bluuebunny/crossref_metadata_2025 huggingface dataset:
Manufacture of fiber‐reinforced composites by microwave assisted pultrusion
Authors: Methven J. M. , Ghaffariyan S. R. , Abidin A. Z.
Published: April 2004

A brief review of the potential for microwave heating in the manufacture of fiberreinforced composites is presented, with particular emphasis on the Microwave Assisted Pultrusion (MAP). Manufacture of a 6 mm cylindrical glass reinforced profile, based on a number of latent‐cure epoxy resins by MAP is described. These materials combine room temperature stability (long pot life) with rapid crosslinking at elevated temperature. The measured line speeds exceeded 2 m/min, using approximately 800 W of applied microwave power in a single mode TM010 cavity operating at 2450 MHz. The measured pulling force was about 250 N, showing a stick‐slip behavior for a crosslinked profile. The ultimate tensile strength and the interlaminar shear strength of the profiles increased after post curing, suggesting that the extent of crosslinking in the MAP die may be diffusion limited.
Processing and properties of injection molded thermoplastic composites reinforced with melt processable glasses
Authors: Young R. T. , Baird D. G.
Published: April 2004

This work was concerned with evaluating the properties of injection molded composites comprising polyetherimide (PEI) and polyetheretherketone (PEEK) reinforced with various lower Tg melt processable phosphate glasses. Composites were produced utilizing a variety of glass and resin combinations in order to ascertain the effects of factors such as glass concentration and viscosity of the components on the mechanical properties of the composite blends. Changes in the rheological and interfacial properties of the blends obtained by varying the resins and phosphate glasses used during processing resulted in a variety of reinforcing morphologies consisting of glass beads, ribbons, and an interpenetrating network structure. The large variations in the glass phase morphologies obtained during injection molding led to composites that displayed a wide range of properties. Generally, it was found that the use of resin/glass combinations that minimized the viscosity difference between the components resulted in composites displaying the best overall mechanical properties. The stiffness of the composites was found to increase with glass concentration with loadings up to 45 vol% glass, leading to moduli 3‐4 times greater than those of the neat resins. While the addition of the phosphate glasses produced significant enhancements in the stiffness of the composite blends, the strength often fell to values 2‐3 times lower than those of the neat resins.
A simplified in‐plane permeability model for textile fabrics
Authors: Yu Boming , James Lee L.
Published: April 2004

In this paper, a simplified in‐plane permeability model for textile fabrics is developed. The model is based on a rectangular unit cell geometry, the one‐dimensional Stokes equation for flow in the channels or gaps between fiber tows, and the one‐dimensional Brinkman equation for flow in fiber tows. Three different textile fabrics are considered in the model: plain woven, 4‐harness, and bidirectional stitched fiberglass mats. The model incorporates the effect of porosity changes on permeability of fiber preforms under compression, which usually occurs in the molding process. To verify the validity of the model, the theoretical values are compared with a set of permeability measurements. Good agreement is found between the model prediction and the permeability measurements in the porosity ranges of ϕ ≤ 0.59 for plain woven fiber mats, ϕ ≤ 0.60 for 4‐harness fiber mats and ϕ ≤ 0.62 for bidirectional stitched fiber mats.
A phenomenological study of the mechanical properties of long‐fiber filled injection‐molded thermoplastic composites
Authors: Stokes V. K. , Inzinna L. P. , Liang E. W. , Trantina G. G. , Woods J. T.
Published: April 2004

Tensile and flexural tests on specimens cut from rectangular injection‐molded plaques show that long‐fiber filled thermoplastic composites are complex, non‐homogeneous, anistropic material systems. Like all fiber‐filled materials, they exhibit through‐thickness nonhomogeneity as indicated by differences between tensile and flexural properties. The in‐plane orientation of fibers in through‐thickness layers causes the material to have in‐plane anisotropic properties. However, these long‐fiber filled materials exhibit an unexpectedly large level of in‐plane nonhomogeneity. Also, the effective mechanical properties of these materials are strongly thickness dependent. The thinnest plaques exhibit the largest differences between the flow and cross‐flow tensile properties. These differences decrease with increasing thickness. A methodology for part design with this class of materials is discussed.
Commingled poly(butylene terephthalate)/unidirectional glass fiber composites: Influence of the process conditions on the microstructure of poly(butylene terephthalate)
Authors: Vendramini J. , Bas C. , Merle G. , Boissonnat P. , Alberola N. D.
Published: April 2004

The microstrocture of poly(butylene terephthalate) (PBT) has been investigated at the different stages of the manufacture of a new generation of composite materials, i.e., commingled Twintex® PBT/glass fiber composites. From differential scanning calorimetry and wide angle X‐ray scattering analysis in addition to density measurements and optical microscopy observations, it was concluded that the different stages of the composite manufacture induce some changes in the crystalline forms of PBT. In particular, the drawing of PBT can promote the formation of the β phase. The analysis of the crystallization kinetics points out the nucleation role played by the fibers. It is concluded that the growth of a transcrystalline region in the vicinity of fibers is promoted by the elevated pressure and temperature used in the manufacturing process of the composites.
Morphological development during injection molding of self‐reinforcing composites. I: Experimental results
Authors: Narh Kwabena A. , Li Zhigang
Published: April 2004

