bluuebunny/crossref_metadata_2025 huggingface dataset:
Environmental effects on the static strength and stress relaxation of a quartz fabric–reinforced, cyanate resin–matrix composite
Authors: Gutman E. M. , Grinberg A. , Ribak E. A. , Petronius I.
Published: April 2004

Environmental effects are an important consideration when one is designing polymer matrix composites (PMC). The mechanical behavior of a quartz fabric–reinforced, cyanate resin–matrix composite was studied by tensile tests, compression tests, and stress relaxation tests. Compression and tensile strength as well as smaller relaxation spectra were obtained after exposing the material to different environments involving printed wire board (PWB) in aviation systems: Skydrol hydraulic oil (25°C), salt spray (5% NaCl at 95°F), alkaline solution (45 g/l 50°C), and nitric acid (304 g/l 25°C). Although only a small amount of environmental liquid was absorbed, it nonetheless had an acute effect on the mechanical properties and lifetime of the material.
Processing conditions for electromagnetic interference shielding effectiveness and mechanical properties of acrylonitrile‐butadiene‐styrene based composites
Authors: Chiang Wen‐Yen , Cheng Kai‐Yuan
Published: April 2004

A conductive plastic was compounded in a twin screw extruder by incorporating conductive carbon fiber (CF) into an acrylonitrile‐butadience‐styrene (ABS) copolymer. The effects of various processing parameters prior to injection molding were investigated; then, the electromagnetic interference shielding effectiveness (EMI SE), fiber length, processability, and mechanical properties of the composite were studied. Results showed that the EMI SE of the composite increased as the final fiber length increased. The longer final fiber was produced by feeding fibers into the ABS melt at 240°C and 60 rpm. A more conductive network was formed by adding lubricants to the composite to reduce fiber damage and increase fiber dispersion. The increase of the fiber content affected processability. When the fiber content was higher than 40 phr (parts per hundred resin) in the composite, the average fiber length shortened. This study shows that better shielding can be obtained by adding a fiber at a rate higher than 30 phr. The best shielding obtained is about 30 decibels (dB).
Interfacial properties of glass fiber/brittle‐ductile dual‐matrix composites using micromechanical techniques and acoustic emission
Authors: Lee Sang‐il , Park Joung‐man , Shin Dong‐woo , Yoon Dong‐jin
Published: April 2004

The interfacial adhesion and microfailure modes of glass fiber‐reinforced brittle unsaturated polyester/modified epoxy composites were investigated via micromechanical techniques and acoustic emission (AE). Various silane coupling agents caused different degrees of interfacial adhesion and subsequent microfailure modes. In the brittle matrix layer, the number of matrix fragments was significantly influenced by the type of silance coupling agents. The more cracks, the higher the interfacial adhesion under both dry and wet conditions. This is attributed to the chemical and hydrogen bondings in two interphases. The results obtained from microdroplet and fragmentation tests were correlated by associating with the AE technique. The sequential occurrence of mainly three groups of AE were as follows: the first group originated mainly from brittle matrix cracking. The second and the third groups resulted in fiber breakage and ductile matrix cracking and debonding. For dual‐matrix specimens the micromechanical tests provide reliable information with regard to the interfacial adhesion and characterize the microfailure modes when combined with the AE technique.
Simulation of compression molding for sheet molding compound considering the anisotropic effect
Authors: Lin Chao‐ming , Weng Cheng‐i
Published: April 2004

An improved model of the anisotropic flow characteristics of SMC (sheet molding compound) during compression molding is developed. This study is intended to complement our previous paper, which was conducted to determine the anisotropic parameters for short fiber reinforced thermosets SMC (16). Our prior study measured flow viscosities and material anisotropy by means of axisymmetric and plane strain compression molding tests. The current study, in order to identify the superior flow model from the choices (1) isotropic, (2) constant anisotropic and (3) varying anisotropic, applies the finite element method to obtain numerical results, which are subsequently compared with experimental results to determine the flow model with the best fit. The anisotropic parameters of the shear directions are determined by use of normal and planar parameters because SMC is planar isotropic. Six varying anisotropic parameters and six viscosity values are estimated during molding experiments, which are conducted at room temperature so that the polymer does not cure. Two‐dimensional molding numerical analyses are carried out to explain two experimental classes, axisymmetric and plane strain compression molding. The load‐levels predicted by the isotropic model, anisotropic model (parameter values fixed) and anisotropic model (parameter values varying) are compared with the experimentally derived values, the results showing that the varying anisotropic model best fits SMC compression behavior.
Rheological characterization of commercial glass mat thermoplastics (GMTs) by squeeze flow testing
Authors: Kotsikos G. , Bland J. H. , Gibson A. G.
Published: April 2004

