bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of moisture on interface modified graphite epoxy composites
Authors: Benatar Avraham , Gutowski Timothy G.
Published: May 2005

Graphite epoxy composites which are exposed to high humidity environments suffer a loss in their shear and transverse properties. In addition to the effect of moisture on the matrix, thermodynamic theories suggest that the interface may suffer irreversible damage. In this study, we investigated if fiber surface treatments could improve fiber matrix adhesion as well as the wet composite properties. Plasma polymerization was used to generate very thin coatings with the polar component of the surface energy varying from 1.8 to 5.3 (dyne/cm)1/2. These coatings were characterized by ESCA and SEM, and a micro‐Wilhelmy technique was used to determine the dispersive and polar components of the surface energy. The uniaxial composites made with the treated fibers were tested wet and dry in short beam shear. It was found, as is well known, that the dry shear properties could be improved by up to 40 percent. Two effective treatments were oxygen etching and polymerization with methylmethacrylate monomer. However, all the wet samples failed at the same stress regardless of the surface treatment used. These results strongly suggest that the shear and transverse properties of the wet composites are dominated by the epoxy and cannot be improved by interface treatments.
Composite lamination—analysis and modeling
Authors: Appelt B. K. , Gotro J. T. , Schmitt G. P. , Ellis E. T. , Wiley J. P.
Published: May 2005

Multilayer printed circuits are a prime example of complex polymer composites. One of the critical steps in manufacturing of the circuit boards is the lamination process, which is frequently simulated by the aborted flows test (AFT). In order to obtain a composite free of voids and good interlaminar adhesion, it is imperative for the resin to flow into metal interfaces as well as intermix in the resin interfaces. On an empirical basis, the AFT is used as a process monitor for making prepreg, the glass fiber reinforced dielectric which supports the circuitry. Under a given set of conditions (temperature profile, pressure, layup), AFT measures advancement and reactivity of prepreg. By using a given batch of prepreg, the optimum laminating conditions can be identified, which are mainly a function of the temperature profile. Here, a calorimetric and rheological study of prepreg cure kinetics during lamination is presented. The flow process is being modeled by calculating the viscosity from combining a temperature dependent viscosity term with a kinetically controlled molecular weight term.
Dynamic mechanical behavior and structure of ternary blends of polypropylene/EPDM rubber/polyethylene
Authors: Kolařík J. , Velek J. , Agrawal G. L. , Fortelný I.
Published: May 2005

The dynamic mechanical behavior of ternary blends of isotactic polypropylene (80–0 percent)/EPDM rubber (20 percent)/high‐density polyethylene (0–80 percent) was investigated in the temperature range from −196 to 100°C by means of a free‐oscillating torsional pendulum. The structure of the blends was examined by a scanning electron microscope on etched surfaces cut by a fractured glass edge in liquid nitrogen. Dynamic mechanical response spectra and microphotographs of the systems studied show that the minor thermoplastic forms the core of EPDM rubber inclusions. At 20 percent rubber in the blends, the inclusions can accommodate from 20 to 30 percent polyethylene or polypropylene. Addition of either thermoplastic not exceeding this limit has almost the same effect on the stiffness, damping, and yield stress of the blends as the addition of the same amount of rubber. Ternary blends with equal or slightly different polypropylene and polyethylene fractions have the structure of interpenetrating phases in which EPDM rubber forms the interface layer.
Determination of the orientation of short glass fibers in sheet molding compound (SMC)
Authors: Kau Hua‐tie
Published: May 2005

A computer based semiautomatic procedure has been developed to measure the fiber orientation on the surface of Sheet Molding Compound (SMC) plaques. The principal orientation direction and the Hermans' orientation descriptor were used to summarize the fiber orientation distribution. The results show that only slight orientation was produced in the SMC plaque with a centrally placed charge occupying 40 percent of the area of the mold. Plaque to plaque, top surface to bottom surface, and region to region variability in the fiber orientation were observed. With a flow length of less than 3 fiber lengths in these plaques, the Hermans' orientation descriptor was in the range of 0.1 to 0.5. The descriptor varied appreciably with increasing sampling area and consistently displayed a sharp transition across the boundary of the original charge area, indicating distinct flow behaviors inside and outside the charge area. Likewise, the principal orientation direction varied with increasing sample area, albeit with some degree of coordination between the top and the bottom surfaces. Finally, the orientation of fibers in the four quadrants of the same plaque differed significantly. Based on the descriptor for each quadrant, the flow could be radial from the center of the plaque, rotational with the top and bottom surfaces in similar angular displacements, or twisted with the two surfaces moving in opposite directions, all indicating local nonuniformity of the flow.
A model for resin flow during composite processing part 2: Numerical analysis for unidirectional graphite/epoxy laminates
Authors: Dave R. , Kardos J. L. , Duduković M. P.
Published: May 2005