The correlation between structure development during injection molding and the modulus of injection molded PET/LCP blends were studied. Process parameters such as injection speed and mold and melt temperatures were varied to determine the effect of these parameters on the tensile modulus and structure development of the blends. The skin/core structure in the cross section of injection molded samples was observed with both optical and scanning electron microscopy techniques. Injection molding experiments show that the thickness of the skin layer increases with decreasing injection speed and decreasing melt and mold temperatures. The trends in morphological developments in the injection molded specimens correlate with the measured tensile moduli.
Numerical and experimental analysis of resin flow and cure in resin injection pultrusion (RIP)
Authors: Ding Zhongman , Li Shoujie , Yang Huan , James Lee L. , Engelen Herbert , Puckett P. M. (Mac)
Published: April 2004

This work presents the results of modeling, numerical simulation, and experimental study of resin flow and heat transfer in the resin injection pultrusion (RIP) process. A control volume/finite element method (CV/FEM) was used to solve the flow governing equations, together with heat transfer and chemical reaction models. Resin viscosity, degree of cure, and fiber stack compressibility and permeability were measured in order to understand their influences on the process. An analytical flow model has also been develped based on the one‐dimensional flow approximation for the resin flow in the injection die. A high‐pressure small‐taper injection die was tested with different line speeds. Experimental data were used to verify the simulation results and the analytical solutions.
Toughening of epoxy/polycaprolactone composites via reaction induced phase separation
Authors: Siddhamalli Sridhar K.
Published: April 2004

A series of melt processable thermoset/thermoplastic blends were prepared by mixing bisphenol‐A diglycidyl ether (Epon‐828)/diaminodiphenyl sulfone (DGEBA/DDS) system with two grades of polycaprolactone resin. Phase separation behavior of the blends was investigated by means of optical microscopy, microstructure by scanning electron microscopy (SEM), and thermo‐mechanical properties. The toughness of polycaprolactone modified epoxies was measured by instrumented falling weight impact (IFWI) testing. Various blend morphologies were observed depending upon the cured epoxy network/thermoplastic composition. Spinodal decomposition as characterized by modulated structure of unique periodicity and phase connectivity was found to be the probable mechanism of phase separation. SEM examination of fracture surfaces indicated a strong adhesion between the epoxy‐rich and polycaprolactone‐rich phases. Optimum improvement in failure energy was obtained for the compositions containing 10‐20% polycaprolactone without significantly compromising the elastic modulus and the thermo‐mechanical stability of the epoxy. In light of morphological evidences, a possible toughening effect was postulated in terms of tearing of the thermoplastic component and induced plastic deformation of the epoxy matrix.
Relaxation of the axial strain induced stresses in double–coated optical fibers
Authors: Shiue Sham‐Tsong
Published: April 2004

The axial strain induced stresses in double‐coated optical fibers are analyzed by the viscoelastic theory. A closed form solution of the axial strain induced viscoelastic stresses is obtained. The viscoelastic stresses are a function of the radii, Young's moduli, relaxation times and Poisson's ratios of the polymeric coatings. If the applied axial strain linearly increases, the induced stresses increase with the time. On the other hand, if the axial strain is fixed, besides the axial stress in the glass fiber, the stresses exponentially decrease with the time. The relaxation of stresses is strongly dependent on the relaxation times of the polymeric coatings. If the relaxation time of the polymeric coating is very long, the viscous behavior of the polymeric coatings will not appear, and the axial strain induced stresses solved by the viscoelastic theory are the same as those solved by the elastic theory. On the other hand, if the relaxation time of the polymeric coating is very short, the relaxation of stresses is very apparent. A compressive radial stress at the interface of the glass fiber and primary coating will result in an increase of the transmission losses, and a tensile interfacial radial stress will possibly produce debonding at the interface of the glass fiber and primary coating. To minimize this interfacial radial stress, the radius, Young's modulus and Poisson's ratio of the polymeric coatings should be appropriately selected, and the relaxation time of the primary coating should be shortened. Finally, the stresses in single‐coated and double‐coated optical fibers are discussed.
Morphology and mechanical properties of direct injection molded polypropylene/thermotropic copolyester blends
Authors: Guerrica‐Echevarría G. , Eguiazíbal J. I. , Nazíbal J.
Published: April 2004

Blends of two grades of polypropylene (PP) with thermotropic copolyester (Rodrun) contents of up to 40% were obtained by direct injection molding at different processing temperatures. In the skin of the molded specimens rather long fibers were seen in blends with low‐viscosity PP, whereas sheets were found when the high‐viscosity PP was the matrix. In the core, the viscosity of the matrix played a more relevant role than the viscosity ratio on the orientation level of the dispersed Rodrun phase. The better mechanical properties of the blends with the low viscosity PP are attributed to the morphology change of the dispersed phase from sheets to fibers when the viscosity of the matrix decreased.
Properties and interfacial bonding for regenerated cellulose–polyurethane/amylose acetate sipn composite films
Authors: Yu Jiahui , Du Yumin , Zheng Hua , Cheng Fangyu
Published: April 2004