The flow behavior of commercial glass mat thermoplastics (GMTs) has been investigated by isothermal axisymmetric squeeze flow testing between parallel circular plates. Tests have been performed over a range of squeeze rates and the analysis is based on simple power law constitutive equations. A key part of the work is determining the relative contributions of shear and extensional flow to the overall squeezing force response. By developing a variational flow model, based on the upper bound theory, the behavior has been found to be predominantly extensional, with shear effects becoming important at very small plate separations. In addition, a predictive model for the radial pressure distribution during squeezing has been developed, based on fiber suspension rheology. Since fibers are generally long or continuous, treating the material as a statistically homogeneous fluid does not yield accurate results in this respect.
Comparison of spectroscopic and dynamical‐mechanical characteristics of some polymer composites on polypropylene base
Authors: Mamedov Sh. V. , Alekperov V. A. , Lenger Y. , Ören D. , Abasov S. A. , Bayramova R. L. , Subasi M.
Published: April 2004

In this study the mutual investigation of the results obtained from the mechanical stability properties and the IR spectra of polypropylene‐based but differently doped low‐density polyethylene (LDPE) and o‐o‐di‐n‐cresylditio phosphorous (CTPh) acid polymer composites is given. As opposed to the studies of the writers in recent years here, dynamical‐mechanical properties are explained not only by the variations in the oxidation peaks but also by the optical density in the peaks of the variations in the groups in amorphous and crystalline regions. The properties are also explained by the spectra variations in the IR region, which, according to Mamedov (2), is important, and by the extreme variations of the polymer properties. IR spectra analyses show that for both composites, (1) the optical density of the peaks in the crystalline regions for LDPE decreases while for PP those of the peaks in the amorphous regions increase; (2) the optical density of the peaks belonging to the CC groups get higher, whereas those of the peaks 1717, 1748 cm−1 diminish; and (3) the dynamical‐mechanical properties change accordingly, and these changed composites have the maximal characteristics, since the added amount is 0.2–0.3% CTPh by mass and 20–30%LDPE by mass. On the basis of the paper by Abdullayev et al. (3), the mechanism of these processes is given.
Fatigue crack growth model and mechanism of a random fiber SMC composite
Authors: Atodaria D. R. , Putatunda S. K. , Mallick P. K.
Published: April 2004

The primary purpose of this study is to investigate the fatigue crack growth rate and mechanism in a random fiber sheet molding compound (SMC) composite. The fatigue crack growth rate in this study was measured using a compliance approach. The fatigue crack growth rate in the SMC studied here shows a good agreement with a new power law model proposed earlier. This new model takes into account not only the effect of the stress intensity factor range, but also the effect of crack growth at various stages of loading using a weight average stress intensity factor. It was observed that this new model can represent the fatigue crack growth rate of the SMC at three different load ratios in a single unifying curve.
An empirical model for the viscosity buildup of hydroxy terminated polybutadiene based solid propellant slurry
Authors: Lakshmi R. , Athithan S. K.
Published: April 2004

The Polar Satellite Launch Vehicle (PSLV) makes use of hydroxy terminated polybutadiene (HTPB) based solid propellant in the booster stages, strap‐ons, and the third stage. An empirical model that describes the change in viscosity of various formulations of propellant slurry with time and temperature has been developed as an extension of the Andrade equation. The model incorporates constants that account for the viscosity of the slurry caused by the combined effect of the continuous binder phase, dispersed particulate solids, and progress of crosslinking between reactive species of the binder and curators. The correspondence between predicted viscosity buildup and measured values obtained during casting of motors is verified and found satisfactory. A method of tailoring some constants, so as to make them batch‐specific, is explained. Several practical applications are described. The model serves as a predictive tool for evaluating the potlife and rheological behavior of the slurry during production of large solid motors. The experimental method and approach used for data analysis can be used for other slurries that are similar in composition to propellant slurry.
The performance of calcium carbonate filled, random fiber composites
Authors: Steenkamer D. A. , Sullivan J. L.
Published: April 2004

This paper describes an investigation into the performance of calcium carbonate filled, random fiber reinforced composites. A series of composite plaques were fabricated by resin transfer molding (RTM) a calcium carbonate filled, unsaturated polyester resin with either an A‐glass continuous strand mat (CSM) or an E‐glass CSM preform. The influence of fiber type and calcium carbonate filler loading level were then evaluated through a combination of experiments and micro‐mechanics based modeling. The results of these studies indicate that the use of the E‐glass reinforcement led to better mechanical properties relative to their A‐glass counterpart. In addition, the composite moduli increased as the calcium carbonate filler level increased; however, the composite strengths were insensitive to filler loading level. Finally, a previously developed micro‐mechanics based model was extended to consider the presence of the filler within the resin matrix. This model was used to predict the tensile moduli and strength as well as the overall shape of the stress‐strain curves. Good agreement was found between the model predictions and the experimental results which suggests that this model could be used as a preliminary design tool to examine the effect of constituent composition on structural performance.
Separation of a thermotropic liquid crystalline polymer from polypropylene composites
Authors: Collier Monty C. , Baird Donald G.
Published: April 2004

