bluuebunny/crossref_metadata_2025 huggingface dataset:
Stochastic analysis of isothermal cure of resin systems
Authors: Padmanabhan S. K. , Pitchumani R.
Published: April 2004

Curing of catalyzed resin systems is an important and critical processing step in the fabrication of reinforced thermosetting composite materials. Strong uncertainties inherent in the associated process and material parameters, however, pose a stiff challenge to robust commercial manufacturing of quality composites in practice. Although deterministic models have been developed over the years to simulate the cure process, analysis of the effects of the parameter uncertainties on the process performance and the product quality variabilities has been the subject of little attention, and forms the focus of this study. This paper presents a methodology for a systematic analysis of the effects of the process and material parameter uncertainties on the isothermal curing of thermosetting resin systems. A stochastic model is developed, and parameteric studies are presented to systematically examine the effects of the uncertainties in the processing temperature and the kinetic parameters on the process output variabilities. Optimum parameter spaces that minimize the variance of the output parameters are identified, as a first step towards robust manufacturing of composites.
Determinant assembled stowage bins—a case study
Authors: Button Scott D.
Published: April 2004

Most commercial aircraft interior stowage bins are assembled using jigs. These assembly jigs are built to strict tolerances to ensure that the assemblies created in them will perform properly. Over the past decade, commercial aircraft structures assembly has been in transition from a method of assembly jig location to a method of part to part indexing, also known as Determinant Assembly. This method is now being investigated for accurate location of composite sandwich panels and metal attachments for commercial aircraft interiors. A background on Determinant Assembly (DA) as it has developed in structures is introduced. Several methods of coordinating composite sandwich panels to metal parts are discussed. Data that substantiates the use of these methods to maintain the prescribed tolerances is presented graphically. The results of building prototype parts using these techniques are presented. The process capability for DA assembled stowage bins is predicted using Variation Simulation Analysis, and those predictions are compared to results obtained with the current assembly jig method.
Mechanical properties of composites using ultrafine electrospun fibers
Authors: Kim Jong‐sang , Reneker Darrell H.
Published: April 2004

The objective of this research was to show the reinforcing effects of nanofibers in an epoxy matrix and in a rubber matrix using electrospun nanofibers of PBI (polybenzimidazole). The average diameter of the electrospun fibers was around 300 nanometers, which is less than one tenth the diameter and 1/100 the cross sectional area of ordinary reinforcing fibers. The ultrafine fibers provide a very high ratio of surface area to volume. The nanofibers toughened the brittle epoxy resin. The fracture toughness and the modulus of the nanofiber (15 wt%)‐reinforced epoxy composite were both higher than for an epoxy composite made with PBI fibrids (17 wt%), which are whisker‐like particles. In an elastomeric matrix, The Young's modulus and tear strength of the chopped nanofiber‐reinforced styrene‐butadiene rubber (SBR) were higher than those of the pure SBR. Micrographs of the fracture surfaces were obtained by scanning electron microscopy (SEM).
A process for prestaging thermosetting towpreg
Authors: Beck R. W. , Colton J. S.
Published: April 2004

A method for prestaging thermosetting towpreg consisting of passing the material through a tunnel oven was developed to address the difficulties inherent in making thick‐walled thermosetting composite parts: aging, moisture uptake, and low resin viscosity during lay‐up; and residual stresses, warping, thermal degradation, resin gradients, and excessive void nucleation during the final cure. AS‐4 carbon fiber in a 3501‐6 epoxy matrix was used to test the continuous prestaging process. With prestaging, significant reductions in towpreg bulk, voids, and total enthalpy were achieved. Cure shrinkage also was achieved. The final towpreg product had good drape and greatly reduced tack. These changes addressed many of the aforementioned difficulties. A relation between residence time at 200°C (the process temperature) versus percent degree of cure (%DOC) was developed. Manufacturing characteristics such as drape, tack, and bulk were related to %DOC. A processing window of 15 to 22 %DOC yielded towpreg with optimal characteristics. Prestaged towpreg would be well suited to manufacturing with advanced fiber placement (AFP) type equipment. Initial trials in industry have been promising.
Low velocity impact behavior of glass/epoxy cross‐ply laminates with different fiber treatments
Authors: Kessler Andreas , Bledzki Andrzej K.
Published: April 2004

This paper deals with the influence of the fiber/matrix adhesion quality on the impact behavior of cross‐ply glass/epoxy laminates. Glass fibers with two different treatments (one to promote and one to prevent adhesion to the matrix) were embedded in epoxy matrix systems and subjected to low velocity impacts at energies below perforation energy. It will be shown that the laminates with good fiber/matrix adhesion are significantely more damage resistant than the plates with poor adhesion. It will be pointed out that the composites with the more brittle matrix system show the lower damage resistance. For all materials, the absorbed energy correlates well with the amount of impact induced damage. Furthermore, an experimental/mathematical model is introduced that gives the possibility of predicting the maximum deflection and maximum force for any given impact energy. Only one experiment is needed for each material to identify the model parameters. These parameters describe the real material behavior and can be used as characteristic values to classify materials with respect to their impact stiffness and damage resistance.
Grip point spacing along the edges of an anisotropic fabric sheet in a biaxial tensile test
Authors: Bassett Richard J. , Postle Ron , Pan Ning
Published: April 2004

