bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of hydrothermal aging on the thermo‐mechanical properties of a composite dental prosthetic material
Authors: Pegoretti Alessandro , Migliaresi Claudio
Published: April 2004

The aim of this investigation was to evaluate the long‐tern effects of aging in water on the physical properties of a new class of commercially available dental polymer composites. The selected product consists of a bisphenol a glycidyl methacrylate (Bis‐GMA) resin diluted with triethylene glycol dimethacrylate (TEGDMA) and reinforced with long E‐glass fibers, specifically developed for prosthetic dental bridges. Samples were prepared according to a standard procedure suggested by the producer, and aged in water at 37°C and 70°C up to 32 weeks. Samples were periodically tested in order to assess their mass variation, static flexural modulus and strength, fatigue resistance, and dynamic mechanical thermal behavior. Experimental results evidenced that aging caused two simultaneous phenomena, having opposite effects on the specimen mass. In fact, composites absorbed a certain amount of water (up to 0.8 wt% at 37°C and 1.2 wt% at 70°C) but at the same time a mass loss was detected, which could be attributed to a release of unreacted monomeric species and fragments generated by polymer chain degradation (especially at 70°C). Flexural strength strongly decreased during aging in water, reaching 80% and 45% of the initial value for samples aged for 32 weeks at 37 and 70°C, respectively. Aging practically does not affect flxural modulus, while a sensible reduction of the material fatigue life was observed. Glass transition temperature and the relative activation energy were markedly influenced by the aging in water with effects related to the water uptake and mass loss phenomena.
Mechanical properties of carbon woven reinforced epoxy matrix composites. A study on the influence of matrix modification with polysulfone
Authors: Mujika F. , De Benito A. , Fernández B. , Vázquez A. , Llano‐Ponte R. , Mondragon I.
Published: April 2004

An experimental investigation has been carried out to study the influence of thermoplastic addition on the mechanical properties of woven carbon fiber/epoxy matrix composites. As toughening agent bisphenol‐A polysulfone, PSu, has been added to the epoxy matrix. Flexural tests haved been performed to characterize the mechanical behavior of unmodified and PSu‐modified bulk tetra‐ and bifunctional epoxy matrices and also for the corresponding woven carbon fiber, CF, composite materials. Three‐point notched flexural tests been used to investigate the influence of polysulfone addition in the mode‐I fracture properties of the bulk epoxy matrices, relating them to their microstructural features investigated by atomic force microscopy (AFM). The double‐cantilever bea (DCB) and the end‐notched flexural (ENF) tests have been applied to characterize the interlaminar fracture toughness of the corresponding composites. For composites, the flexural properties were simmilar independent of the funcetionality of the epoxy matrix and of the thermoplastic content. Nevertheless, PSu addition to the epoxy matrix celarly enhanced the ode‐I and II interlaminar fracture toughness of the corresponding composites, the immprovement being higher for the composites manufactured with the bifunctional epoxy matrix at every thermoplastic content because of the lower crosslink density of the epoxy matrix.
Surface modification of fique fibers. Effect on their physico‐mechanical properties
Authors: Gañán Piedad , Mondragon Iñaki
Published: April 2004

A study has been performed to assess the possibilities of fique fibers (a Colombian native fiber) as reinforcement for polymeric matrices. The fique fibers were treated using different chemical surface treatments. The effect produced on the physico‐mechanical properties of fique fibers have been investigated. The applied surface treatments have been mercerization, and/or chemical agents such as maleic anhydride and acrylic acid, and also silanization. In order to analyze the chemical changes in the treated fibers, Fourier Transform infrared, FTIR, spectroscopy was used along with two related techniques: Attenuated Total Reflection (ATR) and Coupled Microscopy. Additionally, physical and mechanical properties of untreated and treated fique fibers have been studied. A wettability study using contact angle measurements has also been performed for surface free energy determination. The thermal stability of the fibers has been investigated by thermogravimetrycal analysis.
Initial dispersion mechanisms of chopped glass fibers in polystyrene
Authors: Kuroda Melody M. H. , Scott Chris E.
Published: April 2004

Understanding the mechanisms contributing to the initial stage of dispersion for individual fibers from chopped glass bundles is important for optimizing process conditions and obtaining fiber composites with optimal properties. These studies have shown that under model flow conditions, bundles have two separation modes, rupture and erosion. Both of these occur in a stochastic manner and are dependent upon the internal structure of the bundles. Under chaotic flow, both strain rate and total strain to which the material is exposed are important, though the effect of increasing strain rate decreases at higher total strains. To Obtain consistent minimal dispersion in model flows, the maximum nominal stresses required are higher than 104 Pa for simple shear and 106 Pa for squeezing flows. In contrast, stress levels around 104 Pa yield good dispersion under chaotic flow, implying the flow field is a more significant factor than the stresses imposed.
The effect of moisture and loading rate on the interfacial fracture properties of sandwich structures
Authors: Scudamore R. J. , Cantwell W. J.
Published: April 2004