This work is concerned with determining how to effectively recycle wholly thermoplastic composites comprised of a polypropylene (PP) matrix reinforced with a thermotropic liquid crystalline polymer (TLCP). A novel reclamation process was developed in which the TLCP could be recovered from the PP matrix. Reactive extrusion was used to reduce the molecular weight of the PP (Montell 6523) and to facilitate phase separation. The melt was then extruded into a heated mineral oil bath, which separated the TLCP (DuPont HX8000) from the matrix by dissolving the PP. It was found that greater than 70 wt% of the TLCP could be reclaimed from the PP matrix at a purity of greater than 96 wt%. In order to determine the ability to reuse the reclaimed HX8000, injection molded in situ composites were generated and their mechanical properties were determined. When the neat HX8000 component was partially replaced with reclaimed HX8000, the injection molded TLCP/PP composites showed no discernible difference in mechanical properties.
Processing induced residual stress in asymmetric laminate panels
Authors: Theriault R. P. , Osswald T. A. , Castro J. M.
Published: April 2004

In this research, a one‐dimensional finite difference model has been developed to simulate the progression of material properties during the processing of metal‐clad, multi‐layered, fiber mat reinforced, thermoset resins. Using a micro‐mechanical model, the simulation is also capable of predicting the dimensional movement observed during processing and the through‐thickness residual stress distribution within thin laminates that will lead to the development of warpage or curling. The ability to predict the overall movement is quite complex; however, the contributing factors that lead to warpage of epoxy, glass‐fiber mat laminate composites have been experimentally and numerically identified. It has been found that the dominant factor that leads to warpage in asymmetric multi‐layered laminates is the differences in the coefficient of thermal expansion of the individual plies. Thus, by selecting appropriate combinations of the degree of cure and resin content of the thermoset in the individual plies, it is possible to reduce the material property variability of the laminate through thickness. The planar movement of individual plies is a function of the glass‐fiber mat tension during pre‐processing operations. Variability in pre‐processing mat tension can be compensated for after lamination via post‐baking processes.
Use of a simple, inexpensive pressure sensor to measure hydrostatic resin pressure during processing of composite laminates
Authors: Lynch Katherine , Hubert Pascal , Poursartip Anoush
Published: April 2004

A simple and cheap method of measuring the resin pressure within a composite laminate during processing is presented. The method consists of using a small diameter, long needle filled with inert fluid and connected to an external pressure sensor, to measure the resin pressure at a point inside a composite laminate. This method can be used to investigate resin flow, laminate compaction, the control of voids, and in several composite material processing methods such as autoclave processing, hot press curing and resin transfer molding. The sensors are suitable for research and development or troubleshooting, but not for production. Sensor assemblies were developed and tested to show that their response is reproducible, linear and stable with temperature and time. Resin pressure profiles for two AS4/3501‐6 laminates were generated and compared. The experimental results were also compared to the resin flow simulation of a general processing model for composites, COMPRO. It is shown that the resin profile in the laminate is influenced by the presence of the bleeder cloth and the vacuum bag pressure. A significant pressure drop corresponded to the point of minimum viscosity of the resin. Finally, the resin pressure was stabilized when the resin reached gelation.
Prediction and experimental verification of damping in 3‐D braided textile structural composites
Authors: Yim Jong Hee , Jang Bor Z.
Published: April 2004

The material damping characteristics of 3‐D braided textile structural composites were investigated in this paper. A model for predicting damping in these materials was developed based upon the classical laminated plate theory. The model modified the existing models for predicting the static moduli of 3‐D textile structural composites by using the elastic‐viscoelastic correspondence principle. The basic engineering constants were replaced with their corresponding complex forms by applying the elastic‐viscoelastic correspondence principle to them. In this study, the basic damping loss factors (i.e. ηL, ηT, ηLT and ηvLT) were obtaiend by a modified Hashin's theory, Rule‐of‐Mixture Laws, and an indirect method on the basis of empirical works. From complex numerical results, we concluded that axial damping, flexural damping, coupling damping, and in‐plane shear damping coefficients were all functions of the yarn orientation angle and fiber volume fraction in 3‐D braided textile structural composites. Experimental data supported theoretically predicted results.
Effect of coupling agent and filler particle size on melt rheology of polymer‐bonded Nd‐Fe‐B magnets
Authors: Otaigbe Joshua U. , Kim Hyun Seog , Xiao Jun
Published: April 2004

The effect of coupling agents and filler particle size on melt rheology of poly(phenylene sulfide)‐bonded neodymium‐iron‐boron (Nd‐Fe‐B) alloy magnets was studied with oscillatory flow experiments to accelerate efforts to optimize their processing. The minimum viscosity of the polymer‐bonded magnets near 290°C was obtained with NdFeB fillers (106–150 particle size range) that were coupled with a silane coupling agent. All the samples tested followed power‐law fluid flow behavior. Morphological and dynamic mechanical analysis of the samples showed that the beneficial function of the coupling agent may be ascribed to enhanced wetting of the magnetic NdFeB powders by the polymer, improving the processability of the polymer bonded magnets.
Analysis of acoustic emission signals resulting from fiber breakage in single fiber composites
Authors: Giordano M. , Calabrò A. , Esposito C. , Salucci C. , Nicolais L.
Published: April 2004