Composite films were obtained by placing a polyurethane/amylose acetate semi‐interpenetrating polymer network (SIPN) coating onto the surfaces of regenerated cellulose (RC) film. The properties of the composite film, such as tensile strength, 79.9 MPa (in dry state), 49.5 MPa (in wet state), water resistance (R), 0.62, dimensional stability (Sc), 3.0%, and water vapor permeability (P), 5.96 × 10−5 Kgm−2h−1, are better than those of the uncoated RC film or RC film with PU coating. The interfacial strength was characterized with infrared spectroscopy (IR), ultraviolet spectroscopy (UV), transmission electron microscopy (TEM), and electron probe microanalysis (EPMA). The results showed the existence of covalent and hydrogen bonds between the SIPN coat layer and the RC layer. It was also found that the PU prepolymer in the coating layer penetrated into the cellulose bulk, and reacted with the cellulose molecules, which formed another SIPN.
Damage evolution in low velocity impacted unreinforced vinyl ester 411‐350 and 411‐C50 resin systems
Authors: Motuku M. , Janowski G. M. , Vaidya U. K. , Hosur M. V. , Mahfuz H. , Jeelani S.
Published: April 2004

Damage evolution in plaques made of vinyl ester resin systems was investigated as a function of specimen thickness, impact energy level and matrix material. Dow DERAKANE vinyl ester 411‐350 and 411‐C50 resin systems, which have low viscosity and are ideally suited for low‐cost liquid processing techniques like vacuum assisted resin transfer molding (VARTM), were considered for the low velocity instrumented impact testing. Characterization of damage evolution was undertaken using optical microscopy and analysis of impact load histories recorded during the impact event. Radial cracking, perforations at the point of impact (in the form of a truncated cone), and damage resulting from the support constraints were identified as the dominant failure characteristics in both resin systems. Radial cracking, which originated from the bottom surface, was operative in all failed specimens and was attributed to the catastrophic failure due to extensive flexural tensile strength losses. For specimens that could deflect significantly, radial cracking and support‐constraint‐induced damage were the operative failure mechanisms. Radial cracking and through‐thickness shearing led to failure in stiffer plaques. The DERAKANE 411‐350‐vinyl ester resin system was found more damage resistant than the 411‐C50 system.
Extensional processing behavior of thermoplastics reinforced with a melt processable glass
Authors: Young R. T. , McLeod M. A. , Baird D. G.
Published: April 2004

This work was concerned with investigating the processing behavior of thermoplastics reinforced with a melt processable phosphate glass under extensional flows at temperatures used for forming and shaping operations. Injection molded blends consisting of polyetherimide (PEI) and polyphenylene sulfide (PPS) reinforced with 30‐60 wt% phosphate glass were exposed to uniaxial and planar deformation at temperatures above the Tg of the phosphate glass (234°C) to evaluate the effects on the morphology and mechanical properties of the composites. Tensile testing at elevated temperatures (250‐300°C) was used to evaluate the forming behavior and ascertain the conditions most suited for the deformation of the composite blends. A temperature approximately 35°C above the Tg of the P‐glass was found to offer conditions most conducive to the deformation of the PEI/P‐glass blends. The phosphate glass reinforced PEI was found to offer greater retention of properties and smoother surfaces than an E‐glass filled material when exposed to shearfree deformation similar to that seen in a process such as thermoforming. For PPS based composites, the application of planar shearfree deformation near the melting point of the PPS (≈︁ 283°C) resulted in the elongation of the phosphate glass phase which served to enhance the stiffness of the composite blends along the principal deformation direction.
Material systems for rapid manufacture of composite structures
Authors: Kinard Janet , Colton Jonathan
Published: April 2004

A manufacturing process is described that builds complex composite parts using a layered building process in which each layer of pre‐preg composite is laid and cured as the build progresses. In order to employ on‐line curing without molds, resin technologies that provide fast curing at room temperature—ultraviolet curable and epoxy/polyamide—were investigated. UV‐curable resins were tested for their ability to “shadow” cure by exposing carbon fiber composites to ultraviolet light to determine if the cure propagated from areas directly exposed to areas under fibers. Though ultraviolet curing showed advantages in cure time and low volatile production, very minimal “shadow” curing was achieved. A low temperature curing epoxy/polyamide mixture was tested for the effects of cure temperature, cure time, and mix ratio on the final degree of cure (%DOC) and glass transition temperature (Tg). Layers were made using different resin mixtures, partially cured, and used to build layered parts to determine curing characteristics during the lay‐up process. In the epoxy/polyamide mixtures, mix ratio had little effect on the reaction rate but did affect the Tg. A kinetic model was established for the resin epoxy/polyamide system for optimizing processing conditions during fabrication. However, the model failed to correctly predict the fabrication. The reaction of the material was different during the fabrication process than during the isothermal cure due to the presence of oxygen. During the build process, the degree of cure in each layer increased significantly over the prestaged degree of cure in less time than theoretically predicted. However, the final resin properties, such as Tg, were still below the specifications for high performance parts.
Practical guidelines for predicting steady state cure time during sheet molding compound (SMC) compression molding
Authors: Abrams Lisa M. , Castro Jose M.
Published: April 2004