The skin‐core interfacial fracture properties of a number of dry and moistureconditioned sandwich structures have been investigated over a range of crosshead displacement rate using the three point bend sandwich (TPBS) structure. It has been shown that the interfacial fracture toughness, Gc, of a crosslinked PVC system decreases rapidly with loading rate, whereas the toughness of a linear PVC remains roughly constant. In contrast, the interfacial fracture toughness of the balsa core material increased steadily with increasing crosshead displacement rate, an effect that was attributed to the rate dependent properties of the glass fibers in the wake of the primary crack. Prolonged seawater exposure in an aluminum honeycomb structure was found to attack the bond between the epoxy matrix and the aluminum core, facilitating crack advance along the skin‐core interface. Finally, it is concluded that great care should be exercised before selecting sandwich structures for hostile marine environments.
An analysis of stretch forming of thermoplastic composites
Authors: Lee J. H. , Vogel J. H. , Rhee K. Y.
Published: April 2004

Thermoplastic composite sheet materials have found an increasing number of applications through the use of matched‐die compression molding. The combination of high strength, low weight and moderate manufacturing costs makes the material an attractive alternative to sheet metal stampings in a number of applications. However, a significant range of conventional sheet forming techniques may also prove suitable for these materials, provided the effects of strain rate and temperature can be properly understood and exploited, leading to a further reduction of manufacturing costs and an increasing number of potential applications. In this work, limiting strains in biaxial stretch forming were explored using the hemispherical stretch forming test. The materials tested contain 20, 35, and 40 percent by weight of randomly oriented glass fiber in a polypropylene matrix. The forming tests were performed at temperatures ranging from 75°C to 150°C and at punch speeds of 0.01cm/sec, 0.1cm/sec, and 1cm/sec. A rotationally symmetric, anisotropic material model with rate sensitivity was developed and incorporated into an axisymmetric finite element model of the stretch‐forming process. The model parameters were temperature‐dependent, though the temperature distribution in the formed part was assumed to be uniform. Strain distributions in the formed parts are compared to finite element method results, and the results are good up to the point when localized necking begins to dominate the strain distribution. These forming limit strains are compared with predictions based on Marciniak's imperfection theory, with good results.
Lead—Natural rubber composites as gamma radiation shields. II: High concentration
Authors: Gwaily S. E. , Madani M. , Hassan H. H.
Published: April 2004

Natural rubber composites were prepared by the incorporation of different lead concentration, namely: 100, 300, 500, 1000, 1500 and 2000 phr to be used as gamma radiation shields. A non‐linear dependence of the attenuation coefficient upon the lead concentration was observed for both 60Co and 137Cs as gamma radiation sources. Sample containing the ultimate lead concentration (2000 phr) and thickness 1 mm was found to have 0.336 mm and 0.383 mm lead equivalent for 60Co and 137Cs respectively. Electrical and mechanical properties of the investigated composites were also measured.
Investigation of the influence of textile compression on the out‐of‐plane permeability of a bidirectional glass fiber fabric
Authors: Endruweit Andreas , Luthy Thierry , Ermanni Paolo
Published: April 2004

In Liquid Composite Molding (LCM) processes, the out‐of‐plane component of the textile permeability varies with the applied injection pressure because of compression of the textile. A correlation of the compressibility measured on a testing machine and the behavior in a flow channel gives information about the compression for dry and wet preforms in the LCM mold during resin injection. The permeability for injection pressure values between 0.1 MPa and 0.5 MPa has been measured for various initial porosity values with five pressure cycles each. A steady state permeability value has been defined, averaging the permeability values in the pressure range of 0.1 MPa to 0.5 MPa excluding the hysteresis effect of the initial pressure cycle. A relation between this steady permeability and the given initial porosity and injection pressure has been found, replacing the Kozeny‐Carman formula for the description of the permeability of compressed textiles. Based on the same idea, a formula for description of the variations in permeability for increasing pressure in the first pressure cycle has been found. In this empirical formula, the term describing the influence of the injection pressure varies from the one found for the steady state permeability.
A comparison between the static and fatigue properties of glass‐fiber and carbon‐fiber reinforced polyetherimide composites after prolonged aging
Authors: Viña J. , Castrillo M. A. , Argüelles A. , Viña I.
Published: April 2004

The static and fatigue behavior of two fiber‐reinforced composites was characterized after samples were subjected to both natural and accelerated aging. The composites consisted of a thermoplastic matrix (PEI: polyetherimide) reinforced with glass or carbon fabric. Natural aging involved exposing samples to the elements over a period of two years, while accelerated aging was conducted in a saline solution during 200 days. Subsequently, static (tensile and interlaminar shear) and fatigue tests were carried out with the aim of determining the mechanical properties of the materials after exposure. A negligible decrease in the value of these properties was observed, while different behavior was detected depending on the type of aging of the material.
Short fibers as reinforcement of rubber compounds
Authors: López Manchado M. A. , Arroyo M.
Published: April 2004