Acoustic emission techniques are able to sense micro failures in loaded structures for a wide range of materials. Moreover, a spread application of such techniques to structural health monitoring requires further efforts in the signal analysis in order to assess the relationships between the micromechanics of the failure and the signal features as recorded by a transducer. In the case of polymer matrix composites, the main sources of acoustic emission signals are fiber breakage, matrix cracking, and fiber/matrix debonding. In this work, single fiber composites have been tested to produce fiber breakage acoustic emission events. Three different fiber/matrix systems have been investigated: PCA polyster matrix/carbon fiber (6 μm diameter), ECA epoxy matrix/carbon fiber (6 μm diameter), and ECB epoxy matrix/carbon fiber (12 μm diameter). Acoustic emission signals recorded through a wide band transducer have been analyzed in the frequency domain in order to study the amplitude spectra characteristics. The irregular shape of the amplitude spectra suggested the fractal dimension as a feature. The Richardson fractal dimension resulted in a key parameter in the signal identification among the different fiber/matrix systems: DPCA = 1.28 ± 0.02, DECA = 1.46 ± 0.03, DECB = 1.42 ± 0.02. On the basis of this finding, the shape of the amplitude spectra has been analyzed. An algorithm able to extract shape similarities from a class of spectra has been developed. It permits the definition of the representative spectra together with the identification of the relevant frequencies for the fixed fiber/matrix system.
Measurement and prediction of the glass transition temperature during processing of copper‐clad epoxy resin systems
Authors: Theriault R. P. , Osswald T. A. , Castro J. M.
Published: April 2004

The determination of the glass transition temperature of a cured resin system is necessary for the classification of the temperature region of usable service. For the printed circuit board industry, accurate and consistent properties of the copperclad laminates that are used as the substrate are essential. Variations in material properties of the finished laminates may become a hindrance in subsequent steps in the manufacturing of printed circuit boards and more importantly in the failure rate of the boards during service. In this research, a one‐dimensional finite difference method has been implemented with constitutive models to simulate the progression of the degree of cure and glass transition temperature during the processing of copper‐clad, multi‐layered, fiber mat reinforced, epoxy laminates. The numerical results have been compared to experimental measurements and it has been found that the simulation predicts the experimental observations quite well.
Optimization of filling process in RTM using a genetic algorithm and experimental design method
Authors: Kim Byoung Yoon , Nam Gi Joon , Lee Jae Wook
Published: April 2004

In the resin transfer molding (RTM) process, preplaced fiber mat is set up in a mold and thermoset resin is injected into the mold. An important issue in RTM processing is minimizing the cycle time without sacrificing part quality or increasing the cost. In this study, a numerical simulation and optimization process for the filling stage was conducted in order to determine the optimum gate locations. The control volume finite element method (CVFEM), modeled as a 2‐dimensional flow, was used in this numerical analysis along with the coordinate transformation method to analyze a complex 3‐dimensional structure. Experiments were performed to monitor the flow front to validate the simulation results. The results of the numerical simulation corresponded with that of the experimental quite well for every single, simultaneous, and sequential injection procedure. The optimization analysis of the sequential injection procedure was performed to minimize fill time. The complex geometry of an automobile bumper core was chosen. A genetic algorithm was used to determine the optimum gate locations in the 3‐step sequential injection case. Taguchi's experimental design method was also used for determining the pressure contribution of each gate. These results could provide the information on the optimum gate locations and injection pressure in each injection step and predict the filling time and flow front.
Study of the bending induced gap in fiber preforming of woven fiber mats
Authors: Gao Zheng‐Sen , Young Wen‐Bin
Published: April 2004

In the preforming of an RTM process, the configuration of the fiber mats should deform in order to conform to the mold surface. Several deformation modes may occur during a forming process. It can be characterized by factors such as the local surface curvature of the mold, the loading condition and the type of reinforcement. It has been suggested that fiber deformation modes may include fiber stretching, fiber straightening, shear slip, and buckling. As the fiber is bent at the bending edge in the preforming, a small gap may emerge from the outer side owing to the movement of fiber bundles. Studies on the relation between the channel gap formation and the bending angle together with the effect of the fiber content were investigated in order to establish a useful guide for the design of mold with bending edges. A model with a slip factor was proposed to estimate the bent fiber thickness.
Axial impregnation of a fiber bundle. Part 1: Capillary experiments
Authors: Amico S. C. , Lekakou C.
Published: April 2004

This is an experimental study to investigate the axial capillary impregnation of an E‐glass fiber bundle by using two Newtonian infiltrating fluids: (a) silicone oil and (b) epoxy. First, the wetting properties of the systems were measured, i.e. the surface tension of each fluid and the contact angle between the fluid and a single fiber filament. Liquid height and weight data as a function of time were collected from several capillary impregnation experiments of single fiber bundles. A data analysis is presented following either the integral or the differential approach to determine capillary pressure, permeability and porosity. The so‐determined values of capillary pressure were compared to corresponding theoretical values and were generally found much lower, unless the duration of the capillary experiment was around a month. This long‐term experiment brought substantial improvement in the determined values of capillary pressure. The infiltration plots from the experimental data will be further used for validation of theoretical inpregnation curves, which will result in the theoretical prediction of permeability and capillary pressure in Part 2 of this study (1).
Robust design of a composite air spindle
Authors: Chang Seung Hwan , Lee Dai Gil
Published: April 2004

