bluuebunny/crossref_metadata_2025 huggingface dataset:
Use of genetic algorithms to optimize gate and vent locations for the resin transfer molding process
Authors: Mathur Roopesh , Advani Suresh G. , Fink Bruce K.
Published: April 2004

Resin infusion processes are finding increasing applications in the manufacture of composite parts that have geometric and material complexities. In such cases, the placement of gates and vents is non‐intuitive and may require expensive repetitive experimentation. Finite element based resin flow simulation codes have been successfully used for modeling and analysis of the mold filling process. Such filling simulations, when coupled with a search algorithm, can also prove useful for optimal design of the filling process. Genetic algorithms (GAs) mimic natural selection and can efficiently “evolve” near‐global optimal solutions from a large number of alternative solutions. In this paper, GAs are used to optimize gate and vent locations for the resin transfer molding process in order to minimize fill times and dry spot formation. A process performance index or cost function is defined that incorporates the fill time and dry spot formation as primary variables. A part having material and geometric complexities was chosen for a case study. A genetic algorithm and mold filling simulations were used interactively to search for optimal gate and vent locations to locate near optimal solutions. The GA was able to find good solutions using less than 1% of simulations of the possible permutations of gates and vents. The case study was also repeated in the presence of recetracking channels. Again, the optimal locations were found by the GA using less than 1% of all possible combinations. Thus, GAs can be efficiently used for minimization of fill time and dry spot formation through optimal location of gates and vents in RTM processes. However, the optimal location will be a function of the cost function, the choice of which depends on the trade‐offs between different factors and the quality of the part desired.
The effects of preforming induced variable permeabilities on the rtm molding flow
Authors: Lai Chyi‐lang , Young Wen‐bin
Published: April 2004

Forming bidirectional woven fabric into complex shapes consisting of double curvature surfaces leads to rearrangement of the network structure of fiber mats. In this study, the geometric configuration of the fabrics after deformation is modeled using a modified pin‐jointed net model. Deformation of the fabric can result in the local change of the permeability in the reinforcements. In the flow simulation, the prediction of the filling flow highly depends on the correct values of permeabilities. The primary goal of this study is to investigate the effects on filling flow phenomena caused by the deformation of the fiber mats. A general integrated procedure of computer simulation with variable permeabilities for the filling process of arbitrary mold shape is also presented.
Uniaxial elongational viscosity of various molten polymer composites
Authors: Takahashi Tatsuhiro , Takimoto Jun‐ichi , Koyama Kiyohito
Published: April 2004

Uniaxial elongational viscosity of low density polyethylene (LDPE) that was filled with glass bead, glass flake, talc, or glass fiber was measured. The effect of various inorganic fillers on the strain‐hardening property in elongational viscosity was investigated. The strain‐hardening property of LDPE became weaker by the addition of fillers in the order of glass bead, glass flake, and talc. Glass fiber filled LDPE showed a strain‐softening property. The smaller the particle and the larger the aspect ratio, the weaker the strain‐hardening property. Their causes were analyzed from the two terms, i.e., the relaxation spectrum and the damping function, by Bernstein‐Kearsley‐Zapas (BKZ) model. By the incorporation of fillers, the relaxation modules became larger, and the damping function became stronger in the order of glass bead, glass flake, talc, and glass fiber. Recoverable strain was also measured to understand weaker strain‐hardening properties. The degree of recovery became smaller by the addition of fillers. It was found that the smaller the particles and the larger the aspect ratio, the smaller the degree of recovery. It was concluded that weaker strain‐hardening properties of LDPE composite than that of LDPE are caused by stronger damping function and smaller degree of recovery.
A model to predict the strength of short fiber composites
Authors: Van Hattum F. W. J. , Bernardo C. A.
Published: April 2004