The effect of aramid, glass and cellulose short fibers on the processing behavior, crosslinking density and mechanical properties of natural rubber (NR), ethylene‐propylene‐diene terpolymer rubber (EPDM) and styrene‐butadiene rubber (SBR) has been investigated. Two fiber percentages (10 and 20 phr) were added to the rubber. The results have shown that the above‐mentioned fibers, especially aramid fibers, are effective reinforcing agents for these rubbers, giving rise to a significant increase in mechanical properties, such as tensile modulus and strength, and tear and abrasion resistance. Moreover, a significant decrease in the time to reach 97% of curing, t′c (97) is observed, which indicates that the fibers tend to increase the vulcanization rate, regardless of the rubber used. Fibers give also rise to an increase in crosslinking, especially the aramid fibers.
Recycling of the fibrous fraction of reinforced thermoset composites
Authors: Kouparitsas C. E. , Kartalis C. N. , Varelidis P. C. , Tsenoglou C. J. , Papaspyrides C. D.
Published: April 2004

We investigate the feasibility of reusing short fibers recovered from recycled thermoset composites for the production of new composites. Glass fibers were recovered from glass‐polyester composites, and carbon and aramide fibers from epoxy based composites. From the different fractions obtained after grinding, a specific fibrous fraction was selected for reuse. This recycled fraction was first characterized in terms of length and residual matrix content, and then incorporated into virgin polymer matrices to prepare new thermoplastic composites. To evaluate the performance of these composites, tensile tests were initially carried out, and the results were compared with similar measurements performed on pristine composites containing short unused fibers of similar length. In most cases examined, recycling does not adversely affect the mechanical performance of the new composite. This overall behavior is explained in terms of fiber length preservation, fiber dispersion mechanism and fiber‐matrix adhesion.
Self‐reinforced polypropylene/LCP prepregs and laminates
Authors: Liang Y. C. , Isayev A. I.
Published: April 2004

Polypropylenes (PPs) of various molecular weights were mixed with a thermotropic liquid crystal polymer (LCP) to prepare unidirectional sheets (prepregs), quasi‐isotropic and unidirectional laminates. The mechanical properties and the morphology of the prepregs and the laminates at 0° and 90° with respect to the machine direction were investigated as a function of draw ratio, LCP concentration and molecular weight of the PP. The results for prepregs and laminates showed that both drawing and LCP concentration generally enhanced modulus and tensile strength in machine direction. The morphology of LCP changed from spherical or ellipsoidal droplets to elongated fibrils as the draw ratio increased. The diameter of LCP fibrils decreased with increasing molecular weight of the PP matrix, indicating more effective droplet breakup and better mixing in the case of high molecular weight PP.
The influence of cooling rate on the fracture properties of a glass reiforced/nylon fiber‐metal laminate
Authors: Guillén J. F. , Cantwell W. J.
Published: April 2004

The effect of varying cooling rate on the microstructure and resulting mechanical properties of a novel fiber‐metal laminate (FML) based on a glass fiber‐reinforced nylon composite has been investigated. Polished thin sections removed from plain glass fiber/nylon composites and their corresponding fiber‐metal laminates indicated that the prevailing microstructure was strongly dependent on the rate of cooling from the melt. Mode I and Mode II interlaminar fracture tests on the plain glass fiber reinforced nylon laminates indicated that the values of GIc and GIIc averaged approximately 1100 J/m2 and 3700 J/m2 respectively at all cooling rates. The degree of adhesion between the aluminum alloy and composite substrates was investigated using the single cantilever beam geometry. Here, the measured values of Gc were similar in magnitude to the Mode I interlaminar fracture energy of the composite, tending to increase slightly with increasing cooling rate. The tensile and flexural fracture properties of the plain composites and the fiber metal laminates were found to increase by between 10% and 20% as the cooling rate was increased by two orders of magnitude. This effect was attributed to over‐aging of the aluminum alloy plies at elevated temperature during cooling. Finally, fiber metal laminates based on glass fiber/nylon composites were shown to exhibit an excellent resistance to low velocity impact loading. Damage, in the form of delamination, fiber fracture, matrix cracking in the composite plies, and plastic deformation and fracture in the aluminum layer, was observed under localized impact loading. Here, the fast‐cooled fiber metal laminates offered superior post‐impact mechanical properties at low and intermediate impact energies, yet very similar results under high impact energies.
Effects of temperature and strain rate on the tensile behavior of short fiber reinforced polyamide‐6
Authors: Wang Zhen , Zhou Yuanxin , Mallick P. K.
Published: April 2004