The technique of gripping fabric edges using pin grips has been applied in several studies of biaxial mechanical behavior of fabrics. This paper presents a theoretical study of the nonuniform stress distribution caused by the spacing of the grip points. The theory leads to expressions of the shear strain in the fabric specimen and the displacement of the grip points by which the tensile stress nonuniformity caused by the grip method can be calculated. The analysis reveals important factors affecting the stress distribution in a fabric specimen and provides design criteria for new instrument development.
Characterization of mixed fiber nylon composites incorporating composite scrap
Authors: Licea‐claveríe A. , Carrillo F. J. U. , Alvarez‐castillo A. , Castaño V. M.
Published: April 2004

Recycling of engineering thermoplastics and their composites represents both a challenge and an opportunity in today's material science. A study on nylon 6,6 based composites with mixed glass‐fiber and carbon‐fiber reinforcement, including nylon composite scrap in their formulation, is reported. Several formulations were prepared by injection molding and were characterized by stress‐strain‐measurements, impact testing, simultaneous differential thermal analysis and thermogravimetry (DTA‐TG), density measurements and scanning electron microscopy (SEM). No dependence on mechanical properties due to increasing amounts of scrap in the composites was found up to 10.4 wt%. The generated recycled composites generally showed lower mechanical properties as compared with the virgin composite because of a poor matrix‐fiber adhesion.
Measurement of principal permeability with the channel flow experiment
Authors: Weitzenböck J. R. , Shenoi R. A. , Wilson P. A.
Published: April 2004

A framework for channel flow permeability measurement in resin transfer molding (RTM) is developed in this paper. For the channel flow experiment, five possible experimental configurations are identified, of which two have not yet been mentioned in the literature. Starting from effective permeability, the entity measured in the experiment, explicit formulas for principal permeability and its orientation in two and three dimensions are developed. These formulas are applied to (published) experimental results that demonstrate their validity. The practical problems that prevent three‐dimensional permeability measurement are discussed. Next, a framework for two‐dimensional channel flow permeability measurement is proposed. All known channel flow permeability measurement methods are classified according to this new framework, and where appropriate, differences are discussed. An important finding emerging from this comparison is that two different definitions of Darcy's law are used today in permeability measurement.
Processing and properties of new high‐temperature, lightweight composites based on foam polyimide binder
Authors: Yudin V. E. , Otaigbe J. U. , Artemieva V. N.
Published: April 2004

We report a method for making novel, lightweight (ρ = 0.3–1.1 kg/dm3) polymer composites based on high‐temperature foam polyimide binder, carbon fibers, and organic fibers. The density and mechanical properties of the foam composite can be varied over a relatively wide range, depending on the volume contents of the fiber and air pores. The resin's high glass transition temperature of 260°C, coupled with the high thermal stability of carbon or polyimide fibers, contributes to its excellent retention of mechanical properties at elevated temperatures. The temperature at the beginning of weight loss is not lower than 570°C and depends on the kind of fiber felt. The combination of excellent thermal and specific mechanical properties of foam composites together with exceptional thermal stability and processability on conventional molding equipment can provide unusual performance for the new design of advanced materials and structures.
Mechanical and thermal properties of environment‐friendly “green” composites made from pineapple leaf fibers and poly(hydroxybutyrate‐co‐valerate) resin
Authors: Luo S. , Netravali A. N.
Published: April 2004

This paper presents the mechanical and thermal properties of unidirectional, degradable, environment‐friendly “green” composites made from pineapple fibers and poly(hydroxybutyrate‐co‐valerate) (PHBV) resin. Tensile and flexural properties of the “green” composites with different fiber contents were measured in both longitudinal and transverse directions. Compared to those of virgin resin, the tensile and flexural strengths of “green” composites are significantly higher in the longitudinal direction while they are lower in the transverse direction. However, the mechanical properties are lower than those predicted by simple models. Scanning electron microscope (SEM) photomicrographs of the tensile fracture surfaces demonstrate fibers being pulled out from the matrix, the interfacial failure, fiber fibrillation, and the nonunidirectional nature of the “green” composites. The thermal behavior of the “green” composites, studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), showed that the presence of pineapple fibers does not affect the nonisothermal crystallization kinetics, crystallinity, and thermal decomposition of PHBV resin.
Copolymerization kinetics of styrene/vinyl‐ester systms: Low temperature reactions
Authors: Dua S. , McCullough R. L. , Palmese G. R.
Published: April 2004