The material properties of short fiber composites, such as modulus, strength and thermal expansion, are greatly dependent on the fiber orientation state in the composite. Although the orientation pattern is often quite complex, models that allow its prediction have been successfully applied in combination with micromechanical models to calculate moduli and thermal properties. This paper describes the derivation of a model to predict the strength of short fiber composites with arbitrary fiber orientation and fiber length distribution based on classical (micro‐) mechanical theories. The predictions are in good agreement with data measured on short fiber composites with different fiber contents.
Local buckling analysis of composite laminate with large delaminations induced by low velocity impact
Authors: Li Guoqiang , Pang Su‐seng , Zhao Yi , Ibekwe Samuel I.
Published: April 2004

Large delaminations can be induced by a low velocity impact on a laminated composite. The local buckling behavior of a laminated composite containing large delaminations is not as well understood as that with small delaminations. This paper presents analytical modeling of the local buckling behavior of a laminated composite containing a large elliptic delamination embedded near the surface and rested partially on a Winkler foundation. A formula for estimating the elastic modulus of the Winkler foundation is also proposed based on an elastic model. It is shown that the buckled shape predicted by the proposed model is similar to the experimentally observed shape. A small portion of the delaminated sub‐laminate between the delamination tip and the inward portion is found to show slight deflections outward. The first buckling mode takes place at higher mode shapes with a more flexible sub‐laminate, larger delamination, and a stiffer Winkler foundation. Increasing the transverse elastic modulus of the laminate within a certain extent, decreasing the size of the delamination, and enhancing the stiffness of the delaminated sub‐laminate are effective in increasing the local buckling resistance.
Electrically and thermally conductive nylon 6,6
Authors: King Julia A. , Tucker Kenneth W. , Vogt Bryan D. , Weber Erik H. , Quan Congling
Published: April 2004

Increasing the thermal and electrical conductivity of typically insulating polymers, such as nylon 6,6, opens new markets. A thermally conductive resin can be used for heat sink applications. An electrically conductive resin can be used in static dissipative and Electromagnetic Interference/Radio Frequency Interference shielding applications. This research focused on adding various carbon based conductive fillers and a chopped glass fiber to nylon 6,6. These materials were extruded and injection molded into test specimens. Tensile tests as well as in‐plane electrical resistivity, in‐plane thermal conductivity, and through‐plane thermal conductivity tests were conducted. One successful formulation consisted of 10% 3.2 mm chopped E‐glass fiber/15% Thermocarb (high quality synthetic powdered graphite)/5% carbon black/70% nylon 6,6 (all % in wt%). For this formulation, the in‐plane electrical resistivity was reduced from 1015 ohm‐cm (neat nylon 6,6) to 15 ohm‐cm. The through‐plane thermal conductivity increased from 0.25 W/mK (neat nylon 6,6) to 0.7 W/mK. The tensile elongation at failure was 1.4%.
Fracture toughness of fiber‐polymer interfaces estimated from single fiber peel tests
Authors: Alimuddin M. A. , Piggott M. R.
Published: April 2004

Single fibers of carbon, glass, and DuPont Kevlar have been embedded to about half their diameters in an epoxy resin and then peeled off. The work of fracture GI can be estimated from the peel force by a simple formula, so long as the fibers are fully elastic during the peel process. This was true with glass, which had GI = 140 Jm−2 and carbon with GI = 60 Jm−2. With Kevlar, inelastic effects obscured the results. The effect of water on the glass epoxy interface was readily shown. Carbon, however, required high temperature water at the interface to cause significant losses. Strong acids at 25°C had about the same effect as water at 90°C. Organic fluids such as kerosene had very little effect. Similar environmental effects could also be detected with Kevlar, though further work is required here to separate out the inelastic contribution.
Fluid flow reconstruction modeling with application to liquid molding processes
Authors: Walsh Shawn M. , Freese Colin E.
Published: April 2004