The longest part of the molding cycle during SMC compression molding is the curing stage. Thus it is extremely important to be able to predict its duration to estimate the cost of manufacturing a new part. During an SMC molding cycle, the mold surface temperature drops suddenly when it contacts the cold charge. The surface temperature then gradually recovers as heat is conducted from the interior of the mold and the resin releases heat during curing. In general, this exchange of heat remains locally unbalanced, causing a gradual decrease in the local surface temperature. To avoid blistering, the cure time must be increased with consecutive moldings until a steady state value is achieved (tcss). In this paper, we present a series of charts that can be used to estimate the steady state cure time for new parts. These values can then be used to estimate the manufacturing cost.
Effect of resin crosslink density on the impact damage resistance of laminated composites
Authors: Lesser Alan J.
Published: April 2004

It is generally recognized that fiber‐reinforced laminated composites are susceptible to damage resulting from low‐velocity impacts. Over recent years, many strategies have been devised to increase the fracture toughness of resin matrix materials with the aim of improving the composite's overall resistance to impact damage. One popular strategy for enhancing the fracture toughness of thermosets involves increasing the molecular weight between crosslinks, which, in turn, enhances the resins ductility. In this paper, we investigate the efficiency of this toughening approach with regard to resisting damage in composite laminates subjected to low‐velocity impacts. A mechanistic study shows that at least two distinct processes occur during an impact event. First, the laminate experiences a local failure, which resembles a Hertzian fracture process followed by subsequent delamination between the plies. Hertzian fracture occurs once at a critical threshold level initiates laminate damage through the development of a spatially configured array of matrix microcracks, which resemble that of a Hertzian cone together with radial cracks. Further damage accumulates in the laminate by inter‐ply delamination with the size of delaminated area increasing coincident with the impact load. Systematic changes in resin crosslink density show that both damage initiation and accumulation are affected. However, the maximum resistance for damage initiation occurs at a much higher crosslink density than that measured for damage accumulation.
Influence of reinforcing continuous carbon fibers on the viscoelastic properties of the epoxy matrix
Authors: Raghavan J. , Meshii M. , Feng D.
Published: April 2004

The influence of reinforcing continuous carbon fibers (AS4) on the creep and fracture of the epoxy (3501–6) matrix was studied by dynamic mechanical thermal analysis test, creep test and constant strain rate test. While the glass transition temperature (Tg) of the epoxy matrix (512 K) was increased by 5–6°K by the reinforcing fibers, the activation energy for creep/relaxation was the same for both the epoxy and its composites except off‐axis composites, for which the activation energy was higher. An increase in the instantaneous and rubbery moduli and a reduction in tanδmax indicate that the reinforcing fibers stiffen the matrix and reduce the extent of viscous deformation in the epoxy matrix. This is corroborated by the creep results that recorded a reduction in the creep rate and the magnitude of creep of the pure epoxy due to the reinforcing fibers. A consequence of this reduction in energy dissipation by viscous deformation, in the epoxy matrix due to the reinforcing fibers, is a decrease in the toughness (i.e. the total fracture energy) of the epoxy matrix measured by constant strain rate test.
Modeling the imidization kinetics of AFR700B polyimide
Authors: Russell John D. , Kardos John L.
Published: April 2004

AFR700B imidization was monitored using Fourier transform infrared spectroscopy (FTIR) by following the increase in the carbon‐nitrogen bond of imide groups displayed by the 1360 cm−1 band over several isotherms. Imidization occurred until ∼300°C (572°F). This was confirmed by the presence of water up to 300°C (572°F) in thermogravimetric analysis‐mass spectroscopy (TGA‐MS) data. First, reaction kinetics were described by first‐order kinetics with a fast and slow region of reaction. However, this model did not accurately describe the data in the transition from the fast to the slow reaction region. Imidization is kinetically controlled at short times. However, when the monomers rapidly react to form a high molecular weight polymer gel, imidization is controlled by diffusion. Therefore, a second‐order kinetic/diffusion model was used to describe the imidiation kinetics. This resulted in a substantial improvement in the fit of the data. The kinetic/diffusion model can also be used to describe imidization in polyimides other than AFR700B.
Mechanical properties of wood‐fiber/toughened isotactic polypropylene composites
Authors: Park Byung‐Dae , Balatinecz John J.
Published: April 2004