Vinyl‐ester (VE), synthesized by the addition reaction of methacrylic acid and diglycidyl ether of bis‐phenol A (DGEBA) epoxy, and styrene react via bulk free radical chain growth copolymerization to form a crosslinked network polymer. Vital clues regarding the development and the structure of the network were obtained from the study of copolymerization kinetics of styrene/VE systems. Fourier transform infrared (FTIR) spectroscopy was used to obtain the individual fraction conversion rates of both the monomers. The conversion versus time data for both styrene and VE was fit to an autocatalytic kinetic model. The autocatalytic model was found to adequately replicate the kinetic data over the entire life of cure. In this investigation, the effects of styrene concentration, temperature, catalyst concentration, and initiator concentration on cure kinetics of styrene/VE systems were studied. The conversion profiles of both styrene and VE were used to obtain the reactivity ratios of the two monomers. The reactivity ratios were evaluated to be close to zero, suggesting that initially alternating copolymerization is favored over homopolymerization. The cure behavior of vinyl‐ester resins is affected not only by the chemical reactivity of the monomers toward the free radicals, but also by diffusion effects, phase separation, and microgel formation. The interplay of these factors controls the kinetics of cure, thereby affecting the physical and chemical properties of the resulting polymer.
Tensile properties and morphology of PP/EPDM/glass bead ternary composites
Authors: Liang J. Z. , Li R. K. Y. , Tjong S. C.
Published: April 2004

The tensile properties of three types of injection molded glass bead (GB) filled polypropylene (PP)/ethylene‐propylene‐diene monomer (EPDM) ternary composites have been determined at room temperature by using an Instron materials testing machine. The effects of the filler surface treatment, the glass bead (GBI) pretreated with a silane coupling agent and the EPDM (EPDM‐MA) modified with a maleic anhydride, and the filler content on the tensile mechanical properties of the ternary PP composites have been investigated. The Young's modulus (Ec) increases while the yield stress (σyc) and tensile fracture strength (σbc) of the composites decrease with increasing the volume fraction of glass beads (ϕg) when the volume fraction of EPDM is constant (ϕe = 10%). The (Ec) values of PP/EPDM/GBI and ϵbc for PP/EPDM‐MA/GB2 (no surface pretreated) systems are the highest at the same ϕg. The tensile fracture energy (Ebc) and tensile fracture strain (ϵbc) of PP/EPDM/GBI and PP/EPDM/GB2 systems appear to peak at ϕg = 25%. However, the Ebc and ϵbc of PP/EPDM‐MA/GB2 system show little changes with increasing ϕg. The fracture surfaces of ternary composites have been examined in a scanning electron microscope. The correlation between the tensile properties and morphologies of these materials have been discussed.
Electrochemical synthesis and characterization of polypyrrole‐poly(styrene sulfonate) composite coatings on carbon fibers
Authors: Iroh Jude O. , Chen Yan
Published: April 2004

Polypyrrole‐polystyrene sulfonate composite coatings have been formed on carbon fibers by an aqueous electrochemical process. The effects of process parameters such as applied current density, monomer concentration, electrolyte concentration, and the reaction time, on the electrochemical polymerization of pyrrole onto carbon fibers were investigated. The rate of formation of the coatings increased with the current density and pyrrole concentration. It was, however, independent of electrolyte concentration, especially for polystyrene sulfonate concentration [PsSNa] of 0.05–0.25 M. At higher electrolyte concentration [PsSNa] ≥ 0.30 M, the rate of coatings formation increased with electrolyte concentration. The order of the rate of polymerization with respect to the electrolyte concentration was shown to be 0 and 1.0 at low and high electrolyte concentration, respectively. The polymerization potential, Ep, increased with the current density and decreased with pyrrole concentration. SEM micrographs showed that the morphology of coatings varied with the electrolyte concentration. At low electrolyte concentration, the coatings were rough and granular; however, at higher electrolyte concentration the coatings were smooth and uniform. The doping of polypyrrole films by polystyrene sulfonate ion was confirmed by infrared spectroscopy.
Biologically degradable fiber‐reinforced urethane composites from wheat straw
Authors: Borda Jenö , Tesléry Beáta , Zsuga Miklós
Published: April 2004

Urethane type adhesive mixtures containing poly(MDI) were used to prepare biodegradable urethane type composites containing wheat stalks. According to the mechanical properties, i.e., tensile strength, bending strength, and hardness of the composites, the ratio of wheat stalks to sticking mixtures, the technological pressure used, and the size of the wheat stalks were optimized. The mechanism of the possible chemical reaction is also discussed.
Effect of degree of cure and fiber content on the mechanical and dynamic mechanical properties of carbon fiber reinforced PMR‐15 polyimide composites
Authors: Labronici Marcos , Ishida Hatsuo
Published: April 2004

A unidirectional continuous carbon fiber reinforced polyimide composite is fabricated using the PMR‐15 polyimide as a matrix. Mechanical and dynamic mechanical properties of the composite are studied. The effect of variation of the fiber content ranging from 55% to 70% by volume on the properties of the composite is determined. The effects of the laminate thickness and degree of curing of the matrix on the Izod impact strength are examined. The effect of the degree of curing of the matrix on the dynamic mechanical spectrum is studied. Finally, variables such as humidity, environment temperature, vacuum, and pressure during laminate processing, and variation of monomer composition are discussed in terms of change in matrix structure, which is reflected in a variation of Tg between batches.
Rheological behavior of low molecular weight polystyrene composites containing monodisperse crosslinked polystyrene beads
Authors: Park M. , Salovey R.
Published: April 2004