A unique mathematical reconstruction technique is coupled with discrete experimental flow front detection data to provide continuous fluid flow analysis. A fully finite element based methodology for the construction of surfaces from scattered and sparse data is presented and applied to experimental observations derived from a new class of sensors enlisted as monitors of the resin transfer molding (RTM) process. The basic numerical technique is shown to yield interpolant models capable of flow front position and fluid flow velocity reconstruction. The extrapolative/predictive performance is enhanced through the introduction of a new hybrid procedure. Two‐dimensional and quasi‐three‐dimensional examples are used to demonstrate the effectiveness of the techniques.
A new fabrication technique utilizing a composite material applied to orthopedic bracing
Authors: Morrison Bryan J. , Creasy Terry S. , Polliack Adrian A. , Fite Richard
Published: April 2004

An orthopedic brace fabrication and fitting technique was explored as a rapid and cost‐effective alternative process to polypropylene fabrication. Other potential benefits could also be reduced weight and lower profile with the added advantage of allowing profile and firmness variation where desired. The novel process concept is based on the finding that a partially cured composite remains somewhat malleable. Clinicians conceivably can adjust an ankle‐foot brace to the patient directly before fully curing the brace. Flexure experiments assessed both the partial curing concept and the ability to post‐form. Results indicate that partial curing followed by ambient or heated adjustments and full curing compromises the strength by no more than 7%. However, adjustments resulted in a net displacement change averaging only 50% of that desired when heat was applied. It is also possible to post‐form the fully cured brace. This compromises the material strength significantly in the deformed area, but it may be feasible for minor adjustments where applied loads are not extreme. This research demonstrates the feasibility of the process as an alternative to current techniques, especially for patients needing a strong brace.
Heat transfer for pultrusion of a modified acrylic/glass reinforced composite
Authors: Sarrionandia Mariasun , Mondragón Iñaaki , Moschiar Stella Maris , Reboredo María Marta , Vázquez Analía
Published: April 2004

Experimental values of the temperature on the wall and into the die were obtained for the pultrusion of a modified acrylic resin. The equation of continuity, and energy balance, coupled with a kinetic expression for the curing system, are solved using difference method to calculate the temperature and the conversion profiles in the thickness direction in a rectangular pultrusion die. The effects of the process variables (e.g. pulling rate, die temperature, die thickness and content of fibers) on the performance of the pultrusion are evaluated.
Investigation of interfacial structure of couping agent treated fillers by fourier transform infrared spectroscopy and attenuated total reflection‐FTIR spectroscopy
Authors: Liu Yujuan , Yang Rui , Yu Jian , Wang Kunhua
Published: April 2004

The interfacial structures of fillers treated by coupling agents were studied with FTIR and a Attenuated Total Reflection (ATR)‐FTIR spectrometer. The interfacial interaction was investigated by FTIR, and the nature of interaction was proved to be a chemical bonding, which could not be destroyed by extraction. Using a variable angle ATR technique, the average interfacial layer thickness of filler treated with different amounts of coupling agent was determined. A calibration curve was obtained from these studies to determine the optimal amount of coupling agent for different fillers, which was in agreement with the results of mechanical tests. ATR‐FTIR technique is a convenient method to determine the proper amount of coupling agent in composites.
Effects of processing conditions on the fiber length distribution and mechanical properties of glass fiber reinforced nylon‐6
Authors: Yilmazer Ulku , Cansever Murat
Published: April 2004

The effects of processing conditions on fiber length degradation were investigated in order to produce composites with higher performance. Nylon‐6 was compounded with glass fibers in a twin‐screw extruder for various combinations of screw speed and feed rate. Collected samples were injection molded and Izod impact and tensile tests were performed in order to observe the effect of fiber length on the mechanical properties. Also, by using the extruded and injection molded smaples, fiber length distribution curves were obtained for all the experimental runs. Results show that when the shear rate is increased through the alteration of the screw speed and/or the feed rate, the average fiber length decreases. Impact strength, tensile modulus and tensile strength increase, whereas elongation at break decreases with the average fiber length.
Elastic moduli of particulate composites with graded filler‐matrix interfaces
Authors: Ozmusul M. S. , Picu R. C.
Published: April 2004