In this paper numerical results are presented for resin flow during processing of unidirectional graphite/epoxy laminates. Resin pressure and velocity profiles, as well as resin loss, specific permeability, and resulting thickness changes, were computed to examine the effects of one and two‐dimensional flow, initial laminate thickness, and various cure cycles. Input data to the model are also discussed in detail. Analysis of the input data on the stress‐strain behavior of graphite fiber beds showed that the bed consolidation behavior can be divided into three regions. “Free‐bleeding” (O psig and negligible bleeder resistance to flow at the boundaries) during two‐dimensional resin flow leads to rapid decay in resin pressure. Comparison of the predicted results for the resin mass loss and the average final thickness per ply with experimentally determined values shows good agreement.
The effect of pressure on the curing behavior of unsaturated polyester resins
Authors: Lee Dai‐soo , Han Chang Dae
Published: May 2005

The effect of pressure on the curing behavior of unsaturated polyester resin was investigated, both experimentally and theoretically. The resin used was a general‐purpose unsaturated polyester resin and the initiator used was t‐butyl perbenzoate. A series of isothermal runs with differential scanning calorimetry (DSC) were made at various levels of cure pressure. It was found that the rate of cure was retarded under pressure, and that the ultimate degree of cure went through a maximum at a certain pressure as the cure pressure was increased from atmospheric pressure to 6.21 MPa (900 psi). It was interpreted that pressure has two competing effects on the curing behavior of unsaturated polyester resin; one is a free volume effect that hinders the curing reaction and the other is a thermodynamic effect that favors it. Therefore, when the pressure is higher than a certain level, the free volume effect becomes predominant over the thermodynamic effect, the ultimate degree of cure diminishing as the cure pressure is increased beyond that level. Theoretical interpretation of the experimental results is given, using a mechanistic kinetic model developed in our previous publication.
Ternary blends in the toughening of epoxy resins
Authors: Levita G. , Marchetti A. , Lazzeri A. , Frosini V.
Published: May 2005

The fracture properties of two types of ternary blends (amine terminated rubber/Bisphenol‐A/DGEBA resin) have been studied. In the first type, B, setting was obtained by using piperidine as the hardener. In the second type, C, the rubber was used as both the hardener and the toughener. Both types showed an increased fracture resistance over the binary rubber/resin formulations. The drawback of the too high reaction rate shown by type B blends can be eliminated by using type C blends which can be easily B‐staged.
In situ fracture tests in the scanning electron microscope
Authors: Roulin‐Moloney A. C. , Cudré‐Mauroux N. , Kausch H. H.
Published: May 2005

The experimental arrangement for conducting fracture tests in the scanning electron microscope (SEM) are described together with the particular problems in applying this technique to polymers and composite materials. Several examples are given of mechanisms of fracture in epoxy resins both unfilled and reinforced with particulate and fibrous fillers. The paper is illustrated by still photos taken from the video recording of the fracture experiments and by photos taken from the SEM raster.
Thermal expansion of organic and inorganic matrix composites: A Review of theoretical and experimental studies
Authors: Raghava R. S.
Published: May 2005

This paper reviews theoretical and experimental studies conducted in organic, ceramic, and metal matrix composites containing cylindrical, lamellar, and spheroidal inclusions as reinforcements. Mathematical formulations proposed to predict thermal expansion coefficients of fiber, disc, and sphere reinforced organic and inorganic matrix composites have been reviewed. Experimental studies undertaken to confirm theoretical predictions of thermal expansion coefficients of a variety of reinforcement geometry composites have also been discussed.
Orientation development in molding of short‐fiber liquid crystalline polymer composites
Authors: Kenig S. , Trattner B. , Anderman H.
Published: May 2005

The orientation development mechanisms that take place during molding of short‐fiber reinforced liquid crystalline polymer composites, have been analyzed and consequently the orientation development and properties distribution were studied for the case of thin wall large area components. Two factors associated with orientation development were investigated. The first is related to the melt front shape for in‐plane orientation distribution, and the second to the thickness of the skin layer where the alignment of the orientable liquid crystalline matrix and the rigid reinforcing fibers are of importance. The two basic factors were first verified by flow calculations using a computer assisted design (CAD) system and then experimentally validated. Experimental results for thin wall large area plaques have shown that the control over the melt front shape could be achieved with a narrow restrictor‐type gate and that the skin layer thickness is determined by the heat transfer processes during filling of the mold. Furthermore, a clear correlation has been established between the measured tensile properties and the ratio of skin layer to total thickness of the plaque. Such correlations should be generally developed for short‐fiber polymer composites for cases where rigidity and load bearing capabilities are of importance.
Study of epoxy resin—filler interaction
Authors: Petrović Zoran , Stojaković Nikola
Published: May 2005

A bisphenol‐type epoxy resin cured with phthalic anhydride was filled with three types of SiO2 (untreated, silanized and smaller particle size), CaCO3, dolomite and Al2O3. Also, the concentration of silica was varied. Interaction of filler and resin was estimated by measuring the glass transition temperature (Tg) by different methods. Electron microscopy and x‐ray diffraction were used to study the structure of the boundary layer and the bulk polymer. It was found that silica forms strong bonds with the resin and that basic fillers (CaCO3, Al2O3) probably affect the crosslinking process with anhydride as hardener causing lowering of Tg.
A general strength theory for orthotropic fiber‐reinforced composite laminae
Authors: Labossière P. , Neale K. W.
Published: May 2005