Tensile behavior of extruded short E‐glass fiber reinforced polyamide‐6 composite sheet has been determined at different temperatures (21.5°C, 50°C, 75°C, 100°C) and different strain rates (0.05/min, 0.5/min, 5/min). Experimental results show that this composite is a strain rate and temperature dependent material. Both elastic modulus and tensile strength of the composite increased with strain rate and decreased with temperature. Experimental results also show that strain rate sensitivity and temperature sensitivity of this composite change at a temperature between 25°C and 50°C as a result of the glass transition of the polyamide‐6 matrix. Based on the experimental stress‐strain curves, a two‐parameter strain rate and temperature dependent constitutive model has been established to describe the tensile behavior of short fiber reinforced polyamide‐6 composite. The parameters in this model are a stress exponent n and a stress coefficient σ*. It is shown that the stress exponent n, which controls the strain rate strengthening effect and the strain hardening effect of the composite, is not only strain rate independent but also temperature independent. The stress exponent σ*, on the other hand, varies with both strain rate and temperature.
Synergistic effect of carbon fillers in electrically conductive nylon 6,6 and polycarbonate based resins
Authors: Clingerman Matthew L. , Weber Erik H. , King Julia A. , Schulz Kirk H.
Published: April 2004

The electrical conductivity of polymeric materials can be increased by the addition of carbon fillers, such as carbon fibers, carbon black, and synthetic graphite. The resulting composites could be used in applications such as electromagnetic and radio frequency interference shielding and electrostatic dissipation. A significant amount of work has been conducted varying the amount of single conductive fillers in a composite material. In contrast, very limited work has been conducted concerning the effect of combinations of various types of conductive fillers. In this study, three different carbon fillers were used: carbon black, synthetic graphite pareticles, and pitch based carbon fiber. Two different polymers were used: nylon 6,6 and polycarbonate. The goal of this project was to determine the effect of each filler and combinations of different fillers on the electrical conductivity of conductive resins. A 23 factorial design was analyzed to determine the effects of the three different carbon fillers in nylon 6,6 and polycarbonate. The results showed that carbon black caused the largest increase in composite electrical conductivity. The factorial design analysis also showed that combinations of different carbon fillers do have a positive synergistic effect, thereby increasing the composite electrical conductivity.
Thermal diffusivity of lead titanate/polyvinylidene fluoride‐trifluoroethylene nanocomposites by the mirage method
Authors: Aravind M. , Ong C. W. , Chan H. L. W.
Published: April 2004

Thermal diffusivity D of lead titanate/polyvinylidene fluoride‐trifluoroethylene {PT/P(VDF‐TrFE)} nanocomposites has been measured by the skimming configuration of the mirage method. Diffusivity values are calculated by measuring the phase variation of the probe beam deflection signal upon changing both the vertical and horizontal probe beam offset values at different modulation frequencies of the pump beam. The thermal diffusivity of the nanocomposites is found to depend on the volume fraction of the ceramic in the composites. The composites with higher volume fraction of ceramic have higher thermal diffusivity and conductivity. Different models were used to predict the thermal conductivity (k) of composites and the measured values agree quite well with the Looyenga model.
Electrical properties of composites of hard metal carbides in a polymer matrix
Authors: Vilčáková Jarmila , Sáha Petr , Hausneroná Berenika , Quadrat Otakar
Published: April 2004

The electrical conductivity of composites containing hard metal carbide powder in a nonconducting polymer matrix has been studied. In composites with low and medium filler contents, the expansion of the nonconducting matrix with increasing temperature caused interruption of conducting paths and a rapid drop in conductivity (switching effect). In contrast, at high filler loadings (ΦP > 50 vol%), a continuous and slight decrease in conductivity with temperaute inducates a specific composite structure with a dense particle arrangement and a great number of conducting contants, which change only little during heating.
Mechanical properties of Al2O3/polymethylmethacrylate nanocomposites
Authors: Ash Benjamin J. , Rogers Diana F. , Wiegand Christopher J. , Schadler Linda S. , Siegel Richard W. , Benicewicz Brian C. , Apple Tom
Published: April 2004

Alumina/polymethylmethacrylate (PMMA) nanocomposites were produced by incorporating alumina nanoparticles, synthesized using the forced gas condensation method, into methylmethacrylate. The particles were dispersed using sonication and the composites were polymerized using free radical polymerization. At an optimum weight percent, the resulting nanocomposites showed, on average, a 600% increase in the strain‐to‐failure and the appearance of a well‐defined yield point when tested in uniaxial tension. Concurrently, the glass transition temperature (Tg) of the nanocomposites dropped by as much as 25°C, while the ultimate strength and the Young's modulus decreased by 20% and 15%, respectively. For comparison, composites containing micron size alumina particles were synthesized and displayed neither phenomenon. Solid‐state deuterium NMR results showed enhanced chain mobility at room temperature in the nanocomposites and corroborate the observed Tg depression indicating considerable main chain motion at temperatures well below those observed in the neat polymer. A hypothesis is presented to relate the thermal and mechanical behavior observed in the composites to the higher chain mobility and Tg depression seen in recent ultrathin polymer film research.
Key factors affecting permeability measurement in the vacuum infusion molding process
Authors: Hammami Adel
Published: April 2004