The elastic response of plane‐array models of composites reinforced by particles or aligned fibers having graded interfaces with the matrix is analyzed. Such microstructure is representative of a new class of polymer matrix composite materials in which the filler is nanometer‐sized. In such materials, the polymer chains in the matrix are preferentially oriented close to the interface with the relatively rigid fillers, this leading to a graded interfacial layer about each inclusion. The composite elastic moduli are determined based on the properties and geometry of the interfacial graded layer as well as on the moduli of the filler and the matrix, and the volume fraction of filler. Conversion curves are constructed allowing for an equivalence to be established between the present case and that of similar composites without graded interfaces. Based on these conversion curves, standard homogenization algorithms can be applied to determine the overall elastic properties of such composite. The fillers are considered to be stiffer than the matrix, both rigid and of finite stiffness. Results for both sliding and bonded interfaces are presented. The effect of anisotropic material properties in the graded region on the composite moduli is also investigated. The results of the model are compared with published experimental data.
Resin transfer molding of natural fiber reinforced polybenzoxazine composities
Authors: Dansiri Nadsuda , Yanumet Nantaya , Ellis John W. , Ishida Hatsuo
Published: April 2004

Kenaf fiber is incorporated in a polybenzoxazine (PBZX) resin matrix to form a unidirectionally reinforced composite containing 20 wt% fiber by a resin transfer molding technique. Two types of benzoxazine monomer are synthesized and used as resin mixtures: Benzozazines based on bisphenol‐A/aniline (BA‐a) and phenol/aniline (Ph‐a). The effects of varying BA‐a:Ph‐a ratio in the resin mixture and curing conditions on mechanical properties of pure PBZX resin and kenaf/PBZX composites are studies. The Flexural strength of the pure PBZX resin increases with increasing ratio of BA‐a:Ph‐a, curing temperature and curing time, but the impact strength increases only slightly. PBZX resin has lower water absorption and higher flexural modulus, when compared with unsaturated polyester (UPE) resin. PBZX composites with 20 wt% fiber content have lower flexural and impact strengths, but higher moduli compared with UPE composites with the same fiber content.
Mechanical damage and strain in carbon fiber thermoplastic‐matrix composite, sensed by electrical resistivity measurement
Authors: Mei Zhen , Guerrero Victor H. , Kowalik Daniel P. , Chung D. D. L.
Published: April 2004

The volume electrical resistivity of a unidirectional continuous carbon fiber thermoplastic (nylon‐6) matrix composite was found to be an indicator of strain and damage during repeated loading in the fiber direction. The through‐thickness resistivity irreversibly and gradually decreased upon damage (probably fiber‐matrix debonding) during repeated compression or tension. Moreover, it reversibly and abruptly increased upon matrix damage, which occurred reversibly near the peak stress of a stress cycle. In addition, the resistivity increased reversibly upon tension in every stress cycle, and decreased reversibly upon compression in every stress cycle. On the other hand, the longitudinal resistivity irreversibly and gradually increased upon damage. Moreover, it decreased reversibly upon tension in every stress cycle and increased reversibly upon compression in every stress cycle. The through‐thickness resistivity was a better indicator of damage and strain than the longitudinal resistivity.
Optimal design of multi‐direction composite flywheel rotors
Authors: Gowayed Yasser , Abel‐Hady Faissal , Flowers George T. , Trudell Jeffrey J.
Published: April 2004

Design of composite flywheels made with reinforcement in the hoop and radial directions (Multiple Direction Composites MDC) involves a large multitude of parameters such as those related to the flywheel operation, flywheel geometry, material characteristics, material layup, and stress spatial distribution and values. The problem is further complicated because most design parameters are interactive and because one parameter cannot be optimized independent of the other parameters. Optimum structural design of MDC flywheels using nonlinear optimization via a sequential quadratic programming approach and parametric finite element analysis are used to address this problem. Numerical examples are solved to compare the efficacy of the two approaches.
Reverse piezoelectric behavior of carbon fiber thermoplastic‐matrix composite
Authors: Mei Zhen , Guerrero Victor H. , Kowalik Daniel P. , Chung D. D. L.
Published: April 2004