If the spindle shaft of an spindle system is made of high‐stiffness carbon fiber composite material, the spindle system will be dynamically robust because the maximum rotating speed of a spindle (or rotor) system is usually restricted by the critical whirling vibration frequency of the spindle shaft, which is proportional to (EI/m)0.5 (E: Young's modulus, I: 2nd moment of inertia of shaft cross section, and m: mass per unit length of spindle). In this paper, a composite air spindle system composed of a high‐modulus carbon fiber composite shaft, powder‐containing epoxy composite squirrel cage rotor, and aluminum tool holder is introduced. For the optimal design of the composite air spindle system, the stacking sequence and thickness of the composite shaft were determined by considering the fundamental natural frequency and deformation of the system. Then the dynamic characteristics of the composite spindle system were compared with those of a conventional steel air spindle system.
Preformed particle toughening of epoxy‐based film adhesive systems: The effect of particle size and chemistry
Authors: Hayes Brian S. , Seferis James C.
Published: April 2004

A new family of particulate modifiers was incorporated into an epoxy‐based model film adhesive system and the performance was evaluated. The particulate modifiers were selected to include a range of particle sizes, chemistry, and functionality. Thermal analysis, lap shear, and fracture energy tests were performed to characterize the performance of the adhesives. The mechanisms of failure for the adhesives were analyzed in relation to the particle modifier characteristics. Significant differences were found for mode I fracture energy when comparing adhesively joined composite specimens in cocured and bonded situations. Large preformed particle modified adhesives had nearly the same GIC values for both cocured and bonded applications, while the GIC values for the much smaller core‐shell particle modified adhesives differed significantly. All particle modified adhesives provided an improvement in mode II fracture toughness over that of the control such that the laminates failed either in compression (through‐thickness direction) or through delamination of the prepreg plies.
All‐Plant fiber composites. I: Unidirectional sisal fiber reinforced benzylated wood
Authors: Lu Xun , Zhang Ming Qiu , Rong Min Zhi , Shi Guang , Yang Gui Cheng
Published: April 2004

Benzylation of sawdust from China fir was carried out to prepare plastics based on natural resources. It was found that thermoplasticity and mechanical properties of the chemically modified wood flour changed with the substitution reaction conditions. By compounding sisal fibers and the plasticized fir sawdust, unidirectional laminates were manufactured in a method similar to conventional thermoplastic composites. Such an all‐plant fiber composite material is characterized by easy processing, environmental friendliness, and low cost. Instead of chemical heterogeneity of conventional composites, physical heterogeneity of the current natural fiber composite should be favorable for interfacial interaction. However, the reinforcing sisal fibers were not well impregnated by the matrix because of the relatively high viscosity of the benzylated fir sawdust. Further efforts should be made in this area on the basis of the current preliminary work in order to improve mechanical properties of the composites.
Interface modification and characterization in linear low‐density polyethylene highly loaded with aluminium hydroxide
Authors: Wang Genlin , Jiang Pingkai , Zhu Zikang
Published: April 2004

Fire‐retardant linear low‐density polyethylene (LLDPE) composites were prepared by combining this polymer with uncoated and surface treated forms of aluminum hydroxide (Al(OH)3). The poor toughness and ductility of polyethylene highly filled with Al(OH)3 can be significantly improved by addition of a small amount of silicon oil. It is found that silicon oil improves elongation at break of the composite remarkably, but this is accompanied by the deterioration of tensile strength. Silane crosslinked polyethylene substituting for polyethylene as the matrix in Al(OH)3‐filled polyethylene improves the tensile strength of the composite. Fractured surface analysis and limiting oxygen index (LOI) of the composites were also studied. Possible mechanisms accounting for these effects are discussed.
Empirical nonlinear viscoelastic model for injection molded thermoplastic composite
Authors: Hadid M. , Rechak S. , Zouani A.
Published: April 2004

Injection molded components made from fiber reinforced thermoplastics exhibit strong viscoelastic behavior. In the present study, the bending creep tests show that the 43‐wt% glass fiber reinforced polyamide 66 is highly stress‐dependent and requires a nonlinear viscoelastic representation. However, such representation is complex for isotropic materials and is even more arduous for composite materials. In order to overcome this complexity, an empirical approach is used herein to develop a viscoelastic model based on a simple power law with stress‐dependent parameters. The proposed model demonstrates high stress sensitivity and agrees with experimental data over a wide range of applied stress.
Blade geometry effects on initial dispersion of chopped glass fibers
Authors: Kuroda Melody M. H. , Scott Chris E.
Published: April 2004