Steady shear viscosities and dynamic moduli of polymer composites, consisting of crosslinked polystyrene beads and low molecular weight polystyrene matrix, were measured in a cone‐and‐plate rheometer at different temperatures. Viscosities and dynamic moduli were found to be very sensitive to filler loading and measurement temperature. Steady shear viscosities of 30% and 40% loaded low molecular polystyrene composites showed a power‐law behavior over the entire range of shear rates. Storage and loss moduli were initially linear with frequency on double logarithmic plots, with limiting slopes of 0.3 and 0.1. At high concentration of filler particles, they showed a flat plateau at low frequencies, indicating that these systems exhibit a yield behavior. A 20% PS composite loaded with beads of high crosslink densities resulted in poor dispersion of beads as a result of poor dispersion of particles. PS beads 1.16 μm in diameter showed a higher viscosity. It is due to the apparent increase in loading resulting from broken particles. At low measurement temperature, filler effects were suppressed by high viscosity matrix and showed a similar rheological behavior to high molecular weight by PS matrix. We suggest that rheological behavior reflects the state of dispersion of beads in the matrix.
Processing and composition dependence of the structure and mechanical properties of polycarbonate/ultrax blends
Authors: De Gauna B. E. Ruiz , Gaztelumendi M. , Nazábal J.
Published: April 2004

Polycarbonate (PC)/Ultrax blends were obtained by injection molding changing both the Ultrax content and the injection temperature, speed and pressure. The change of injection speed or pressure did not lead to significant change of morphology or properties. An injection temperature of 300°C allowed some reaction to take place between the components and the presence of some reacted Ultrax in the PC‐rich phase, but the mainly spherical morphology obtained lead to poor mechanical properties. At a lower injection temperature of 280°C, however, reactions were almost not detected, but fibrillar structures were produced, mainly because of the higher matrix viscosity. This gave rise to important increases not only in modulus and tensile strength but also in flexure properties and creep dimensional stability.
Three‐dimensionally woven and braided composites. I: A model for anisotropic stiffness prediction
Authors: Pochiraju Kishore , Chou Tsu‐Wei
Published: April 2004

A model for the prediction of anisotropic elastic stiffness of 3‐D textile structural composites based on the preform architecture and the preforming process parameters is presented. The methodology presented in this paper uses a preform model to represent the fiber microstructure and employs a micro‐mechanical analysis to determine the macroscopic anisotropic stiffness. Representative volume elements (macro‐cells) are identified for three‐dimensionally woven and braided preforms and geometric descriptions of the path and interactions between the tows within the cells are generated. This geometric modeling enables the estimation of the fiber volume fraction and the directional distribution of the fiber in the composite. The macro‐cell is further decomposed into simpler elements whose stiffness is determined from tow geometry and the stiffness of the fiber/matrix constituents. The macroscopic stiffness of the textile composite is obtained from the macro‐cell definition using an Effective Response Comparison (ERC) technique. Numerical examples are presented illustrating the developed methodology. Experimental characterization of several textile composites and the correlation of the experimental data with model predictions are considered in a companion paper.
Modeling fiber slippage during the preforming process
Authors: Lai Chyi‐lang , Young Wen‐bin
Published: April 2004

Until now, the pin‐jointed net model has been a widely used method for the prediction of preforming shape for woven fabrics. However, it deals only with pure trellising (or shear) and neglects other possible deformation modes. In some cases, there is evidence showing that other deformation modes have relevance to, and effectiveness in forming the final shape. A model considering only the shear mode will not result in an adequate prediction. In this study, a slippage model is developed to modify the pin‐jointed net model. Macrostructure of the fabrics is included in the slippage model in order to capture more deformation modes. Comparisons of experimental and simulation results are presented. It is shown that the numerical prediction using the slippage model has better agreement with experimental measurements.
Effect of cyclic moisture absorption desorption on the mechanical properties of silanized jute‐epoxy composites
Authors: Gassan Jochen , Bledzki Andrzej K.
Published: April 2004

The goal of this paper is to discuss the influence of water absorption‐desorption cycles on the mechanical properties of natural fiber reinforced plastics. Therefore, epoxy resins with jute wovens as reinforcement with untreated and silane treated fibers were investigated. Silane treatment of fibers led to increased tensile, flexural strength, and Young's modulus of composites with up to 30%. Absorption‐desorption cycles of fibers changed the fracture mechanisms of fibers without having significant effects on the tensile strength of the fibers. Light microscopic investigations showed that absorption‐desorption cycles of composites led to the debonding of resin from fibers as well as to cracks in the adjacent resin. Because of these mechanisms, tensile strength and Young's modulus decrease, independent of the quality of fiber resin adhesion. For dynamic loadings, storage cycle induces damages, further bringing about a decreased dynamic modulus and an increased progress in damage with increasing load cycles during the first two environmental cycles, being constant afterwards.
Visualization and quantification of forced in‐plane flow through deformed porous media
Authors: Friedman Henry L. , Johnson Robert A. , Gusev Vladimir , Neimark Alexander V. , Buvel Dianne , Salem David R. , Parnas Richard S.
Published: April 2004