The reverse piezoelectric effect was observed in the through‐thickness direction of a continuous carbon fiber nylon‐6 matrix composite. The piezoelectric coupling coefficient was 2.2 × 10−6 m/V, as determined up to an electric field of 261 V/m. The effect was largely reversible upon turning off the field and was unaffected by field reversal. It is attributed to the electric dipoles in the nylon‐6 matrix.
Production of leather‐like composites using short leather fibers. II. Mechanical characterization
Authors: Madera‐Santana T. J. , Aguilar‐Vega M. J. , Márquez A. , Vázquez Moreno F. , Richardson M. O. W. , Cruz Machin J. L.
Published: April 2004

Leather‐like composites were prepared by addition of chemically modified short leather fibers (SLF) into a plasticized polyvinyl chloride (pPVC) matrix. The fibers were subjected to chemical modification by emulsion polymerization to achieve good interfacial adhesion between SLF and the pPVC matrix. The SLF with chemical modification were obtained from three different reaction conditions where these SLF have different percentages of grafted and deposited PMMA polymer onto the fiber surface. The incorporation of the SLF into the thermoplastic matrix was carried out using a torque‐rheometer and the composites obtained were molded by compression. Tensile and tear mechanical tests were performed on composite samples, and the morphology of the fractured surfaces was analyzed using scanning electron microscopy (SEM). The results show that the incorporation by grafting of polymethyl metacrylate (PMMA) onto the fibers produced a significant improvement of their interfacial adhesion to pPVC, promoting the compatibilization between the fiber surface and matrix. The findings are discussed and interpreted in terms of enhanced adhesion at phase boundaries. Overall, the results confirm that it is possible to produce modified leather composites based on a pPVC matrix, which exhibit relatively high tensile strength, tear resistance and flexibility. These composites are very suitable candidate materials for applications in the footwear industry.
Evaluation of the formability of plastic/Zn22Al/plastic sandwiched structures by gas blowing
Authors: Tsao L. C. , Su T. L. , Chuang T. H.
Published: April 2004

For the manufacturing of electromagnetic interference (EMI) shielding enclosures for portable 3C (computer, communication, and consumer electronics) products, a superplastic Zn22Al thin sheet is inserted in between plastic plates and formed by in‐mold blowing with gas pressure. In order to evaluate the formability of a plastic/Zn22Al/plastic sandwiched structure, free bulging tests are conducted in the temperature range between 135°C and 200°C in an infrared furnace. The results show that the ABS/Zn22Al/ABS sandwiched structure can be free bulged at 150°C with 0.21 MPa of gas pressure at a forming rate similar to that of the monolithic Zn22Al sheet. The PC/Zn22Al/PC sandwiched structure cannot be effectively bulged until the forming temperature is raised to 185°C. The optimized forming condition for PC/Zn22Al/ABS is either at 150°C with a gas pressure of 0.35 MPa or at 165°C and a gas pressure setting of 0.21 MPa. Also, the thickness distributions for Zn22Al and a variety of plastic/Zn22Al/plastic specimens after free bulging at various temperatures with various gas pressures are placed in comparison in this study.
Separation of gelation from vitrification in curing of a fiber‐reinforced epoxy composite
Authors: Bilyeu Bryan , Brostow Witold , Menard Kevin P.
Published: April 2004

Prepregs of a mixture of the tetrafunctional epoxy tetraglycidyl 4,4‐diaminodiphenyl methane (TGDDM) and the tetrafunctional amine 4,4'‐diaminodiphenylsulfone (DDS) were characterized with temperature‐modulated DSC (TMDSC) as well as dynamic mechanical analysis (DMA). The baseline shift of the glass transition was separated from the curing exotherm by using temperature‐modulated and step scan DSC temperature scans. Likewise, the baseline shift in heat capacity due to vitrification was isolated using TMDSC isotherms. Using the TMDSC glass transition temperature, degree of conversion, and vitrification results, combined with the gelation data generated from DMA, a time‐temperature‐transformation (TTT) diagram was constructed, providing information necessary for optimization of industrial processing of the epoxy prepreg. Thus, effects of storage, preprocessing, and postprocessing on the overall curing process are taken into account.
Consolidation of carbon fiber laminae during polymer‐matrix composite fabrication, studied by electrical resistance measurement
Authors: Wang Shoukai , Chung D. D. L.
Published: April 2004