The composites industry, under increased environmental constraints, is seeking to shift from existing open mold manufacturing processes for composite parts. A promising manufacturing technology known as the vacuum infusion molding process is gaining acceptance among composite‐parts manufacturers since it involves low tooling cost and allows complete elimination of volatile organic compounds (VOC). The process is similar to the resin transfer molding process; however, in the vacuum infusion technique, a polymeric film, often referred to as vacuum bag, replaces the stiff mold cover. The film is sealed against the lower half of the mold, at the periphery. Air expelled from the mold cavity results in the compaction of the reinforcement by the atmospheric pressure present on the outer side of the polymeric film. Finally, resin impregnates the mold cavity, usually through a resin distribution channel. The process is mainly developed for large‐scale structures, where material cost is an important parameter and users cannot afford any production pitfalls. Among process parameters that affect resin flow in the vacuum infusion molding process is the permeability of the reinforcement stack, which has to be measured and evaluated taking into consideration the requirements of the process. A possible approach is the definition of a parameter that defines the maximum infused length, and this parameter will take into account the structure of the reinforcement, the resin viscosity, the fiber volume fraction and inlet geometry.
Study of electrophysical characteristics of lead‐natural rubber composites as radiation shields
Authors: Gwaily S. E. , Hassan H. H. , Badawy M. M. , Madani M.
Published: April 2004

The dependence of the attenuation of both gamma rays and neutrons on lead concentration in leaded‐natural rubber composites incorporated with paraffin wax and boron carbide as radiation shields was investigated. Moreover, the dc conductivity and swelling characteristics of the rubber vulcanizates and the effect of aging on them were studied. It was found that samples of 18 mm thickness incorporated with 20 phr of B4C and 60 phr of paraffin wax can reduce the fast neutron beams by about 50%. For slow neutrons, the reduction was about 46%. The attenuated γ‐dose increased exponentially with increasing lead content.
Control of flow in resin transfer molding with real‐time preform permeability estimation
Authors: Nielsen D. R. , Pitchumani R.
Published: April 2004

Variabilities in the preform structure in situ in the mold are an acknowledged challenge to achieving reliable preform saturation in liquid molding processes. Physical models offer an effective means of deriving real‐time process control decisions so as to steer the resin flow in a desired manner, which ensures complete preform saturation. An important parameter influencing the fidelity of the simulations is the preform permeability, which is a strong function of the preform microstructure. A model‐based control strategy that incorporates the ability to determine and utilize local permeability information in real‐time is of much value, and forms the focus of the paper. An intelligent model‐based controller is developed that uses virtual sensing of permeability to derive optimal decisions on controlling the injection pressures at the mold inlet ports in a resin transfer molding (RTM) process. The controller employs an artificial neural network, trained using process simulation data, as an on‐line flow simulator, and a simulated annealing algorithm to optimize the injection pressures on‐the‐fly during the process. Preform permeability is virtually sensed during the process, based on the flow front velocities and the local pressure gradient along the flow front, estimated using a fuzzy logic model. The controller, implemented on an RTM process, is shown to be able to accurately steer the flow fronts through various preform configurations.
Compressive damage in two‐step braided thermoplastic composites
Authors: Kuo Wen‐Shyong , Ko Tse‐Hao , Fang Jiunn
Published: April 2004

This work examines compression‐induced damage in a 3D, braided carbon/nylon 6 composite. The two‐step braiding was used to form the fabric, and the yarn used was a carbon tow commingled with nylon 6 powder. About 82% of overall fibers were arranged along the axial direction, and the rest were the braiding fibers. A matched mold was designed for melting the resin and pressing the fabric into a predetermined thickness for the resulting composite. The molding temperature was varied to examine its influence on the compressive properties. The compressive tests were carried out by using an end‐supported, end‐loaded fixture designed for thicker specimens. The results showed that the molding temperature greatly affected the composite modulus and strength. Microscopic observations revealed that the induced damage modes differ from those in 3D thermosetting composites. Miniscopic failure of the yarns appears to be the dominant mode. Two types of damage were observed. One is miniscopic kink‐band caused by the through‐thickness shear stress; the other is the folding of the axial yarns into a corrugation form. These modes are related to the 3D structure and the interfacial bonding condition. Some key features of the capability of energy absorption of this material are also discussed.
A unified approach to determine principal permeability of fibrous porous media
Authors: Weitzenböck J. R. , Shenoi R. A. , Wilson P. A.
Published: April 2004

This paper presents a unified approach (UA) to determining permeability in liquid composite molding. The UA applies both to the channel flow and the radial flow experiment and it unifies the data processing of almost all known permeability measurement methods. It permits principal permeability to be calculated from experimental measurements which are carried out in an arbitrary co‐ordinate system. This paper describes the UA and its main features. Further, the UA is applied to a number of experimental results for radial flow and channel flow measurements to demonstrate its validity. In addition, the algorithms of current permeability measurement methods are classified according to the UA, and, where necessary, differences are discussed. Some data processing algorithms were found to contain weaknesses or shortcomings.
Surface treatment of CFRP composites by Ar+ irradiation to improve bonding strength between aluminum and CFRP composites
Authors: Rhee Kyong Y. , Choi Nak‐Sam , Park Soo‐Jin
Published: April 2004