This study investigates the mechanical properties of wood‐fiber/toughened PP composite modified by physical blending with an EPDM rubber to improve impact toughness. Wood‐fiber thermoplastic composites were prepared with a modified PP matrix resin, employing high shear thermokinetic compounding aided with maleated PP for the fiber dispersion. The addition of EPDM improved the impact toughness, while it reduced stiffness and strength properties. To compensate the non‐plane strain fracture toughness, the specimen strength ratio (Rsb) was adopted as a comparative measure of fracture toughness. The strength ratio increased with the addition of EPDM, while it decreased with increasing wood‐fiber concentration. The work of fracture increased with EPDM level except at large wood‐fiber concentration. The effectiveness of the impact modification was assessed with the balance between tensile modulus and unnotched impact energy as a function of wood‐fiber concentration. EPDM rubber modification was moderately effective for wood‐fiber PP composites. The examination of fracture surfaces showed twisted fibers, fiber breakage, and fiber pull‐out from the matrix resin.
Flow near the permeable boundary of an aligned fiber preform: An experimental investigation using laser doppler anemometry
Authors: Gupte Sunil K. , Advani Suresh G.
Published: April 2004

The flow of fluid at the interface of an aligned fiber bed and an open flow is the governing phenomenon in a number of processes of industrial importance. Traditionally, this has been modeled by applying Brinkman's modification of Darcy's law to obtain the velocity profile in planar geometries in terms of an additional parameter called “apparent viscosity.” to test this ad hoc approach, a detailed experimental investigation of the flow was conducted using Laser Doppler Anemometry (LDA) in the close vicinity of the permeable boundary of aligned fiber preforms. The performs used in the experiments consisted of cylindrical fibers aligned in one direction. Two cases were investigated. In the first case, the axis of alignment was in the direction of flow and in the second case the axis of alignment was perpendicular to the flow direction. A Hele‐Shaw cell is partially filled with a fibrous preform such that an open channel flow is coupled to the Darcy flow inside the fiber bed through the permeable interface of the bed. The unfilled portion of the Hele‐Shaw cell acts as an ideal porous medium of known in‐plane permeability, which is much higher than the permeability of the fibrous/porous bed. Modeling this flow situation using a Hele‐Shaw cell is appropriate because most composite parts are long and wide in comparison with their thickness. When a viscous fluid is injected at a constant flow rate through the above arrangement, a steady state coupled flow is created. This coupling of the open flow and the Darcy flow through the fibrous bed occurs through the boundary layer zone inside the fibrous bed. Using LDA, steady state velocity profiles are accurately measured in the boundary layer zone by traversing the fibrous bed at a suitable location. For aligned fiber beds, the depth of the boundary layer zone inside the bed was found to be of the order of the mold depth, which is much larger as compared to the Brinkman's prediction. This finding indicates the presence of a length scale that is much larger than the known length scale \documentclass{article}\pagestyle{empty}\begin{document}$ \sqrt K $\end{document}, where K is the permeability of the bed made up of aligned cylindrical fibers. Based on this finding, the depth of the boundary layer thickness is incorporated in the Brinkman's solution through a boundary condition. This results in a model that compares well with the experimental data for the planar geometry and the fibrous beds considered here.
Cure monitoring of the liquid composite molding process using fiber optic sensors
Authors: Woerdeman Dara L. , Spoerre Julie K. , Flynn Kathleen M. , Parnas Richard S.
Published: April 2004

Fluorescence has been demonstrated to be an accurate tool for monitoring resin cure. It is measured using an evanescent wave fiber‐optic sensor. An economical optical fiber sensor has been developed with a refractive index greater than 1.6, permitting evanescent wave monitoring of epoxy resins. The fluorescence wave‐length‐shift, which has been correlated with monomer conversion, is monitored during the liquid molding process. Unidirectional glass fabrics with volume fractions from 40% to 60% were injected with epoxy resin at a variety of driving pressures and cured at several temperatures. Several composite parts were fabricated to test the effects of vacuum pressure, injection rate, cure temperature, and fiber fraction on the performance of the sensor. The sensitivity of the evanescent wave fluorescence sensor to the condition of the resin system was also examined. The sets of resin/hardener samples were subjected to rigorous chemical analysis to determine the extent of their differences.
A new bismaleimide system for resin transfer molding
Authors: Aijuan Gu , Guozheng Liang , Zhongming Li
Published: April 2004

A new bismaleimide (BMI) resin system, designated 4503A, suitable for resin transfer molding (RTM) has been developed. It is prepared by employing allyl methyl phenol and diallyl bisphenol A as reactive diluents for BMI. The processing properties of 4503A were investigated by time‐temperature‐viscosity curves, gel characteristics, and differential scanning calorimetry (DSC). Data show that the injection temperature of 4503A prepolymer can be as low as 75°C, while its viscosity is only 0.34Pa · s. In addition, after being maintained at 75°C for 12h, the viscosity is <0.95Pa · s. The cured 4503A resin has a glass transition temperature (Tg) and a heat deflection temperature (HDT) of 266 and 232°C, respectively; properties include tensile strength of 81MPa, flexural strength of 108MPa, and G1C of 213J/m2. Other properties of a composite based on 4503A system and woven glass cloth are also discussed. Regarding short beam shear (SBS) strength, for tests at 150 and 180°C, 80 and 61% of the original room temperature (RT) strength is retained. A similar strength retention (86 and 68%) is noted for flexural properties.
Morphological effects on the transverse permeability of arrays of aligned fibers
Authors: Papathanasiou T. D. , Lee P. D.
Published: April 2004