Fluid flow in liquid molding processes and in other applications involving porous media is often characterized with a permeability tensor and modeled by d'Arcy's law. The permeability is a sensitive function of pore structure, which, in deformable materials, is influenced by compression and extension. The majority of previous permeability measurements on composite reinforcement fabrics did not account for deformations imposed on them by corners and curves in the mold. In the present study, transparent molds were designed with a single 90° bend in which the gap between plates was held constant throughout the flow path. Thus, the effects of fabric curvature on permeability were investigated independently of the effects of fabric compression in the thickness direction. A new experimental system was developed to visualize and quantify fluid flow in fabrics mounted in transparent molds. The reported measurements were conducted with fluid flowing through a (vertical) flat region of fabric, around the 90° curve, and then along a second (horizontal) flat region. Permeability was found to be reduced by the imposed curvature for a nonwoven polyester and three‐dimensional woven glass fabric. We were able to quantify the effect of curvature on permeability for the former, but not for the latter because of an enduring, dominant nonlinear entrance resistance. For the glass, comparison of two flow rates indicates that the faster flow is characterized by a two stage filling process, whereas, at the slower rate, the liquid front fills all pores at the same time.
Processing and properties of resol and epoxy blends for resin transfer molding
Authors: Manfredi L. B. , Claro J. A. , Kenny J. M. , Mondragón Egaña I. , Vázquez A.
Published: April 2004

Blends of epoxy and resol matrices for resin transfer molding (RTM) have been prepared and characterized with the double aim of improving the processability of resols and the fire resistance of epoxies. The reaction rate of the overall system is enhanced by the exothermic reaction between the epoxy and an aliphatic amine, which also promotes the resol reaction. In addition, the mechanical behavior of the resol is improved as the shear stress increases with the addition of epoxy resins. However, blending also produces a reduction of the intrinsic fire resistance of phenolic resins. The study demonstrated the feasibility of producing blends for matrices of composites processed by resin transfer molding. The recommended range for the epoxy content was determined through a balance of processability, fire resistance, and mechanical properties of the composites obtained. Moreover, the effects of fiber surface modifications were studied as a way to promote higher shear stresses in the epoxy‐resol matrix composite without jeopardizing the fire resistance properties.
Use of acoustic emission to characterize corrosion fatigue damage accumulation in glass fiber reinforced polyester laminates
Authors: Kotsikos G. , Evans J. T. , Gibson A. G. , Hale J.
Published: April 2004

The acoustic emission technique has been used to characterize fatigue damage accumulation in glass fiber woven roving (0/90°) polyester laminates after prolonged exposure in sea water. Comparisons were made with fatigue tests of “as‐received” laminate under similar loading conditions. Pre‐exposure has been found to substantially reduce the fatigue strength of the composite. Acoustic emission monitoring during fatigue testing has shown that the amplitude distribution of the acoustic events shifts from predominantly low amplitude (40–55 dB), associated with matrix cracking, in as‐received specimens, to intermediate amplitude (55–75 dB) associated with delamination and debonding after pre‐exposure. Optical microscopy of fatigued samples has verified these failure mode changes. The number of recorded high amplitude events (≥ 80 dB) associated with fiber fracture is the same in both cases, which indicates that the glass reinforcement is unaffected by pre‐exposure.
Toughening mechanisms in rubber‐modified glass fiber/unsaturated polyester composites
Authors: Sjögren B. A. , Berglund L. A.
Published: April 2004

An important case of composite failure is the leakage of pipes and pressure vessels subjected to internal pressure. The primary damage mechanism leading to leakage is transverse cracking parallel to the fibers. Effects of matrix toughening on transverse cracking strains were therefore investigated for GF/UP cross‐ply laminates with matrices of different liquid rubber content. The strain to the first transverse crack was increased from 0.2 to 0.6% with 10 wt% rubber. Debonding occurred at similar strains in GF/UP and GF/UP‐rubber. However, whereas debonding was almost simultaneous with transverse cracking in GF/UP, gradual growth of debonds to short cracks took place initially in GF/UP‐rubber. This was followed by slow extension of short cracks to a critical flaw size corresponding to unstable growth.
Hydrolytic aging of polysiloxane networks modeling the glass fiber epoxy‐amine interphase
Authors: Salmon L. , Thominette F. , Pays M. F. , Verdu J.
Published: April 2004