Thin, orthotropic fiber‐reinforced composite laminae under plane stress loading conditions are considered. The essential conditions that must be satisfied in developing phenomenological strength criteria for such materials are first established. A new, parametric failure criterion is then presented. It is shown that this entirely general formulation encompasses all previously proposed failure theories (e.g., tensor polynomial criterion) for orthotropic laminae. Examples of application and the inherent advantages and drawbacks of the proposed parametric approach are discussed. In the past, the formulation of strength criteria for composite materials has closely paralleled the development of yield criteria in metal plasticity. The parametric approach advocated here for the failure of composite materials was inspired by a general formulation proposed by Budiansky (1) for the plastic yielding of sheet metals.
Future prospects for wood cellulose as reinforcement in organic polymer composites
Authors: Zadorecki Paul , Michell Anthony J.
Published: May 2005

Wood cellulose, a versatile and renewable natural resource, has potential for use as a reinforcement for synthetic organic polymers. During the past 80 years a number of materials using the reinforcing properties of wood cellulose have found major markets. Forms of wood cellulose proposed as reinforcements include: wood fibers, cellulose fibers, microfibillar, and microcrystalline cellulose. Recent attention has been given to them as fillers/reinforcements in thermoplastics and elastomers. Most cellulosic composites derive their existence from their comparatively low materials cost and the filling rather than reinforcing properties of cellulose. However, cellulose chains have a potential stiffness much higher than glass and in the same range as superstiff aramid fibers. This paper examines the state of the art of combining wood cellulose with synthetic organic polymers to from composites and considers new ways for better using cellulose's reinforcing potential.
The processing of fiber reinforced thermoplastics using co‐rotating twin screw extruders
Authors: Wall David
Published: May 2005

For short fiber reinforced thermoplastics the effect of fiber length and the fiber/matrix bond on the ultimate properties of the material are well understood. But, how the process conditions under which the composite is made affect the fiber length and the fiber matrix bond has not been so thoroughly reported in the literature. A study of this relationship has been made on a co‐rotating twin screw extruder incorporating 30 percent by weight of glass fiber into nylon 66 to make the composite.

Experimental results are presented to show how machine variables such as mixing configuration and screw speed affect the material properties and process efficiency. The properties of the composite were assessed by measurement of the fiber length distribution and tensile strength.

From the results in the text it is possible to select machine variables to give desired production conditions. Although most of the work concerns the twin screw extruder, some of the consequences of subsequent injection molding are also shown.
Properties of carbon/graphite fibers modified by plasma polymerization
Authors: Dagli Gulcin , Sung Nak‐Ho
Published: May 2005

Carbon fibers and pyrolytic graphite blocks were treated with plasma of acrylonitrile (AN) and styrene (ST) monomers, using an induction‐coupled, RF‐plasma reactor. Both substrates were stable towards plasma, leading to a deposition of thin, coherent coatings of 400Å∼1000Å thickness. Both monomers produced surfaces which are substantially more polar (γc=54 dynes/cm for AN and 40 dynes/cm for ST) than the untreated surfaces (γc=32 dynes/cm). ESCA and IR studies indicate that the plasma polymers contain a high concentration of oxygen (12 percent in PPAN and 17.8 percent in PPST), in the form of CO, COOH, COC, and OH groups. Also, treated fibers exhibited slightly higher tensile strengths than the untreated counterparts, suggesting that the plasma coatings effectively heal some of the surface flaws of the fiber. The abundant surface polar groups combined with the improved tensile properties of the plasma treated fibers make them attractive reinforcements for advanced composite materials.
Effects of water sorption on the physical properties of PET, PBT, and their long fibers composites
Authors: Bastioli C. , Guanella I. , Romano G.
Published: May 2005

The behavior of poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT) on water aging has been studied above and below the glass transition temperature (Tg). The aging process is caused by: degradation of the matrix and an increase in crystallinity above Tg, and microcavitation at the amorphous/crystalline interface below Tg. Such behavior well explains the deviation of the sorption kinetics from the Fickian model. The apparent water diffusion coefficients and the transport activation energies of PET and PBT have been calculated at temperatures above and below Tg. The mechanical behavior of the two polymers on water aging has been measured by means of fracture mechanics and Izod impact tests at different stress concentration factors. An increase of toughness of PET at short aging times has been shown by mechanical tests and SEM analysis fracture surfaces of differently aged samples. Izod tests of PET and PBT composites reinforced by long glass fibers have shown the contribution of fibers to the total fracture energy.
Random glass mat reinforced thermoplastic composites. Part I: Phenomenology of tensile modulus variations
Authors: Stokes Vijay K.
Published: May 2005

The mechanical properties of random continuous glass mat reinforced composites, as determined by standard tensile tests, are known to have a very large scatter. To understand this scatter, new test procedures were developed to map the local tensile elastic moduli in a large plaque at 12.7‐mm (½‐in) intervals. Surprisingly, the tensile modulus in these materials can vary by a factor of two over the 12.7‐mm distance. The elastic modulus is shown to vary by a factor of three in a 150 × 305‐mm (6 × 12 in) plaque. Expressions have been obtained for the average moduli measured by tensile and bend tests. These expressions have been used to compare measured flexural moduli with values predicted by using measured tensile moduli.
Melting behavior of high performance composite matrix polymers: Poly(phenylene sulfide)
Authors: Cebe Peggy , Chung Sengshiu
Published: May 2005

