresses in a Graphite/PEEK Composite

A plane-strain, linear elastic finite element model with temperature-dependent matrix properties was developed to analyze residual stresses in graphite/PEEK composites. The residual stress model takes into account the mismatch of the thermal expansion coefficients and the crystallization shrinkage of the matrix. Good agreement between the reported transverse normal stress data and the modeling result was observed in the analysis of a [040] T APC-2 laminate processed at a 35°C/s surface cooling rate. Furthermore, [010/906] T APC-2 laminates were manufactured at different cooling rates to verify the model. The induced residual thermal deformations were measured by a shadow moiré system. The model estimated the out-of-plane displacement of the non-symmetrical laminates accurately. The optimum processing cycles, which minimize the residual stresses and maximize the mechanical properties of composite materials, were found to be different for different lay-ups. Therefore, in order to minimize the residual stresses and to optimize the mechanical performance of the composite laminates, an appropriate processing cycle must be chosen for each specific lay-up.

Production of controlled networks and morphologies in toughened thermosetting resins using real-time, in situ cure monitoring

Based on knowledge of the chemical reactions and morphology, significant changes can be made in the morphology of a toughened dicyanate thermosetting resin through the intelligent manipulation of the cure cycle and real-time knowledge of the conversion of the system. Fourier transform near infra-red spectroscopy using fibre-optic sensors was employed to follow such reactions. Various cure cycle changes resulted in similar degree of cure, thermal stability and solvent resistance, but yielded a 20% change in neat resin toughness associated with the morphologies. The morphological variety was shown not only to occur within reasonable cure cycle variations for neat resin, but were also induced through a processing change in a graphite-reinforced composite containing this resin. Design of custom or gradient morphologies to provide specific mechanical properties is now feasible with this technology. These same approaches could be adapted to the custom manufacture of optical and/or damping properties. This manipulation is not limited to the processing of toughened thermosetting resins.