The effect of micro‐structure on the effective transverse permeability (K) of unidirectional arrays of cylindrical fibers is investigated computationally using the Boundary Element Method (BEM). By converting the original problem into an integral equation on the boundaries of the domain of interest, the BEM allows the straightforward discretization of complex, multi‐connected domains with a fraction of the effort required by domain methods. The equations of Stokes flow are solved in cells containing up to 36 individual fibers located either in uniform arrays (square and hexagonal) or in random locations and the effective permeability is calculated from the corresponding pressure drop and flow rate. Random placement of particles results in the generation of individual microstructures whose permeability shows a scatter around a mean value. This scatter increases significantly as the porosity (ϕ) of the cell is reduced. Statistical averages are compared to analytical and numerical predictions for the permeability of a perfect square array. It is found that a fully random structure exhibits a permeability slightly higher than a perfect square array for very high porosity values (ϕ > 0.8), with this trend disappearing for ϕ ≤ 0.8 and the averages coinciding with the result of the perfect square array. Additionally, the effect of random perturbations in the location and size of fibers around mean values corresponding to perfect square packing of mono‐sized fibers is investigated. It is found that such variations have little effect on K at the high‐porosity end (ϕ > 0.8), but they result in a noticeable increase in the effective permeability (as compared to a perfect square array) at the limit of low porosity (ϕ > 0.5).
Analysis of two‐regional flow in liquid composite molding
Authors: Ni Jun , Zhao Yang , James Lee L. , Nakamura Sho
Published: April 2004

This study investigates the two‐regional flow in Liquid Composite Molding (LCM) with emphasis on the race tracking phenomenon. An equivalent permeability is introduced to describe the flow capacity in the fiber free region. A lumped permeability is also used to further simplify the flow modeling by averaging the flow across the flow direction. Both the equivalent permeability approach and the lumped permeability approach were verified with experiments. It is found that they are capable of modeling the race tracking effects in LCM.
Experimental characterization of autoclave‐cured glass‐epoxy composite laminates: Cure cycle effects upon thickness, void content, and related phenomena
Authors: Thomas Matthew M. , Joseph Babu , Kardos John L.
Published: April 2004

This article presents results from over 100 experimental autoclave curing fiber‐glass‐epoxy composite laminate curing runs. The primary objective was to verify shrinking horizon model predictive control—SHMPC—for thickness and void content control, using readily available secondary measurements. The secondary objective was to present and analyze the extensive experimental results obtained through this verification. Seven series of curing runs (16 per series) were performed, with cure settings governed by partial‐ or full‐factorial orthogonal array based design of experiments. Through t‐tests and two‐way analysis of variance, it was found that pressure magnitude had the largest influence on laminate thickness and void content, while first hold duration/temperature, pressure application duration, and run delay influenced void content more than thickness. Thinner laminates with lesser void contents resulted from pressure application before the second temperature ramp. Prepreg age also affected thickness and void content. Photomicrographs revealed not one large void, but void clusters. Interrupted autoclave cure cycles revealed that significant laminate thickness reduction occurred during all curing cycle stages. The percentage of resin weight loss through laminate sides increased with pressure magnitude and application duration. Ten test curing runs indicated that SHMPC met difficult thickness targets while minimizing void content.
Recycled content in polymer matrix composites through the use of A‐glass fibers
Authors: Steenkamer D. A. , Sullivan J. L.
Published: April 2004

The utility of recycled A‐glass (primarily composed of soda‐lime‐silicate) fibers as reinforcement for structural composites has been studied. A series of plaques of unsaturated polyster composite were resin transfer molded with an A‐glass continuous strand mat (CSM) and a control with E‐glass CSM. The influence of fiber volume fraction on the physical and thermo‐mechanical properties of the resultant composites were investigated, both before and after environmental exposure. At the maximum fiber fraction considered (nominally 29 vol%), the use of A‐glass reinforcement lowered the warp direction tensile modulus from 8.6 to 7.6 GPa and strength from 139 to 100 MPa, relative to the control. Similar results were observed for both the flexural and the tensile properties, irrespective of fiber fraction and test direction (warp vs. weft), for the A‐glass reinforcement. Environmental exposure was found to affect equally the properties of A‐glass and E‐glass fiber reinforced composites. Based upon microscopic analyses and constituent properties, the lower mechanical properties of the A‐glass fibers composites have been linked to the lower properties of A‐glass fibers relative to E‐glass fibers. The experimental results were also used to test a micro‐mechanics models for random fiber reinforced composites. Reasonable correlation was found between the experimental results and the theoretical predictions. To offset their lower mechanical properties, A‐glass fibers could be used as a reinforcement in composite applications by simply increasing the fiber fraction relative to their E‐glass counterpart.
The concentrated emulsion approach to toughened polymer composites: A review
Authors: Ruckenstein Eli , Li Hangquan
Published: April 2004