The consolidation of carbon fiber epoxy‐matrix laminae during composite fabrication by lamination (involving consolidation, curing and cooling) was found to hastened and to occur to a greater extent by increasing the pressure. The consequence of better consolidation remained after curing and subsequent cooling. A higher pressure resulted in a lower throughthickness resistivity in a composite after curing and cooling. The extent of consolidation was quantitatively described by N/No (the ratio of the number of fiber‐fiber contacts between adjacent laminae at a given time, divided by that at the start of consolidation), which was calculated from the measured through‐thickness electrical resistivity. From the variation of N/No during heating in the process of consolidation, three stages of consolidation were observed. In the first stage, N/No increased very gradually; in the second stage, N/No increased abruptly; in the third stage, N/No increased at a moderate rate.
Relationship between processing and properties of biodegradable composites based on PCL/starch matrix and sisal fibers
Authors: Cyras Viviana P. , Iannace Salvatore , Kenny José M. , Vázquez Analía
Published: April 2004

Biocomposites were produced using polycaprolactone and starch as matrix, and sisal fibers as reinforcement. The matrix is a biodegradable commercial product called MaterBi‐Z, which is based on a polycaprolactone and starch system. The relationship between processing conditions and properties is reported. An alkaline treatment was performed in order to improve the adhesion and the compatibility of the fiber with the matrix. The effect of the treatment on the tensile properties and morphology was determined. Fiber content enhances the tensile properties of the biodegradable composite. The experimentally observed tensile properties (modulus and tensile strength) of short sisal fiber reinforced MaterBi‐Z matrix composites with different fiber loading are compared with the existing theories of reinforcement. SEM photomicrographs of the fractured composite surfaces are also analyzed.
Temperature and loading rate effects in the mode II interlaminar fracture behavior of carbon fiber reinforced PEEK
Authors: Berger L. , Cantwell W. J.
Published: April 2004

The combined effect of varying loading rate and test temperature on the mode II interlaminar fracture properties of AS4/carbon fiber reinforced PEEK has been investigated. End notch flexure tests have shown that this thermoplastic‐based composite system offers a very high value of interlaminar fracture toughness at room temperature. Increasing the test temperature leads to a reduction in the mode II interlaminar fracture toughness of the composite, with the value at 150°C being approximately one half of the room temperature value. In contrast, increasing the crosshead displacement rate has been shown to increase the value of GIIcby up to 25%. A more detailed understanding of the effect of varying temperature and loading rate on the failure mechanisms occurring at the crack tip of these interlaminar fracture specimens has been achieved using the double end notch flexure (DENF) geometry. Here, extensive plastic flow within the crack tip region was observed in all specimens. It is believed that the rate sensitivity of GIIcreflects the rate‐dependent characteristics of the thermoplastic resin.
The packing stress of impregnated fiber mats
Authors: Servais Colin , Michaud Véronique , Månson Jan‐Anders E.
Published: April 2004