We have been studying crystallization and melting behavior of high performance semicrystalline polymers to increase our understanding of the effects of processing on microstructure. Questions on development of morphology center around whether single or dual populations of crystals exist after melt processing, and how these crystals melt creating the multiple endotherms usually observed. We have used differential scanning calorimetry (DSC) to examine endothermic response in poly(phenylene sulfide), PPS, after various processing treatments. These include isothermal melt crystallization and crystallization from the rubbery amorphous state. For the first time, we report endothermic response of solution crystallized mats of single crystals. Our results on melt crystallized PPS are explained by a model in which different crystal populations exist in the material at room temperature prior to DSC scanning. Ability of the material to reorganize during the scan depends upon the initial state of perfection of the crystals.
Thermoforming of high performance thermoplastic composites
Authors: Muzzy John D. , Wu Xiang , Colton Jonathan S.
Published: May 2005

Matched‐die forming of a 90° bend in a quasi‐isotropic 16 ply laminate of ICI's APC‐2, containing 61 volume % Hercules AS‐4 continuous carbon fiber in poly(etheretherketone) (PEEK), is described. To uncouple and characterize elementary deformation phenomena, a series of simple tests are developed. Single ply bending of APC‐2 tape up to 420°C has been performed to characterize elastic bending. Transverse flow, axial interply shear flow, and z direction elasticity have been characterized in an instrumented matched die mold. Interply shear flow is characterized using a simple shear test fixture. Transverse flow and interply slip are relatively easily induced. Precautions should be taken to avoid fiber buckling and to provide for laminate reconsolidation.
The Iosipescu shear test method as used for testing polymers and composite materials
Authors: Adams Donald F.
Published: May 2005

The Iosipescu shear test, developed initially in the 1960s for use with homogeneous, isotropic structural materials such as metals, has been adapted during the past ten years for use with composite materials. Much of this development work has taken place at the University of Wyoming, where a very efficient test fixture configuration has been developed, and extensive test results have been generated. Many other research groups have also adopted the test method during the past five years. As a result, the American Society for Testing and Materials (ASTM) is presently in the final stages of making the Iosipescu shear test an ASTM standard. The test method is described and its range of applications indicated, including examples of typical data.
Fiber length distribution effects on the fracture of short‐fiber composites
Authors: Carling M. J. , Williams J. G.
Published: May 2005

The effect of fiber geometry on the fracture behavior of short glass fiber‐reinforced nylons was examined. In particular a comparison was made between conventional short‐fiber composites (with feedstock made by an extrusion process) and the newer pultruded feedstock materials which have longer fiber length distributions but larger fiber diameters. The toughness of the material was measured over a wide range of loading rates, from impact conditions, using Kc and Gc type tests, to long term, slow crack growth tests. The moisture content of the materials was varied from dry to saturated. It was found that there was little difference in fracture toughness when the materials were dry. However, when moist, the longer fiber material showed significant improvements over the conventional short‐fiber‐reinforced material. This behavior is explained in terms of the changes in fiber geometry.
Polypropylene/low density polyethylene blends with short glass fibers. II: Effect of compounding method on mechanical properties
Authors: Ramos M. Arroyo , Belmontes F. Avalos
Published: May 2005

Polypropylene/low density polyethylene blend matrices have been reinforced with short glass fibers in order to study their tensile, flexural, and impact behavior. Two‐roll milling and twin‐screw extrusion compounding methods were used to incorporate the fiber within the polymer matrices, and standard test samples were prepared by injection molding. The effects of matrix composition and fiber concentration on mechanical properties were investigated keeping in mind the matrix and fiber morphology, the latter being intimately dependent upon the compounding method employed.
The dependence of interfacial shear strength on matrix and interphase properties
Authors: Rao Venkatesh , Drzal Lawrence T.
Published: May 2005

Experiments were conducted to determine the dependence of the interfacial shear strength on the bulk material matrix properties using model compounds based on epoxy/amine chemistry. AS4, carbon fibers were used as the subject for these measurements with both a difunctional epoxy (DGEBA) system as well as a tetrafunctional epoxy (MY720) system. Amine curing agents were carefully chosen to produce matrices which produced a range of matrix properties from brittle, elastic to ductile, plastic. The fiber‐matrix interfacial chemistry was constant throughout this study by always using a stoichiometric amount of curing agent. The results indicate that, for both the difunctional as well as the tetrafunctional epoxy system, the interfacial shear strength (as determined by the fragmentation test) decreases nonlinearly with decreasing modulus of the matrix. Linear elastic analysis yields a nearly linear relationship, for both systems, between the interfacial shear strength and the product of strain to final break and the square root of the matrix shear modulus. A linear relationship is also found between the difference in test temperature and glass transition temperature of the cured matrix and the interfacial shear strength. Additionally, the failure mode is seen to remain interfacial as the ductility of the matrix changes.
Optimizing fiber wetout for composites manufacturing
Authors: Cercone Larry
Published: May 2005