The effect of surface treatment of carbon fiber reinforced plastic (CFRP) composites on the T‐peel strength and the shear strength between CFRP and aluminum panels was studied. The surface of the composite panel was treated with Ar+ irradiation under oxygen environment. The optimal Ar+ ion dose was determined by measuring the changes of contact angle and surface energy as a function of ion dose. T‐peel tests and SLS tests were performed using irradiated CFRP/aluminum specimens and unirradiated CFRP/aluminum specimens to determine the treatment effect by Ar+ irradiation under oxygen environment on the T‐peel strength and shear strength of CFRP/aluminum composites. The results showed that contact angle on the surface of the composite panel was reduced from ∼80° to ∼8° and the surface energy increased from 31 ergs/cm2 to 72.4 ergs/cm2 with an ion dose of 1017 ions/cm2. T‐peel strength and shear strength are significantly affected by the surface treatment of composite panel. T‐peel strength and shear strength improved 650% and 56%, respectively, when the treatment was made with an ion dose of 1016 ions/cm2. SEM examination showed that the improvement of bonding strength was attributed to the uniform spread and fracture of epoxy adhesive.
A structure and property based process window for void free thermoset composites
Authors: Eom Yongsung , Boogh Louis , Michaud Véronique , Månson Jan‐anders
Published: April 2004

A process window providing guidelines to minimize internal stress levels and to prevent void formation during cure of thermoset composite materials is presented. A model taking into account the applied pressure and the level of stress borne by the fiber assembly was introduced to calculate the hydrostatic internal stress state in the resin during cure. Based on the fundamental mechanisms of matrix shrinkage and evolution of viscoelastic properties under the given processing conditions, the internal stress in the resin was calculated as a function of fiber volume fraction, fiber stacking sequence, applied pressure and resin conversion. This level of stress is compared to a criterion for void initiation in the resin. A process window was hence constructed for preventing void formation during cure. Composite laminates with different stacking sequences and fiber volume fractions were cured with different applied pressures within and out of the process window boundaries. The composite void contents were measured and correlated perfectly with the process boundaries. This process window construction taking into account the material vis‐coelastic properties and the composite architecture is a unique tool for determining optimum process condition of composite laminates.
A gas flow method for determination of in‐plane permeability of fiber preforms
Authors: Um Moon‐Kwang , Daniel Isaac M. , Childs Brian W.
Published: April 2004

Resin flow plays a crucial role in many composite manufacturing processes. The most important parameters used in modeling and designing mold filling are the permeability of the fibrous preform, which is a kind of flow conductance and a property of the reinforcement, and the viscosity of the resin. The extent reaction, or degree of cure, is also important and causes change of chemical during mold filling. To determine the permeability of fiber preform searchers have been using liquid flow analysis. In this study, a new scheme for determining permeability using gas flow is proposed. In conventional liquid flow methods, radial propagation of the polymer into a porous medium is measured and used to determine permeability, whereas in the gas flow method, the several different preform geometries is measured and used. The effectiveness of the gas flow method was verified by comparing it with conventional methods.
Tearing mode interlaminar fracture toughness of composite materials
Authors: Hwang Shun‐Fa , Hu Chiao‐Lin
Published: April 2004

The Simplified Split Cantilever Beam (SSCB) is proposed in this work and compared with the Split Cantilever Beam (SCB) to obtain the tearing mode interlaminar fracture toughness. The materials considered are single‐fiber system composites and interply hybrid composites. For interply hybrid composites, three different types of stacking sequence for SSCB specimens, which are [0/0//0],[0/0//0]. and [0/0//0], are tested to compare their suitability. Finite element analysis combined with a modified crack closure integral has been applied to separate the different components of the strain‐energy release rate. In addition, the method of compliance calibration was used to calculate Gc values. The effects of crack growth, initial crack length, specimen width, and number of glass fiber plies were also studied. The results show that SSCB testing has a more dominant Mode III component and more stable Gc values than SCB testing. For SSCB testing, the crack growth and the specimen width for the range considered have no clear effects on the interlaminar fracture toughness, but the initial crack length should be carefully selected to obtain corrected values. The tearing mode interlaminar fracture toughness of interply hybrid composites is higher than that of carbon/epoxy composites, and the three different types of stacking sequence considered are all suitable to approximate the Mode III interlaminar fracture toughness for interply hybrid composites.
Conducting aluminum‐filled nylon 6 composites
Authors: Pinto Gabriel , Jiménez‐Martín Ana
Published: April 2004