The effects of blade geometry on the initial dispersion of chopped glass fiber bundles in polystyrene during batch mixing are investigated. Roller, banbury, cam, and sigma blades are evaluated for dispersive efficiency and differences in mechanism. Processed samples are characterized by image analysis to focus upon the initial stages of dispersion. Sigma blades are found to be the most effective blades for initial chopped glass dispersion, while banbury blades prove the least effective. Analytical treatment based on the average time for a volume to pass through high stress regions does not explain the results; chopped glass fiber dispersion does not depend solely upon passage through narrow‐gap, high‐stress regions.
Surface characterization of flax, hemp and cellulose fibers; Surface properties and the water uptake behavior
Authors: Bismarck Alexander , Aranberri‐Askargorta Ibon , Springer Jürgen , Lampke Thomas , Wielage Bernhard , Stamboulis Artemis , Shenderovich Ilja , Limbach Hans‐Heinrich
Published: April 2004

The surface characteristics of several natural fibers—flax, hemp and cellulose—were investigated using scanning electron microscopy, BET‐surface area and zeta (ζ‐) potential measurements. ζ‐Potential measurements using the streaming potential method were performed in order to study the water uptake behavior as well as the surface properties of several natural fibers. The influence of different flax‐fiber separation methods and several modifications, like industrial purification, and such a treatment followed by alkaline purification as well as polypropylene grafting on the fiber surface morphology, surface area and time‐ and pH‐depending ζ‐potentials were studied. The time‐dependence of the ζ‐potential, measured in 1 mM KCl solution, offeres and alternative possibility to estimate the water uptake behavior for nearly all investigated natural fibers. The water uptake data derived from the ζ‐potential measurements (ζ = f(t)) were compared with data from conventional water adsorption studies for some chosen examples.
Flow‐enhancing layers in the vacuum infusion process
Authors: Andersson H. M. , Lundström T. S. , Gebart B. R. , Långström R.
Published: April 2004

The current trend towards increased use of vacuum infusion molding for large surface‐area parts has increased the interest in an advanced modeling of the process. Because the driving pressure is limited to 1 atmosphere, it is essential to evaluate possible ways to accelerate the impregnation. One way of doing this is to use layers of higher permeability within the reinforcing stack, i.e. flow‐enhancing layers. We present an experimental investigation of the flow front shape when using such layers. The through‐thickness flow front was observed by making a number of color marks on the glass‐mats forming the reinforcing stack, which became visible when the resin reached their position. The in‐plane flow front was derived from observations of the uppermost layer. It turned out that existing analytical models agree very well with the experiments if effective permeability data is used, that is, permeability obtained from vacuum infusions. However, the fill‐time was nearly twice as long as predicted from permeability data obtained in a stiff tool. This rather large discrepancy may be due to certain features of a flexible mold half and is therefore a topic for further research. The lead‐lag to final thickness ratio is dependent on the position of the flow front and ranges form 5 to 10 for the cases tested. Interestingly the lead‐lag has a miximum close to the inlet.
Optimization of tubular double lap‐joint configuration
Authors: El‐Hady Faissal Abd , Kandil Naglaa
Published: April 2004

The potential of using composite materials with a polymer base in the automotive industry has grown rapidly. However, one of the main problems encountered is bonding the composite part to metal conventional parts. This paper studies a proposed tubular double lap joint to bond a composite shaft to a metallic end. The automotive driveshaft will be taken as an application throughout this paper. A mathematical model of this joint is presented and a finite element solution has been carried out in order to validate the mathematical model. The optimization objective is to minimize the shear stress level in the adhesive layer. The results show that one can obtain up to 50% reduction in the level of adhesive shear stress. This can be achieved through an optimum selection of lap geometry.
Scratch behavior of nanoparticle Al2O3‐filled gelatin films
Authors: Li Tao , Chen Quan , Schadler Linda S. , Siegel Richard W. , Mendel John , Irvin Glen C.
Published: April 2004

Nanoscale alumina/protein gel composite films with 10–50 wt% filler were subjected to scratch testing and compared to micron‐scale alumina/protein gel composites. Atomic force microscopy (AFM) was employed to examine the width, depth, and morphology of the as‐produced scratches. The results show that the scratch depth in gelatin films and the tearing within the scratch decrease dramatically with the addition of the nanoscale (average particle size, 13 nm) alumina fillers. It was found that refining the particle size distribution of the Al2O3 powder (average particle size, 10 nm, with no particles larger than 70 nm) further reduced the scratch depth and width, while improving the dimensional integrity and surface roughness of the nanocomposites. Aging of the gelatin films improved the intrinsic scratch resistance of both filled and unfilled gelatin films. In comparison, micron‐size Al2O3 filler not only increased the scratch width, but also resulted in poor particle dispersion and low transparency of the films. The optical clarity of the nanofilled composites was much higher than that of the composites with micron‐size filler.
The role of fiber bundle interaction forces in non‐isothermal compression molding of thermoplastic composites
Authors: Törnqvist Ronnie , Servais Colin , Ebinger Jean‐Christophe , Månson Jan‐Anders E.
Published: April 2004