Decreased mechanical strength in a fiber reinforced plastic part can often be traced to poor or incomplete impregnation of the reinforcing fiber with the matrix. To properly understand the impregnation process in the design of new composites manufacturing machinery (specifically in unidirectional tape machines), or to optimize wetout in existing machine designs, the raw process materials and their relationship within the process environment must be examined. The critical factors are: resin viscosity vs. temperature; the work of adhesion between the fiber and resin; and the problem of forcing the resin to completely penetrate a fiber bundle. If these factors are known, nip rolls can be designed to meet a specific process envelope, or in the case of existing equipment, the existing process envelope for specific fiber/matrix combinations can be manipulated for maximum fiber wetout.
Effect of advancement on epoxy prepreg processing—a dielectric analysis
Authors: Day David R. , Shepard David D.
Published: May 2005

With the relatively recent trend toward adaptive process control during processing of structural composites, there is need for an understanding of the nature of variations during cure that are encountered. Part‐to‐part variations can arise because of differences in thermal exposure arising from part placement or part thickness, or from chemical differences due to formulation or advancement. Microdielectric sensors are particularly well suited for monitoring thermoset cures from initial point of flow, through the liquid region, and on into the solid state. In this study, microdielectric sensors are used to systematically study the influence of resin advancement on cure behavior. Relationships between advancement and critical cure phenomena such as flow temperature, point of viscosity minimum, and reaction end point are discussed.
Monitoring the composite curing process with a fluorescence‐based fiber‐optic sensor
Authors: Levy R. L. , Schwab S. D.
Published: May 2005

A novel cure sensor, based on the combination of fiber‐optic fluorometry and the viscosity/degree‐of‐cure dependence of the epoxy resin fluorescence, was developed to provide a reliable low‐cost cure‐monitoring sensor for control of composite manufacturing. The capability of the first‐generation sensor to monitor chemorheological changes that occur during the autoclave cure of carbon‐epoxy laminates was successfully demonstrated. An improved second‐generation sensor, which simultaneously monitors the changes in the epoxy fluorescence intensity and the wavelength of maximum emission, was also developed. The changes in the sensor intensity and wavelength signals, as a function of cure time, provide characteristic profiles that reveal the main chemorheological events of the cure cycle; these signals follow the changes in the degree‐of‐cure to completion. The configuration of the optrode‐laminate interface strongly affects the signal profiles and their reproducibility. A “resin cavity” optrode‐laminate interface, which improves reproducibility, was developed and tested.
Minimum‐gage, maximum‐stiffness graphite/thermoplastic spacecraft structures
Authors: Garvey R. E. , Andriulli J. B. , McKeever J. W. , Rudness R. V.
Published: May 2005

Advanced thermoplastic matrix composite materials have been studied for use in aircraft applications since about 1980. This Strategic Defense Initiative Organization/Air Force project was initiated in 1987 to study the suitability of this class of materials for use in spacecraft applications. This paper will report on the following work in progress: development of ultrahigh‐modulus, pitch‐fiber‐reinforced thermoplastic composites; development of thin‐ply graphite/thermoplastic materials for gage‐limited spacecraft applications; and producibility demonstrations for generic spacecraft structures including open frames and T and L members, circular and square‐tube truss configurations, and stiffened‐skin modules.
Impact damage in a composite material
Authors: Reed P. E. , Beva L.
Published: May 2005

Damage development in plates of a glass fiber‐reinforced polypropylene is recorded, using short pulse photography, during instrumented falling weight tests using the excess energy approach. It is seen that the damage is progressive throughout the test but its initiation cannot be detected either by freeze‐frame photography or visual observation. Specimens are therefore subjected to low‐energy impact followed by microscopic observation of the tensile face. It is found that the initial damage mechanism is cracking of the matrix at the fiber‐matrix interface, the crack propagating along the fiber. The finite element method is used to estimate the stress distributions at damage initiation in 4‐ply and 8‐ply samples cut from unidirectional and cross‐ply plaques. The computed results indicate that initial damage occurs when the transverse tensile stress reaches a critical value.
Development and evaluation of surface treatments to enhance the fiber‐matrix adhesion in PAN‐based carbon fiber/liquid crystal polymer composites. Part I: Coupling agent and amine surface treatments
Authors: King Julia A. , Buttry Daniel A. , Adams Donald F.
Published: May 2005