This work is concerned with the preparation and characterization of composite materials prepared by compression molding of a mixture of aluminum flakes and nylon 6 powder. The electrical conductivity, density, hardness and morphology of composites were investigated. The electrical conductivity of the composites is < 10−11 S/cm unless the metal content reached the percolation threshold, beyond which the conductivity increased markedly by as much as 1011. The volume fraction of conductive filler at the percolation threshold was calculated from experimental data, by fits to functions predicted by the percolation theory. Decreasing the average particle diameter of filler leads to increased percolation threshold (it varies from 23 to 34 vol% for the three different fillers studied) and decreased maximal conductivity of composites. The density of the composites was measured and compared with values calculated assuming different void levels within the samples. Furthermore, it is shown that for certain sizes of particle filler, the hardness decreases initially with the increase of metal concentration, possibly because of poor surface contact with the nylon matrix, but, starting from a certain value, there is a hardness increase. For the smallest particle filler, the hardness of samples is not influenced by the presence of the filler.
Isothermal and non‐isothermal consolidation of carbon fiber towpregs
Authors: Nunes J. P. , Brito A. M. , Pouzada A. S. , Bernardo C. A.
Published: April 2004

The purpose of this work is to establish and validate a model to predict the isothermal and non‐isothermal consolidation behavior of unidirectional towpregs, made of carbon fibers and a thermoplastic polymer. A Finite Difference Method program was devised based on a set of analytical expressions developed in a previous work. The program allows the determination of pressure‐displacement curves at constant press closing speed and fixed or variable molding temperature, allowing comparison with experimental data obtained in the same conditions. There was good agreement between theoretical predictions and experimental results for both the isothermal and non‐isothermal consolidation. However, in the former case, the agreement was limited to low closing speeds and long pre‐heating times. In the latter case these limitations did not exist, showing that the non‐isothermal model is a good tool for the optimization of the consolidation process.
Impact behavior of aramid fiber/glass fiber hybrid composites: The effect of stacking sequence
Authors: Park Rohchoon , Jang Jyongsik
Published: April 2004

Aramid fiber/glass fiber hybrid composites were prepared to examine the effect of stacking sequence on the impact behavior of thin laminates. The effect of position of the aramid layer on the impact properties of hybrid composites was investigated using driven dart impact tester. The delamination area and fracture surface of hybrid composites were analyzed for correlation with impact energy. The addition of glass layer to aramid layer reduced the impact resistance of hybrid composite due to the restriction in the deformation of aramid layer. The position of aramid layer resulted in variations in the impact behavior of hybrid composites. When the aramid layer was at the impacted surface, the composite exhibited a higher impact energy. This was attributed to the fact that the flexible layer at the impacted surface in thin laminates can experience larger deformation. In three‐layer composites, the aramid fiber‐reinforced composite (AAA) exhibited the highest total impact energy due to high impact energy per delamination area (1EDA) in spite of low delamination area. Aramid fiber and glass fiber‐reinforced composites showed a different impact behavior according to the change of thickness. This was attributed to the difference in the energy absorption at interface between laminae.
Modeling and simulation of the hole pressure problem
Authors: Wu G. H. , Ju S. H. , Wu C. C.
Published: April 2004

Whenever a polymeric or viscoelastic liquid flows over a depression in a channel, the phenomenon of hole pressure occurs. Modeling and simulation for the flow of an Oldroyd‐B type fluid passing over a transverse slot is considered here. This type of fluid is a good model for some polymeric liquids. In the model, it is assumed that the extra stress of the solution is the sum of the contribution from the solvent and the contribution from the polymer molecules. The results obtained in the study are computed by using the elastic‐viscous split‐stress finite element method incorporating the non‐consistent streamline‐upwind scheme (EVSS/SU finite element method). As a verification of the numerical scheme, the algorithm is first applied to compute the corresponding flow of the upper‐convected Maxwell fluid model, a special case of the Oldroyd‐B model. Hole‐pressure is evaluated for various Deborah numbers (De), and compared with that derived from the Higashitani‐Pritchard (HP) theory. The agreement between the two is found to be satisfactory for creeping flow in the De range for which the Higashitani‐Pritchard theory is valid. Subsequently, hole‐pressure and characteristics of this flow problem are predicted. Futhermore, the effects of fluid elasticity, inertia, and slot geometry on hole‐pressure are investigated.
Dynamic mechanical behavior of LCP fiber/glass fiber–reinforced LLDPE composites
Authors: Hashmi S. A. R. , Kitano Takeshi , Chand Navin
Published: April 2004

Liquid crystalline polymer (LCP) fibers and glass fibers have been used to rein force linear low density polyethylene (LLDPE) by using an elastic melt extruder and the compression molding technique. The impact behavior of hybrid composites of different composition is compared and is explained on the basis of the volume frac tion of the fibers. Addition of glass fibers decreases the Izod impact strength LLDPE. The impact strength of the composites increases when glass fibers are placed by LCP fibers. Dynamic mechanical α and β relaxations are studied and effect of variation of fiber composition on these relaxations is reported in the tem perature range from −50 to 150°C at 1 Hz frequency, a relaxation shifts toward higher temperatures with addition of fibers in LLDPE. Addition of fibers increases the storage modulus of LLDPE.
Structure and properties of PP/SBS/glass fiber hybrid composites. Past II: Property measurement
Authors: Saujanya C. , Radhakrishnan S.
Published: April 2004