The parameters affecting the packing stress of fiber mats for melt impregnation, resin transfer molding and compression molding are systematically investigated. An analytical equation based on local bending of fibers, which was previously derived for non‐impregnated networks, is applied to composite mats of dispersed planar bundles impregnated with molten polypropylene. It is shown that many simultaneous mechanisms interact during packing of impregnated bundle mats, in particular when the mats are needled. These include in‐plane bending of the bundles, compaction of the fibers within a bundle, and buckling, slippage or breakage of the out‐of‐plane fibers. In order to identify and decouple these features, aspect ratio of the bundles, lubrication, needling intensity and packing history are varied. A microstructural experiment is also developed to evaluate the extent of bundle spreading. It is found that dispersed fibers or bundles roughly follow the equation based on local bending, but that needled bundle networks deviate from the power law behavior. Three regions were identified. The first is attributed to self‐loading of the mats and to buckling of the out‐of plane fibers. The second region is due to slippage and breakage of the out‐of‐plane fibers and depends on the loading history and on the needling intensity. The third region is due to packing of the in‐plane bundles, which do not really bend, or spread under load, but are locally compressible, owing to misalignment and waviness of the individual fibers forming the bundles. In compression molding, the influence of the in‐plane reorientation of the initially out‐of‐plane bundles on the packing stress is observed.
In situ elevated‐temperature drift spectroscopyof γ‐aminopropyltriethoxysilane as a condensed phase and a coated film on silica surface
Authors: Noobut W. , Koenig J. L.
Published: April 2004

It is the purpose of this study to determine the structure of γ‐amlnopropyltriethoxysilane (APS) on the surface of silica. Diffuse Reflectance spectroscopy (DRIFT) was employed to study the molecular structure of the partially condensed (APS) and Al's‐coated silica at 90°C. The condensed APS and Al's‐coated silica were prepared at the natural pH (10.3) of the APS and in the ambient environment. At room temperature, amino groups of the condensed APS and the APS‐coated silica form the amine carbonate and amine bicarbonate salts. Upon heat treatment, the amino salt structures are eliminated, leaving the amino groups in non‐bonded form.
Flow front measurements and model validation in the vacuum assisted resin transfer molding process
Authors: Mathuw R. , Advani S. G. , Heider D. , Hoffmann C. , Gillespie J. W. , Fink B. K.
Published: April 2004

Through‐thickness measurements were recorded to experimentally investigate the through thickness flow and to validate a closed form solution of the resin flow during the vacuum assisted resin transfer molding process (VARFM). During the VART'M process, a highly permeable distribution medium is incorporated into the preform as a surface layer and resin is inftised Into the mold, under vacuum. During Infusion, the resin flaws preferentially across the surface and simultaneously through the thickness of the preform, giving rise to a three dimensional‐flow front. The time to fill the mold and the shape of the flow front, which plays a key role in dry spot formation, are critical for the optimal manufacture of large composite parts. An analytical model predicts the flow times and flow front shapes as a function of the properties of the preform, distribution media and resin. It was found that the flow front profile reaches a parabolic steady state shape and the length of the region saturated by resin is proportional to the square root of the time elapsed. Experimental measurements of the flow front in the process were carried out using embedded sensors to detect the flow of resin through the thickness of the preform layer and the progression of flow along the length of the part. The time to fill the part, the length of flow front and its shapes show good agreement between experiments and the analytical model. The experimental study demonstrates the need for control and optimization of resin injection during the manufacture of large parts by VARTM.
Prediction of failure and fracture mechanisms of polymeric composites using finite element analysis. Part 1: Particulate filled composites
Authors: Ghassemieh E. , Naseehi V.
Published: April 2004

Micro‐mechanical analysis of polymeric composites proves a powerful means for the quantitative assessment of their bulk behavior. In this paper we describe a robust finite element model (FEM) for the micro‐structural modeling of the behavior of particulate filled polymer composites under external loads. The developed model is applied to simulate stress distribution in polymer composites containing particulate fillers. Quantitative information about the magnitude and location of maximum stress concentrations obtained from these simulations is used to predict the dominant failure and crack growth mechanisms in these composites. The model predictions are compared with the available experimental data and also with the values found using other methods reported in the literature. These comparisons show the range of the validity of the developed model and its predictive potential.
Analytical modeling of tensile strength of particulate‐filled composites
Authors: Li Guoqiang , Helms Jack E. , Pang Su‐Seng , Schulz Kurt
Published: April 2004