Several surface treatments, using both commercially available coupling agents and reagents containing multiple amines, were applied to commingled continuous as‐received AS4 carbon reinforcing fiber/liquid crystal polymer (LCP) matrix fibers. Unidirectional composites (normally 60 vol% carbon fiber) were prepared from as‐received and treated commingled fibers and characterized. To estimate the effect the effect of the treatments on fiber‐matrix adhesion, short beam shear (SBS) tests were conducted, the failure surfaces were examined, and spectroscopic studies wee performed. The mean SBS strength of the as‐received unidirectional AS4 carbon fiber/LCP matrix composite system was 49 MPa. The best coupling agent and amine treatments yielded increases in composite shear strength of ∼ 10 to 20%, relative to the as‐received AS4/LCP system. For the amine treatments, ESCA and FTIR analyses suggested of both the carbon and LCP fibers may have caused the increased adhesion. Moreover, SEM analysis of the failure surfaces of SBS specimens from composites prepared with the treated fibers may have caused the increased adhesion. Moreover, SEM analysis of the failure surfaces of SBS specimens from composites prepared with the treated fibers (both with coupling agents and amines) showed that strong fiber‐matrix adhesion was present. That is, failure occurred in the LCP matrix material.
Development and evaluation of surface teratmetns to enhance the fiber‐matrix adhesion in PAN‐based carbon fiber/liquid crystal polymer composites. Part II: Electrochemical teatments
Authors: King Julia A. , Buttry Daniel A. , Adams Donald F.
Published: May 2005

Various electrochemical surface treatments, designed to improve the fiber‐matrix adhesion, were applied to commingled as‐received continuous AS4 carbon reinforcing fiber/liquid crystal polymer (LCP) matrix fibers. Unidirectional composite plates (norminally 60 vol% carbon fiber) were fabricated and short beam shear (SBS) tests were conducted to provide an indication of the effect of the treatments on AS4 carbon fiber‐LCP matrix adhesion. The mean SBS strength of the as‐received AS4/LCP system was 49 MPa. Three elecrtrochemically polymerized coatings‐poly‐para‐phenylene (PPP), polyphenylene oxide (PPO), and polyaniline (PA)‐were applied. Only the PPO coating, however, had a beneficial effect on the composite shear strength. The coating enhanced the shear strength by 14% relative to the as‐received AS4/LCP system. Scanning electron Microscope (SEM) observations of the PPO treated shear failure surface indicated the presence of a strong fiber‐matrix bond. That is, the failure location was in the LCP matrix material.
An improved self‐consistent method for estimating the permeability of a fiber assembly
Authors: Cai Z. , Berdichevsky A. L.
Published: May 2005

This paper presents an improved self‐consistent method for estimating the permeability of an aligned fiber assembly, in both longitudinal and transverse directions. In this method an insertion is assumed to include open space surrounded by densely packed fibers. This improvement allows us to describe effectively the permeability of dense structures containing distributed voids. As used in self‐consistent methods, the insertion is placed into a homogeneous medium with an unknown permeability. Stokes flow and Darcy flow are then considered, respectively, at different regions. Boundary and interface conditions as well as two consistency conditions, including the total amount of the flow and the dissipation energy, are applied accordingly. The permeability is solved from these considerations. This improved permeability model captures the flow characteristics of a fiber bundle. In the longitudinal flow case, the openings within a bundle due to disturbance dominate the flow path. In the transverse flow case, the gaps between neighboring fibers govern the flow resistance. The derived expression for the transverse permeability contains two variables, the averaged fiber volume fraction and the maximum packing efficiency, which adequately describe the status of a fiber bundle. These two variables can also be measured experimentally. The predictions agree with available data reported. The result for the longitudinal flow shows not only the influence of these two parameters, but also the very strong effect of the openings within the bundle on the permeability. This explains the significant differences between the data of idealized packings, such as square and hexagonal packing, and those measured from real fiber bundles. The comparison also provides an estimation of the average opening sizes within a fiber bundle as a function of fiber volume fraction. Numerical simulation results of previous studies are also used to verify this approach.
Epoxy polymer modification with metals
Authors: Kurnoskin A. V.
Published: May 2005

The modification of epoxy polymers with some metal cations, viz., Zn2+, Fe3+ and Mn4+, has been studied. These cations were introduced into the epoxy matrices by hardening of diglycidyl ether of bisphenon‐A with the chelates of the metal salts with some aliphatic amines. Studies center on the dependence of the mechanical strength and heat resistance of the metal containing epoxy polymers with Zn2+ and Fe3+ increases the strength and heat resistance significantly. These metal‐containing hardeners have excellent processability and can be used to produce high strength and high thermal resistance thermosets.
Preparation and properties of optically transparent, pressure‐cured poly(methacrylate) composites
Authors: Weaver Kevin D. , Stoffer James O. , Day Delbert E.
Published: May 2005

The objective of this work was to develop optically transparent glass fiber reinforced poly(methyl methacrylate) (PMMA) composites by matching the refractive index of the glass fiber reinforcement to that of the PMM, A matrix, thereby minimizing the quantity of light which is normally scattered and reflected at the interface of dissimilar materials. A pressure curing process is described for preparing composites 10 by 15 by 0.6 cm thick and containing up to ∼ 35% glass fiber. By curing at 65°C under 6.9 MPa N2 for 18 H, composites have been produced with optical quality surfaces and flexural strengths more than seven times that of commercial PMMA of the same thickness. A transparent composite containing 10.4 vol% of 13 μm diameter fiber had an 84% optical transmission (92% maximum for PMMA) at 600 nm and 25°C.
Modeling resin transfer molding of polyimide (PMR‐15)/fiber composites
Authors: Chan Albert W. , Hwang Sun‐tak
Published: May 2005