It has previously been shown that polycondensates of trialkoxysilanes (AlkO)3SiR could chemically simulate coupling agent layers located at the interphase of amine‐crosslinked epoxy/glass fiber composites (1) and that a humid environment modifies the degree of condensation of the network. Although it is generally believed that water will inevitably hydrolyze the polysiloxane structure and destroy the interphase (2), the authors have demonstrated that the siloxane links of the network evolve toward an equilibrium state. This state depends on the chemical structure of the organic chain that can react with the matrix. For example, in the case of aminopropyltriethoxysilane, the siloxane equilibrium concentration is low enough to allow total hydrolysis of the polymer. Conversely, propylsilane network stability could be explained by a very high siloxane equilibrium concentration. In this article, one of the previously studied systems has been selected: GPS (glycidylpropylsilane), in which the coupling function is an epoxide group (glycidyl). In an epoxy/glass fiber composite, this group is expected to react with an amine group belonging to the epoxy network. Aniline has been used here to model the GPS‐epoxy network bonding. This reaction modifies the chemical nature of the organic chain branched on the silicon and then potentially displaces the siloxane equilibrium. A gravimetric method, size exclusion chromatography (SEC), Fourier transform infrared spectroscopy (FTIR), 29Si nuclear magnetic resonance (NMR), and 13C NMR have been used. The results are that, when exposed to hot water, the GPS and the GPS‐aniline networks evolve contradictorily. Although GPS tends to hydrolysis, GPS‐aniline tends to condensation. This article analyzes the compatibility of the different behaviors with the simple kinetic model reported in a previous paper (1) and the importance of this phenomenon concerning the aging of the glass/matrix interphase of composite materials.
Numerical simulation of resin transfer molding using linear boundary element method
Authors: Schmidt F. M. , Lafleur P. , Berthet F. , Devos P.
Published: April 2004

Using mass conservation and Darcy's law to describe flow through isotropic porous media leads to a Laplace equation, which may be solved numerically at each time step using the boundary element method. For anisotropic porous media in which the permeability matrix is symmetric, the problem can be solved in the same way by rotating and stretching the coordinates. The numerical model has been compared with analytical solutions and experimental data in the case of radial front flows and with finite element for a frontal injection.
Three‐dimensionally woven and braided composites. II: An experimental characterization
Authors: Pochiraju Kishore , Chou Tsu‐wei
Published: April 2004

The results of an experimental characterization of the elastic and failure behavior of several 3‐D textile composites under tension, compression, shear, and bending loads are reported in this paper. Composites reinforced with several textile preforms comprising of layer‐to‐layer weaves, through‐the‐thickness weaves, and four‐step circular braids were characterized. The preform architectures examined included variations in tow size, pitch for the woven composites, and braid angle in the braided composites. The experimental methods, stress‐strain relationships and descriptions of the failure modes of the test specimens are described. Correlations of the predictions from a model described in the companion paper with the experimental results are also presented.
Mechanical and thermal behavior of poly(acrylonitrile‐butadiene‐styrene)/polycarbonate blends reinforced with potassium titanate whiskers
Authors: Tjong S. C. , Jiang Wei
Published: April 2004

Poly(acrylonitrile‐butadiene‐styrene) (ABS)/polycarbonate (PC) blends reinforced with potassium titanate (K2Ti6O13) whiskers were prepared in a twin screw extruder followed by injection molding. The whiskers were pretreated with tetrabutyl orthotitanate prior to compounding. The tensile, dynamic mechanical, impact, morphology and thermal properties of the blends were studied. Tensile tests showed that the modulus of ABS/PC/K2Ti6O13 blend increased markedly with increasing whisker content. However, the variation of the modulus of ABS/PC/K2Ti6O13 blend with PC content followed a sigmoidal relation. In addition, the tensile strength of the blends containing 20 wt% PC tended to increase markedly with increasing whisker content. But the impact strength of the blends containing 20 wt% PC decreased rapidly with increasing whisker content. Dynamic mechanical analyses (DMA) results indicated that the storage modulus of the blends increased markedly with increasing K2Ti6O13 whisker content. Differential thermal analysis and thermogravimetric measurements showed that potassium titanate whiskers tend to induce chemical decomposition of PC during blending of the PC/whisker blends. However, the incorporation of ABS into PC was beneficial to reduce the PC decomposition during compounding with the whiskers.
Processing and properties of co‐injected resin transfer molded vinyl ester and phenolic composites
Authors: Gillio Emanuele F. , McKnight Geoffrey P. , Gillespie John W. , Advani Suresh G. , Bernetich Karl R. , Fink Bruce K.
Published: April 2004