The mechanical properties viz. impact and tensile modulus of hybrid composites of PP/SBS/glass in the composition range 0‐40 wt% of glass fibers (GF) were studied. Both impact and tensile modulus increased with respect to glass content at a certain composition of the blend. Synergistic effects were seen in the value of elongation and toughness at a certain composition of the glass content. The above findings could be associated with the high aspect ratio and nucleation ability of GF together with good interfacial adhesion between the GF and SBS, rather than the PP matrix. These have been supported by the investigations on structures and morphologies of the hybrid composites.
Determination of elastic constants of transversely isotropic materials by combining vibration testing and numerical method
Authors: Hwang Shun‐Fa , Chang Chao‐Shui , Chang Kung‐Lin
Published: April 2004

Since it is nondestructive and can produce global properties, the method combining vibration testing and numerical analysis is a potential approach to determine elastic constants of materials. In order to simplify the modeling process and reduce the complexity of the numerical analysis, both the direct determination method and the inverse determination method are presented in this work. In the direct determination method, two special natural frequencies of an X mode and a ring mode are obtained by the vibration testing of a plate with special dimensions decided from another arbitrary plate. Five natural frequencies obtained from both plate specimens can be used to determine four elastic constants of transversely isotropic materials by substituting them into the existing formulas. In the Inverse determination method, the combination of finite element analysis and optimum design is adopted to inversely calculate the elastic constants. Thin plates of steel and carbon/epoxy composites are selected to demonstrate the procedures and to compare the application of both methods. The results indicate that the elastic constants predicted by both methods are in good agreement with the reference values. Even though the steel plate is treated as a transversely isotropic material, these predicted constants also satisfy the conditions of isotropy. However, it is always difficult and cumbersome to find the X and ring modes in the direct determination method, especially for composite materials. Therefore, the inverse determination method is generally recommended because of its single test and versatility.
Effects of water, salt solution and simulated concrete pore solution on the properties of composite matrix resins used in civil engineering applications
Authors: Chin Joannie W. , Aouadi Khaled , Haight Michael R. , Hughes William L. , Nguyen Tinh
Published: April 2004

One of the obstacles hindering the acceptance of polymer composites in civil engineering applications is the susceptibility of the polymeric matrix to degradation that is initiated by moisture, temperature, and corrosive chemical environments. The objective of this study was to characterize chemical and physical changes in polymer matrix resins following exposure to these environments. Resin systems studied were vinyl ester and isophthalic polyester, both of which are proposed for use in construction applications. Unreinforced free films were exposed to water, alkaline and saline environments at ambient and elevated temperatures for extended periods of time. Changes in strength and thermophysical properties were evaluated through tensile testing, dynamic mechanical thermal analysis (DMTA) and thermogravimetric analysis (TGA). Chemical degradation of the polymers was characterized using Fourier transform infrared (FTIR) spectroscopy. Energy dispersive X‐ray (EDX) analysis of specimens following exposure was carried out to determine if ion diffusion into the bulk polymer occurred. Only minor changes in the glass transition temperatures of the polymers were observed after prolonged exposure at elevated temperature, but more substantial changes were noted in tensile strength, particularly in the case of the isophthalic polyester. Examination of the polymers following immersion in salt solution and alkaline solution showed essentially no ionic penetration into the bulk, with the exception of specimens that were visibly degraded. Spectroscopic analysis of chemical structure prior to and following exposure revealed varying degrees of ester hydrolysis.
Analysis of an image processing method for fiber orientation in polymer composites
Authors: Vahlund C. Fredrik , Gebart B. Rikard
Published: April 2004

The errors in fiber orientation distribution measurements of compression molding materials have been investigated, for generated as well as for real distributions. Because of the size of the sample, only a finite number of fibers are visible in a picture leading to a statistical error in the measurement. A method is proposed to express this error as function of the number of visible fibers and the number of subintervals the distribution is divided into. Studies of the error In a Fraunhofer/FT based fiber orientation distribution analysis have been performed, including effects of increasing number of fibers (fiber‐fiber intersections) in generated pictures and investigation of the errors in real fiber images. All steps in a fiber orientation distribution analysis of SMC/GMT kind of material are described, including suitable equipment, image enhancement methods and investigation of the errors present in analysis of a real image as well as comparisons to hand–measured data.
Effect of reinforcement structure on compaction behavior in the vacuum infusion process
Authors: Hammami A.
Published: April 2004

The Vacuum infusion molding process is gaining Increased popularity for its relatively short cycle time, low equipment cost and low labor requirements. Furthermore, for large and complex structures, it offers a high potential of integration In addition to the complete elimination of volatile organic compound (V.O.C.) emissions, which are considered to be a major health concern for employees within the composite Industry. Successful implementation of the process, especially for large structures such as boat hulls where large amounts of raw materials are involved, requires the completion of the impregnation phase in the shortest time possible. This can be achieved by the integration of a highly permeable layer to the reinforcement stack. However, this will strongly affect the compaction behavior of the resulted stack. This paper investigates the effect of the reinforcement structure on the overall compaction behavior. More specifically, the investigation is oriented toward the effect of the flow enhancement layer on the compaction response.