The non‐isothermal flow during compression molding of thermoplastic‐based composites having a planar, randomly oriented fiber structure has been investigated. Local interactions at the bundle‐bundle level were measured and a constitutive equation was derived to describe the shear and squeeze flow of the composites. This equation contains only micro‐mechanics‐related variables, and accounts for the temperature dependence of the interaction forces. The flow of the material under industrial processing conditions can thus be related to the deformation mechanisms of the reinforcement structure at the scale of the individual fibers Good agreement was obtained between the predicted and experimentally observed theological behavior of the composites in both shear and squeeze flow. The use of this approach to analyze the industrial molding of complex‐shaped parts is dis cussed.
Effects of glass fibers and polypropylene/glass fiber hybrid fibers on the kinetics and mechanical properties of epoxy composites
Authors: Punchaipetch Prakaipetch , D'Souza Nandika Anne , Brostow Witold , Akinay Ali E. , Reed Jonathan
Published: April 2004

Curing reactions of diglycidyl ether of bisphenol F (DGEBP‐F) and pre‐catalyzed methyltetrahydrophthalic anhydride (MTHPA) with benzyl triethyl ammonium chloride (BTEAC) were studied and effects of glass fibers evaluated. The influence on the kinetics of glass fibers and a hybrid blend of maleated polypropylene + glass fibers is studied. Isothermal and dynamic kinetic parameters are determined by differential scanning calorimetry (DSC). Applicability of the autocatalytic model is investigated. The model serves well in the range of degrees of conversion between 25 and 80%. At high conversion rates the diffusion control becomes apparent. Glass fibers accelerate the curing, shortening the time needed to reach the maximum reaction rate; this is reflected in lower activation energies for curing in comparison to the neat resin. The effects observed can be explained by a reaction between the amine group present on the fiber surfaces and the epoxy glycidyl groups. The result of both isothermal and non‐isothermal curing of resin + glass fibers commingled with polypropylene are close to those for the neat resin. The reinforcement increases the elastic modulus 12 times, the tensile strength 2 times, and the impact strength 285 times. The glass fibers + commingled polypropylene reinforcement provides comparable mechanical properties as glass fibers alone when normalized with respected to the density fraction of the fibers.
Influence of chemical structure of polyimide prepolymer on rheo‐mechanical properties of polyimide foam composites
Authors: Otaigbe J. U. , Yudin V. E. , Artemieva V. N.
Published: April 2004

A study is reported of the effect of varying chemical composition of foam polyimide prepolymers (H‐complex) on the structure and properties of foam polyimide resins and their composites with aromatic polyimide fiber felt. The melt rheological behavior of the H‐complex was found to be strongly dependent on its chemical composition and structure reorganization in the melt which is akin to mesophasic structural transition states in liquid crystal polymers. Changing the diamine part of the H‐complex from 4,4‐methylene dianiline to 1,3‐diaminobenzene led to decreased steady shear viscosity at temperatures and shear rates ranging between 95° and 105°C and 0.01 and 1 s−1, respectively. Additionally, changing the dialkyl moiety from dimethyl to diethyl in 3,3′,4,4′‐benzophenonetetracarboxylic dialkyl ester (precursor to the H‐complex) increased gelation time of the H‐complex, enhancing processibility of the foam polyimide composites. The results indicate that novel, lightweight foam polyimide composites with enhanced thermomechanical properties for beneficial uses can be prepared through chemical modification of the H‐complex prior to its thermal imidization in the presence of the aromatic polyimide fiber felt.
Structure and properties of PP/SBS/glass fiber hybrid composites. Part I: Crystallization behavior and morphology
Authors: Saujanya C. , Sainkar S. , Radhakrishnan S.
Published: April 2004

The structure development, crystallization behavior and morphology were studied in hybrid composites made with polypropylene/styrene‐butadiene‐styrene (PP/SBS) containing glass fibers (GF). The techniques applied include optical polarizing microscopy, X‐ray diffraction and scanning electron microscopy (SEM). The isothermal crystallization studies of PP/SBS/GF revealed that SBS as well as GF could act as good nucleating agents for PP. However, the nucleation efficiency of GF is much higher than that of the SBS. At high concentrations of SBS (> 30%), the crystallization rate decreases even in the presence of GF. This was attributed to the strong bonding between SBS and glass, giving rise to an intermediate layer of SBS between PP and GF, which was confirmed from the SEM of the fractured morphology of the injection molded hybrid composite. The orientation of GF was observer in the injection molded samples for certain compositions, and this was accompanied by an increase of intensities of particular diffraction peaks in the XRD.
In‐plane compression of constrained preforms
Authors: Norman David A. , Kim Jae‐Ho , Robertson Richard B.
Published: April 2004

Preforms constructed from a plain‐weave, glass fabric were compressed in‐plane within a fixture that mimicked the constraints of a closed mold. Typically, a gap was left between the bottom of the preform and the floor of the fixture; upon Initial compression, the preform slid within the fixture, which allowed the friction between the preform and the fixture wall to be measured. The preform began to compress as it contacted the floor of the fixture. The deformation was proportional to the applied stress until a critical stress was reached. Above this stress, the preform sustained damage in the form if localized buckling and a corresponding decrease in mechanical integrity. The in‐plane compressive behavior varied with system parameters, such as preform geometry, fabric orientation, and clamping stress and was shown to be strongly dependent on friction of the preform against the fixture wall. A model was developed to describe the contribution of preform friction with the fixture wall to the in‐plane compressive behavior of constrained preforms.
Detecting sub‐surface cracking in laminated membranes using infrared imaging
Authors: Mackin Thomas J. , Vernon Paul J.
Published: April 2004