Vacuum Assisted Resin Transfer Molding type processes have been proven to be cost effective manufacturing techniques for large composite structures. However, their use has been limited to a single resin system. A large variety of composite structures require multiple resins to serve different purposes while being integrated into a single structure. Co‐Injection Resin Transfer Molding (CIRTM) is a new manufacturing process, developed at the University of Delaware's Center for Composite Materials in collaboration with the U.S. Army Research Laboratory, that enables the user to manufacture multi‐layer hybrid composite parts in a single processing step (1). In this paper, CIRTM is used to manufacture a dual layered structure consisting of a vinyl ester layer for structural integrity and a phenolic layer for fire, smoke, and toxicity protection. The two resins are simultaneously injected into a mold filled with a stationary fiber bed and are co‐cured. Resin separation is maintained by a 0.0254 mm (0.001 in) thick polysulfone film sandwiched between two layers of 0.165 mm (0.0065 in) thick adhesive. A Differential Scanning Calorimeter (DSC) is used to select the optimum cure cycle for all of the materials. Mechanical testing is used to evaluate the performance of the interphase formed between dissimilar materials. Short beam shear (SBS) is used to evaluate the overall quality of the part produced. Double cantilever beam (DCB) is used to quantify the fracture toughness of the interphase, and the wedge test is used to evaluate the durability of the interphase. Experimental results show that co‐injected, co‐cured materials offer properties equivalent, or in some cases, superior, to those provided by single injection resin composites. This case is used to develop and present a methodology that can be followed to co‐inject different resins.
Effects of interlaminar stresses on damping of 0‐degree unidirectional laminated composites
Authors: Yim Jong Hee , Jang B. Z. , Suhling J. C. , Gillespie J. W.
Published: April 2004

The principal aim of this paper is to formulate a general model for predicting damping in composites on the basis of the concept of strain energy‐weighted dissipation. In this model, the effects of interlaminar stresses on damping have been included in addition to the effects of in‐plane extension/compression and in‐plane shear. Validation of the model was confirmed by performing damping measurements on 0° unidirectional composite beams with varying length and thickness. The results of theoretical predictions of damping in laminated composites were found to compare favorably with experimental data. The transverse shear (σxz) reveals to have a considerable effect on the damping mechanisms in 0° unidirectional polymer composites. However, the other interlaminar stresses (σyz, σz) were shown to have little influence on damping in composite beam.
Polymer composites prepared by compression molding of a mixture of carbon black and nylon 6 powder
Authors: Pinto Gabriel , López‐gonzález Cipriano , Jiménez‐martín Ana
Published: April 2004

The results of an experimental study on the effect of processing variables and filler concentration on the electrical resistivity of conductive composites based on nylon 6 filled with carbon black are reported. A typical percolation behavior in the effect of electroconductive filler content on the resistivity was found. The electrical resistivity of the composites is > 1012 ohm°Cm unless the carbon black content reaches the percolation threshold at ∼9 wt%, beyond which the resistivity decreases markedly by as much as twelve orders of magnitude. Two parameters of molding process—temperature and time—were shown to have a notable effect on the resistivity of composites, whereas pressure has no influence on this property in the pressure range considered. There is no sharp variation in the density due to the onset of percolation, and the hardness of samples is not influenced by the presence of the filler.
Development of an isothermal lubricating layer model for injection pultrusion of electronic prepregs
Authors: Hogade Ranjeet , Lee Ming , Castro Jose M. , Lee L. James
Published: April 2004

The current process for manufacturing electronic prepregs uses solvent‐based resin systems. Solvents are environmentally unfriendly and contribute to voids in the pre‐preg, which are a source of product variability. Prepreg inconsistencies are one of the major sources of scrap on the board shops. To overcome these drawbacks, a solvent‐less process is currently being developed in our laboratory. The process is based on the concept of injection pultrusion or continuous resin transfer molding (RTM). The centerpiece of the process is an impregnation die. Glass fabric and resin are fed into the die where fiber impregnation and partial resin cure occurs depending on the material chemo‐rheology. The B‐staging is finished in an oven located immediately after the die. A key to the success of the process is being able to predict the pulling force. Fabrics used in electronic prepreg manufacturing are relatively fragile; they could be damaged if the pulling force is too high. A model to predict the pulling force, based on the formation of a continuous lubricating layer between the die surface and the moving fabric, has been developed and experimentally tested using model fluids.
AC conductivity of carbon fiber–polymer matrix composites at the percolation threshold
Authors: R̈ímská Zden̈ka , Křuesálek Vojtěuch , S̈pac̈ek Josef
Published: April 2004

This paper reports results on experimental investigation of the conductivity behavior of carbon fiber filled polymer composites at the percolation threshold. Two types of carbon fiber‐epoxy matrix composites have been studied and comparison of the measured data has been made. These two types of composites differ in the surface modification of carbon fibers (in one case the surface of carbon fibers is covered with polymer beads using the microencapsulation technology, in the other their surface stayed unmodified). Experimental data reveal that surface modification of carbon fibers influences greatly the DC conductivity (percolation threshold moves to higher concentrations) but does not influence the AC electrical properties. From the frequency dependence of conductivity upon fiber concentration it becomes clear that it is not possible to predict the high frequency conductivity (electromagnetic interference shielding properties) based on the DC conductivity. Percolation behavior of conductivity as a function of conductive filler concentration is typical only for DC or low frequency AC conductivity. The percolation threshold gradually vanishes for high frequencies of electromagnetic field. The temperature dependence of electrical properties has also been studied. Composites with concentration near the percolation threshold show the switch‐off effect (at the specific temperature the DC conductivity drops by several orders of magnitude). This switch‐off effect does not occur for high frequency AC conductivity.
Composite machining damage quantification using thermoelastic stress analysis
Authors: Horn Gavin P. , Mackin Thomas J. , Kurath Peter
Published: April 2004