Polyimide (PMR‐15)/fiber composites are important structural components in many high temperature applications requiring high performance lightweight materials. Most composites are fabricated from “prepregs,” but resin transfer molding (RTM) is an alternative route to the formation of a resin‐saturated perform, further processing of which yields the final part. A simple model is proposed for the RTM of “thin” composite parts. Numerical solution of the model is accomplished through integration of the equation s formulated using he finite element method. This approach accounts for the changing shape of the flow front during RTM. The resulting computational schemes are numerically stable and efficient.
The development of an epoxy resin system for the injection molding of long‐fiber epoxy composties
Authors: Ishida H. , Zimmerman D. A.
Published: May 2005

The Combination of reaction injection molding and pultrusion has resulted in a new processing technique, RIM‐Pultrusion, Which has been used to produce a thermoplastic epoxy prepreg. This prepreg has been used to produce a long‐fiber injection molded phenoxy/carbon fiber composite with near‐Zero void content. A heat‐activated curing system has been developed, which allows injecton molding of the prepreg to form a thermostet long‐finer epoxy/carbon finber composite. The RIM‐ pultrusion conditions for producing an injection moldable prepreg are described. Capillary rheomety is used to study the epoxy resin to determine the proper molar ratio for RIM‐Pultrusion. The long‐fiber epoxy compostie is analyzed with dynamic mechanical analysis (DMA) and Fourier transform infrared spectroscopy (FTIR). Also., the impact strength and solvent resistance of the long‐fiber composite are examined. The properties of the thermoset long‐fiber epoxy xomposite are compared to those of a thermoplastic injection molded long‐fiber phenoxy composite.
The fracture behavior of rubber‐toughened, short‐fiber composites of nylon 6,6
Authors: Pecorini T. J. , Hertzberg R. W.
Published: May 2005

This study critiques the use of both rubber particles and short‐glass fibers for the improvement of polymer fracture toughness (Kc). Although dry neat nylon is brittle with only a moderateKcvalue (4.2 Mpa√m), additions of either second phase produce risingKR‐curves and associated highKcvalues (8.1 Mpa√m for rubber‐toughened nylon, and 10.0Mpa√m for 17 vol% Glass‐fiber neat nylon). In the rubber ‐modified resin, the highKcvalue is associated with extensive plastic blunting at the crack tip. In the fiber‐reinforced neat resin,Kcis improved due to a combination of fiber‐bridging and increased strength, the latter being associated with additional load carrying capacity of the fibers. When both rubber and fibers are added, however, no further increase inKcis noted (Kc= 9.3 Mpa√m for 17 vol% glass‐fiber rubber‐modified nylon). The extent of ductile blunting in the rubber + fiber resin is not as great as in the rubber‐only resin. Furthermore, the fracure strength of the rubber + fiber resin is not as high as the fiber‐only resin. The net result is a balance of properties for the rubber‐toughened composite.
The devolatilization of polyimide fiber composites: Model and experimental verification
Authors: Yoon In Seon , Yang Yubo , Duduković Milorad P. , Kardos John L.
Published: May 2005

A one‐dimensional devolatilization model is developed for autoclave processing of themroplastic polyimide, graphite‐reinforced, composites. The model rests on the assumption that continous gas passages are formed during processing due to partical crystallization of the polymer in the course of polymerization. Both DSC and optica microscopy indicate that crystallinity develops in the Avimid‐III polyimide resin during polymerization, which in turn facilitates formation of cracks. These cracks heal upon removal of volatiles and polymer melting. Model‐Calculated temperature profiles in the laminate during polymerization, and the predicted removal rates of volatile species (water, ethanol, NMP) are compared to experimental miniautoclave data for the DuPont Avimid K‐III/IM6 system. Good agreement is obtained when the volumetric mass transfer coefficient required by the model is assumed to vary proportionally to the inverse of the changing resin viscosity. The model allows the investigation of desired cure cycle parameters for different laminate thicknesses.
Interfacial toughness and its effect on compression strength in polycarbonate/carbon fiber composites
Authors: Stone Paul R. , Nairn John A.
Published: May 2005

Processing Polycarbonate/carbon fiber composites for long times at high temperatures significantly improved adhesion between the matrix and the fibers. The interfacial properties were studied by measuring transverse fracture toughness, observing fracture specimens by scanning electron microscopy, and by monitoring composite cross‐sections using atomic force microscopy. The processing treatment provided an ideal method for varying the properties of the interface without changing any other properties. We used this method to study the effect of interfacial properties on the axial compression properties of unidirectional composites. Both the compression strength and compression modulus increased significantly as the fiber/matrix adhesion improved. We concluded that improving interfacial adhesion increased compression properties by inhibiting fiber microbuckling.
Self‐reinforced thermoplastic‐LCP prepregs and laminates
Authors: Isayev A. I. , Joldengreber Y. , Viswanathan R. , Akhtar S.
Published: May 2005