A series of tough polymers were prepared by combining flexible with rigid chains, using the method of concentrated emulsion polymerization. The tough materials obtained can be classified into four types: [a] those prepared via the polymerization of a monomer containing a dissolved elastomer, starting from its concentrated emulsion in water; [b] those prepared via heterogeneous (interfacial) crosslinking of two partially polymerized concentrated emulsion in water containing incipient latexes; [c] those prepared as semi‐interpenetrating or AB network latexes, starting from a concentrated emulsion in water and [d] those prepared by mixing two partially polymerized concentrated emulsions and completing the polymerization. A concentrated emulsion differs from the conventional emulsion in that the volume fraction of the dispersed phase is higher than that of the most compact arrangement of monosize spheres (0.74) and can be as high as 0.99. The cells of a highly concentrated emulsion are no longer spherical, but polyhedral in shape, compactly packed and separated by thin films of continuous phase. As a result of such structure, a high polymerization rate and a high molecular weight can be achieved, the size (in the colloidal range) of the flexible phase can be controlled and the latter phase can be uniformly dispersed in the rigid one. Consequently, the concentrated emulsion method constitutes a suitable pathway to toughened composites. Owing to the compact packing of cells, the concentrated emulsion polymerization method is particularly suitable for cases in which reactions occur at the cell interface. For the materials of type [d], these reactions generate quasi‐block copolymers, which compatibilize (via “auto‐compatibilization”) the components of blends.
Cationic polymerization of diglycidly ether of bisphenol a resins initiated by benzylsulfonium salts
Authors: McGowen John A. , Mathias Lon J.
Published: April 2004

Benzylsulfonium salts are latent thermal cationic initiators that dissociate on heating to form benzyl cations that can initiate polymerizations. This paper describes the cure behavior of commercial epoxy resins using 1 ‐(p‐methoxybenzyl)tetrahydrothiophenium hexafluoroantimonate [2]. The thermal cationic cure of bisphenol A diglycidylether (DGEBA) resins was investigated using differential scanning calorimetry (DSC). DSC revealed complex cure behavior as indicated by multiple exotherms. The resins cured rapidly at low initiator concentrations with gelation occuring in 3.5 and 1.5 min at 75 and 85 °C, respectively, at conversions of epoxy groups α = 0.2–0.3. Cure kinetics were evaluated from both dynamic and isothermal DSC measurements. The effects of initiator concentration, isothermal cure temperature and heating rate on the cure behavior and mechanisms, especially involving potential termination pathways, are discussed.
Effect of isothermal aging on post‐imidization and glass transition temperature of LaRC‐IA polyimide resin
Authors: Jordan Kelvin M. , Iroh Jude O.
Published: April 2004

The isothermal aging of partilly imidized NASA Langley Research Center, LaRC‐IA polymide resin containing 70 wt% N‐methyl pyrrolidone, NMP was performed in a vacuum oven at 65, 95, 135, 165 and 200°C for 0.5, 1.0, 1.5 and 2 h. The weight loss and chemical changes that occurred during aging was determined gravimetrically and by FTIR spectroscopy, respectively. The imide absorption peak at 1778 cm−1, increased in intensity as the aging temperature was increased from 65 to 200°C. The expulsion of NMP (70 wt%) was completed after ∼2.5 h of aging at 135°C. Additional weight loss ≤4 wt%, after the expulsion of NMP, was attributed to post‐imidization. The imide carbonyl peak absorption at 1721 and 1778 cm−1, respectively, were broadened after aging at 200°C. The broadening of the imide absorption peaks was marked by the disappearance of the amide peak near 1660 cm−1 and is attributable to post‐imidization of the partially imidized polyamic‐acid. Dissolution of the polyimide aged at T ≥ 165°C in dimethyl formamide, DMF, was unsuccessful even after long times of stirring (∼12 h) at elevated temperature (T ∼ 85°C). The DSC thermogram for the LaRC‐IA resin showed a series of broad endothermic peaks between 150–180°C and narrow endothermic peaks at 210°C. The low temperature endotherm disappeared after aging at T ≥ 135°C for t ≥ 1 h. The high temperature endotherm decreased with increased aging temperature and time. The glass transition temperature of the polyimide increased with increased aging temperature and time.
Processing models and characterization of thermoplastic composite wound parts
Authors: Lin Hong‐Ru , Advani Suresh G.
Published: April 2004