A new experimental method is presented for quantifying machining damage in polymer matrix composites. The method consists of capturing infrared images of machined samples and using thermoelastic stress analysis to quantify damage from the machining event. A modified stress concentration factor is presented as an easily measured and useful damage parameter. Circular holes were drilled into the center of plate specimens fabricated from a commercially available glass fiber reinforced composite. A standard drill bit, brad point drill bit, and abrasive water jet machining were the three machine tools investigated. Infrared images were used to quantify the machining damage by assigning a thermoelastic stress analysis based stress concentration factor (mSCF) to each machined hole. The mSCF was then used to rank the damage inherent to each machining method. Optical and electron microscopy were utilized to identify the types of damage associated with the three machining methods. Finally, each sample was fatigued to failure to substantiate the IR results. The ranking of damage based upon the mSCF was in good agreement with the fatigue lifetime rankings: higher mSCF is associated with shorter fatigue lifetimes.
Numerical simulation of three‐dimensional fiber orientation in injection molding including fountain flow effect
Authors: Han Kyeong‐Hee , Im Yong‐Taek
Published: April 2004

It is essential to predict the nature of flow field inside mold and flow‐induced variation of fiber orientation for effective design of short fiber reinforced plastic parts. In this investigation, numerical simulations of flow field and three‐dimensional fiber orientation were carried out in special consideration of fountain flow effect. Fiber orientation distribution was described using the second‐order orientation tensor. Fiber interaction was modeled using the interaction coefficient CI. Three closure approximations, hybrid, modified hybrid, and closure equation for CI=0, were selected for determination of the fiber orientation. The fiber orientation routine was incorporated into a previously developed program of injection mold filling (CAMPmold), which was based on the fixed‐grid finite element/finite difference method assuming the Hele‐Shaw flow. For consideration of the fountain flow effect, simplified deformation behavior of fountain flow was employed to obtain the initial condition for fiber orientation in the flow front region. Comparisons with experimental results available in the literature were made for film‐gated strip and centergated disk cavities. It was found that the orientation components near the wall were were accurately predicted by considering the fountain flow effect. Test simulations were also carried out for the filling analysis of a practical part, and it was shown that the currently developed numerical algorithm can be effectively used for the prediction of fiber orientation distribution in complex parts.
Axial impregnation of a fiber bundle. Part 2: Theoretical analysis
Authors: Amico S. C. , Lekakou C.
Published: April 2004

This paper considers the numerical simulation of the axial capillary infiltration of an aligned fiber bed and the assessment of the input permeability and capillary pressure data by validating the simulation results with experimental data from Part 1 (1) of this study. Theoretical predictions of capillary pressure are close to the capillary pressure value determined from a long‐term experiment and are used as input data in the infiltration simulations. The Carman‐Kozeny equation is recommended for the prediction of permeability and the validation of the numerical simulations with experimental data for two infiltrating liquids, silicone oil and epoxy, yields recommended values for the empirical Kozeny constant for different ranges of fiber volume fraction.
Compaction of fiber reinforcements
Authors: Batch Gibson L. , Cumiskey Sean , Macosko Christopher W.
Published: April 2004

In resin transfer molding, dry fiber reinforcements are compacted as the mold closes before injection of a curable resin matrix. This paper presents experimental data of compaction pressure as a function of fiber volume fraction. Data are presented for woven roving mats, random fiber mats, loose fiber rovings for pultrusion, and uniaxial or biaxial roving mats. These data are fit to a mathematical model derived in an Appendix. Experimental data are also given for six combinations of reinforcements. We use the compaction model of each constituent layer to predict the average volume fraction assuming that fiber layers do not interact. However, we see that most combinations of reinforcements have fiber volume fractions greater than expected at pressures under 50 psi, indicating a synergistic packing between the layers of different composition.
An optimization procedure for the pultrusion process based on a finite element formulation
Authors: Coelho R. M. L. , Calado V. M. A.
Published: April 2004

Composite materials are manufactured by different processes. In all, the process variables have to be analyzed in order to obtain a part with uniform mechanical properties. In the pultrusion process, two variables are the most important: the pulling speed of resin‐impregnated fibers and the temperature profile (boundary condition) imposed on the mold wall. Mathematical modeling of this process results in partial differential equations that are solved here by a detailed procedure based on the Galerkin weighted residual finite element method. The combination of the Picard and Newton‐Raphson methods with an analytical Jacobian calculation proves to be robust, and a mesh adaptation procedure is presented in order to avoid integration errors during the process optimization. The two earlier‐mentioned variables are optimized by the Simulated Annealing method with some constraints, such as a minimum degree of cure at the end of the process, and the resin degradation (the part temperature cannot be higher than the resin degradation temperature at any time during the whole process). Herein, the proposed objective function is an economic criterion instead of the pulling speed of resin‐impregnated fibers, used in the majority of papers.