Self‐reinforced sheets (prepregs) have been prepared by stretching extruded sheets made of thermoplastic (TP) and a thermotropic liquid crystalline polymer (LCP) blend. The sheets are formed by extrusion through a coathanger die, device. Processing at this stage is done at a temperature at which both components in the blend are melt processable. These prepregs are laid up in multi‐layers in a direction parallel to the stretching direction or in the direction of 45° with respect to each previous layer. The lay‐ups are compression molded into unidirectional or isotropic laminates at temperatures below the melt processing temperature of the LCP. Various pairs of TP and LCP have been studied. These include polypropylene and an LCP based on p‐oxybenzoyl, terephthaloyl and hydroquinone moieties, polyphenylene oxide (PPO) and polystyrene/PPO alloy and a LCP based on 6‐oxy‐2‐naphthoyl and p‐oxybenzoyl moieties. Mechanical properties of the prepregs and laminates were measured and compared with those obtained from injection molded samples. Surprisingly, tensile strength and modulus of isotropic laminates are found to be higher than those of injection molded samples in the flow direction. Morphlogical studies of the prepregs and laminates indicate the presence of well‐defined LCP fibers in various thermoplastic matrices.
Coating carbon fibers with a thermoplastic polyimide using aqueous foam
Authors: Chary Rajendran R. , Hirt Douglas E.
Published: May 2005

Many techniques have been developed to coat carbon fibers with thermoplastic resins to form a prepreg. These techniques include melt impregnation and solution/slurry coating. The applicability of these techniques, however, may be restricted by high melt viscosities or significant drying times. Recent emphasis has been towards a dry powder coating technique using a fluidized bed. This technique may be limited by difficulties in fluidization of the polymer powders. To overcome these difficulties, a technique has been developed to continuously coat carbon fibers with thermoplastic resins using aqueous foam as a carrier medium. The polymer is slurried in a surfactant solution, into which air is dispersed using a foam‐generating device. A predetermined amount of this foam is then applied to a moving carbon fiber tow using a foam application unit. The tow is then pulled through ovens where the foam is collapsed and the power fused to the fibers. Other aspects that are addressed include the stability of foams in the presence of powders, the mechanical properties of the composites formed using foam prepregging, and the effect of surfactant on composite properties.
Permeability characterization. Part 1: A proposed standard reference fabric for permeability
Authors: Parnas Richard S. , Howard J. Grant , Luce Thomas L. , Advani Suresh G.
Published: May 2005

A three‐dimensionally woven fabric is proposed as a standard reference material for permeability characterization. The 3‐D woven fabric requires care in cutting and handling, although it is more robust than 2‐D woven or braided fabrics. If prepared carefully, the permeability of the 3‐D woven fabric can be measured reproducibly within 15% in either radial flow or saturated 1‐D flow geometries. The material was characterized for permeability in radial, unsaturated and saturated 1‐D, and through‐thickness flow geometries. The transient results demonstrated the importance of structural heterogeneity on the unsaturated flow behavior, and agree qualitatively with a simplistic model of flow in heterogeneous unsaturated porous media. The effects of heterogeneity were manifested in the proposed SRM by an increasing trend in the “unsaturated permeability.” Experiments were also conducted with a random mat that displayed transient flows dominated by wicking. The effects of wicking on the macroscopic flow behavior were manifested by transients in the “unsaturated permeability” in which a decreasing trend was observed.
Permeability characterization. Part 2: Flow behavior in multiple‐layer preforms
Authors: Luce Thomas L. , Advani Suresh G. , Howard J. Grant , Parnas Richard S.
Published: May 2005

Within the resin transfer molding (RTM) process, flow is generally characterized by the progression of a distinct, nonuniform flow front into the preform as a function of time. The flow front progression introduces unsaturated characteristics into RTM flow fields. As a result, the definition of an effective in‐plane permeability (Keff) is used to determine the permeability of actual preforms as they fill with fluid. This Keff expression expands upon the original definition of Darcy's law by generalizing its applicability to unsaturated creeping flows. Results of experimentally obtained Keff for flow in single‐layer preforms have been detailed for two common RTM materials, a random mat and a 3‐D weave, in Part 1. In this paper (Part 2), we characterize the unsaturated and saturated permeabilities of multiple‐layer preforms constructed from the random mat and 3‐D weave materials characterized in Part 1. This work identifies the apparent permeability characteristics of a specific unsaturated multiple‐layer flow that demonstrates behavior inherent to this important class of heterogeneous flows. Also, parallels are drawn between the unsaturated permeability behavior of complex 3‐D weave materials and multiple‐layer preforms.
Thermal dispersion in resin transfer molding
Authors: Dessenberger Richard B. , Tucker Charles L.
Published: May 2005

This investigation focuses on the effects of thermal dispersion in resin transfer molding (RTM). A set of volume‐average balance equations suitable for modeling mold filling in RTM is described and implemented in a numerical mold filling simulation. The energy equation is based on the assumption of local thermal equilibrium and includes a dispersion term. Thermal dispersion is an enhanced transport of heat due to local fluctuations in the fluid velocity and temperature away from their average values. Nonisothermal mold filling experiments are performed on a center‐gated disk mold to investigate and quantify dispersion effects. Good agreement is found between the experimentally measured and numerically predicted temperatures, and a function for the transverse dispersion coefficient in a random glass fiber mat is determined. The results indicate that thermal dispersion is important in RTM processes and must be included in simulations to obtain accurate predictions.