This paper presents a new experimental method that utilizes the thermoelastic effect to detect sub‐surface cracks in a laminated polymer membrane. A highly accurate infrared camera is used to measure the thermoelastic and dissipational heat signatures associated with bi‐axial fatigue loading of membranes. Changes in these thermal signatures arise whenever cracks form in any layer of the laminate, including fully embedded layers, thereby providing a novel method for experimentally measuring the initiation and growth of damage in sub‐surface layers. The proposed method is illustrated using a model 3‐layer system of EVOH sandwiched between two polyurethane layers. Bi‐axial fatigue loading was used to initiate cracking in the central EVOH layer without damaging the outer polyurethane layers. Cracking in the central layer resulted in a distinct thermal signature that was plainly visible using the proposed method.
A T spin lattice relaxation study of the molecular motions of a semicrystalline polyimide matrix
Authors: Russell Vernon M. , Koenig Jack L.
Published: April 2004

The kHz molecular motion of a semicrystalline polyimide was studied through 13C solid state NMR spectroscopy. Variable temperature T experiments were performed on a Bruker msl‐300 spectrometer for various B1 applied radio frequency fields. Similar activation energies of motion for different carbon atoms in the flexible spacer gave evidence of a group motion of this molecular segment. It was also found that aromatic activation energies were higher than those of aliphatic carbons.
Novel isocyanate‐based matrix resins for high temperature composite applications
Authors: Sendijarevic Vahid , Sendijarevic Aisa , Frisch Kurt C. , Reulen Paul
Published: April 2004

Polyoxazolidone composites were prepared from polymeric isocyanate (PAPI 901) and epoxides (Epon 828 and DEN 431) in the presence of an oxazolidone‐forming catalyst, triphenylantimony iodide. The effects of isocyanate to epoxide equivalent ratio, type of epoxide, and amount of fiberglass reinforcement on the composite properties were studied as well as the effects of post‐curing temperature and time. Increasing the fiberglass content of the polyoxazolidone composites resulted in an improvement of the thermal and mechanical strength properties. The heat deflection temperature of all polyoxazolidones was > 250°C. The retention of the tensile strength at 150°C was excellent, ∼90% or higher. Polyoxazolidone composites based on DEN 431 at 1.2 isocyanate to epoxide equivalent ratio with 70 wt.% of fiberglass and post‐cured at 150°C for 48 h exhibited the best properties. According to the results of DMA, TMA and DSC, the maximum operating temperature for polyoxazolidone composites is around 200°C. The TGA data showed that the decomposition temperature was ∼330°C.
Emergence of fibrillar composites due to chaotic mixing of molten polymers
Authors: Liu Y. H. , Zumbrunnen D. A.
Published: April 2004

A technique is presented in which fine fibers (2 μm–20 μm) emerge within a cavity containing randomly distributed molten pellets of low‐density polyethylene (LDPE) and poly(ethylene‐stat‐vinyl acetate) (EVA) or polystyrene (PS) and LDPE as a result of three‐dimensional chaotic mixing. Initially coarse mixtures processed with this technique adopted fibrillar morphologies because of the interaction of interfacial tension and flow structures that are characteristic of chaotic mixing. Similar fibrillar structures were obtained for inverse viscosity ratios and different interfacial tensions, suggesting that chaotic mixing is a robust process to produce fibrillar composites. Mechanisms for the emergence of fibrils during chaotic mixing are presented. The unique characteristics of this novel technique are highlighted and possible applications in the polymer processing industry are discussed.
Acousto‐ultrasonic evaluation of carbon‐carbon composites at different processing stages
Authors: Raju P. K. , Vaidya U. K.
Published: April 2004

The final form of a carbon‐carbon (C/C) composite is a consequence of a series of complex processing stages, during which significant changes occur in material microstructure and properties (thermal, physical, and mechanical). Owing to the uncertainties in the processing steps, material integrity and quality assurance is a matter of concern. The present study focuses on acousto‐ultrasonic (AU) evaluation of plain and eight‐harness (8H) satin weave fabric reinforced C/C composites. Studies pertaining to ex‐situ AU testing of C/C composites at various stages of processing are presented. The sensitivity of AU in identifying such defects as fiber breakage and misaligned fibers is also considered. AU signatures were interpreted in the time and frequency domains through well‐known approaches, such as measurement of stress wave factor (SWF) and spectral moment computation. The results indicated strong qualitative and quantitative correlations between the parameters obtained from time and frequency domain features of the AU signal and the evolving microstructure of carbon‐carbon at various processing stages. An important application of this work will be in the establishment of AU for in‐situ process monitoring of C/C composites.