Squeezing flow was adopted to model the processing of thermoplastic composites in filament winding. It is found that the resin flow is facilitated as a result of the slip condition, thus reducing the process times. An apparatus with a constant dead weight was employed to characterize the rheology of the thermoplastic composite in filament winding. The system is carbon fiber (AS4) reinforced with the polyetherketone‐ketone (PEKK) thermoplastic matrix. It is found that resin flow is dominant in the transverse fiber direction and there is insignificant flow along the fiber direction. An empirical equation of viscosity for AS4/PEKK as a function of temperature was obtained based on the model. For mechanical properties, either in a 4‐point bending test or a short beam shear test, most of the hoop wound rings failed at the inside radius or on the compression side. Insufficiently applied heat and a cold mandrel in the beginning of the winding process may result in low degree of consolidation on the compression side. The average void content of the hoop wound ring is lower than that of the preimpregnated tow because of void compression through resin flow during the winding process.
Three‐dimensional models for predicting the modulus and yield strength of polymer blends, foams, and particulate composites
Authors: Kolařík Jan
Published: April 2004

Two models are introduced to predict mechanical properties of two‐component isotropic systems: (i) the cross orthogonal skeleton (COS) with co‐continuous components and (ii) the cubic orthogonal skeleton (three perpendicular plates–3PP) with one component continuous and one discontinuous. Simultaneous prediction of the modulus and yield (or tensile) strength is based on phase continuity parameters (calculated for the two models) which serve as input data in an equivalent box model. At yielding (or breaking), the upper and lower bounds are distinguished which are related, respectively, to interfacial adhesion sufficient and insufficient for the transmission of stress inducing plastic deformation. The moduli and the upper bound of yield (or tensile) strength of the COS model, which is applied to polymer blends, are monotonic functions of composition. If partial or complete debonding occurs before yielding (or breaking), the corresponding strength passes through a minimum as a function of composition. The 3PP model renders an approximate prediction of the modulus and yield strength of closed‐cell foams and particulate composites. Predicted dependences reasonably agree with those of other models and/or with experimental data from literature.
Evaluation by torque‐rheometer of suspensions of semi‐rigid and flexible natural fibers in a matrix of poly(vinyl chloride)
Authors: Ayora Martha , Ríos Rolando , Quijano Jafet , Márquez Alfredo
Published: April 2004

The flow properties of three different PVC‐natural fiber suspensions were evaluated using a torque‐rheometer. The fibers used were extracted from henequen leaves, coconut husk, and bovine leather. They have distinct mechanical properties that produce suspensions with different flow behaviors. Nevertheless, the experimental results show that, at the rates employed, the behavior of the suspensions can be described by a power‐law (\documentclass{article}\pagestyle{empty}\begin{document}$ \sigma = m\dot \gamma ^n $\end{document}). The flow (n) and consistency (m) indexes of each suspension were evaluated using an approximate technique previously developed. The evolution of these indexes as a function of the volume fraction of fibers (Φ) are studied and discussed. It was found that, at diluted concentrations, the value of the flow index unexpectedly increases with the concentration of fibers, especially in those suspensions containing the more flexible fibers. However, at elevated concentrations it substantially diminishes. To model this behavior an empirical function for n(Φ) is proposed. The index of consistency (m) presents a continuous increment with the concentration of fibers. To model the evolution of this parameter with the content of fibers, we have defined a relative consistency index mr(mr = ms/mo, where ms and mo are the suspension and matrix indexes) in a similar form to that established for the relative Newtonian viscosity. The function mr(Φ) is modeled using several equations used to describe the behavior of this last viscosity (Maron and Pierce, Mooney, Thomas, Sudduth and Polynomial models). Among them, the Sudduth and Polynomial equations were those that better fit the behavior of these type of suspensions. Finally, to test the previous methodology, the theoretical torque values obtained were compared with experimental data for each suspension. Excellent agreement among them was found.
Optimal cure cycles for the fabrication of thermosetting‐matrix composites
Authors: Rai Neeraj , Pitchumani Ranga
Published: April 2004

Polymer‐matrix composites using thermosetting resins as the matrix are increasingly finding use. However, a major impediment to their widespread commercial use is the high cost associated with their manufacture, arising from the long processing cycle times. This paper addresses the problem of determining cure temperature and pressure variations with time for a time‐optimal manufacture of thermosetting‐matrix composites subject to practical constraints. The optimal cure cycles are determined using the nonlinear programming scheme of sequential quadratic programming combined with a physical model base to simulate the process phenomena. The optimized cycles are shown to improve upon the manufacturer‐recomended cycles as well as the improved cycles reported in the literature. The optimization results are reported for a wide range of resin materials, product specifications, and process constraints to illustrate their effects on the optimal cure cycles. Parametric studies are presented in terms of dimensionless groups to assess the combined effects of the product and process variables on the optimal cycles in a generalized manner.
Manufacturing of composite links by structural reaction injection molding
Authors: Castro Jose , Conover Steve , Wilkes Charles , Han Kerang , Chiu Hung‐Tzu , James Lee L.
Published: April 2004

An innovative approach has been developed to fabricate composite parts with complicated geometry. The part is first divided into several simple substructures and each substructure is produced by an optimal process. These preformed substructures are then assembled on a specially designed tool and transferred into a mold cavity for resin impregnation by structural reaction injection molding (SRIM). A composite link is used as an example in this study. The mold filling pattern is simulated using a 3‐D computer model. The simulated results compared well with the experimental results when race tracking and fiber impingement effects were considered in the model.