Based on Christen sen and Lo's (11) three‐layer sphere model, a two‐layer built‐in model is proposed to represent particulate‐filled composites. Following Papanico‐laou and Bakos' (14) procedure for a particle embedded in an infinite matrix model and using the rule‐of‐mixtures approach, formulations estimating the tensile strength of particulate‐filled composites are developed. Unlike Fapanicolaou and Bakos' formulations, the formulations developed in the present paper can characterize the effect of particle size, particle size distribution, and particle clustering on the tensile strength of the composites. A reasonable agreement is found between the predicted tensile strength and the experimental results found in the literature. Parameters affecting the tensile strength of particulate‐filled composites are discussed via the calculated results.
Analysis of heat transfer in autoclave technology
Authors: Antonucci Vincenza , Giordano Michele , Inserraimparato Sabato , Nicolais Luigi
Published: April 2004

In autoclave technology, polymer based composites are manufactured under the application of pressure and heat. The heat transferred between the energy carrying fluid and the bag‐composite‐tool element activates exothermic curing reactions, leading to composite consolidation. The convective heat transfer mechanism is the most relevant aspect controlling the rate of chemical and physical transformations associated with composite curing. Moreover, the fluidodynamic regime that results from the interactions between the autoclave and the tool geometry, even if totally predictable in theory, is unattainable in practice. In this study, the heat transfer phenomena occurring during the autoclave manufacturing cycle have been analyzed. The assumption of a negligible through‐the‐thickness thermal gradient led to simplified energy balance equations. In this case, the thermal evolution of the manufacturing elements has been completely determined by two parameters: the global convective heat exchange coefficient, setting the rate of the heat transfer between the autoclave environment and the bag‐composite‐tool element, and the adiabatic temperature rise, establishing the relevance of the polymerization exotherm. A scaling analysis has been performed in order to identify the dimensionless parameters controlling the autoclave process. The developed semitheoretical methodology has been extensively tested by comparison with experimental data from an industrial autoclave.
A kinetic model for free‐radical crosslinking co‐polymerization of styrene/vinylester resin
Authors: Yang Huan , Lee L. James
Published: April 2004

Vinylester resin is a major thermoset polymer used In low temperature composite manufacturing processes, such as the Seemann Composite Resin Infusion Molding Process (SCRIMP). In this study, the effect of temperature on the cure kinetics of vinylester resin in the range of 35 to 90°C was investigated using a differential scanning calorimeter (DSC) and a Fourier transform infrared spectrometer (FTIR). A mechanistic kinetic model was developed to simulate the reaction rate and conversion profiles of vinylester vinylene and styrene vinyl groups, as well as the total reaction action rate and conversion. Experimental results from DSC and FUR at different temperatures were compared with model predictions. The glass transition temperature of the vinylester resin cured at different temperatures was identified and used to monitor the final conversion change. A series of SCRIMP molding experiments were conducted. The developed kinetic model in conjunction with a heat transfer model was used to simulate the temperature and conversion changes inside the SCRIMP‐molded composites.
Vacuum‐assisted resin transfer molding using tackified fiber preforms
Authors: Shih Chih‐Hsin , Liu Qingfang , Lee L. James
Published: April 2004

A low cost composite fabrication process—tackified SCRIMP—is described for fabricating aerospace‐grade composites based on tackification and vacuum‐assisted resin transfer molding (VARTM). Tackification based on a commercial tackifier (FT 500 from 3M) was used to make the net‐shape fiber preform. It was found that tackifier concentration and application conditions play important roles in governing the moldability of tackified fiber preforms. An epoxy resin (PR 500 from 3M) was used in the VARTM process‐SCRIMP at high temperatures. Experimental results show that composites with high fiber content (> 60% by volume) can be manufactured at low cost using tackification. Effects of tackification methods on composite dimension control, void content and mechanical properties were investigated and compared in both RTM and SCRIMP.
Novel eco‐friendly biodegradable coir‐polyester amide biocomposites: Fabrication and properties evaluation
Authors: Rout J. , Misra M. , Tripathy S. S. , Nayak S. K. , Mohanty A. K.
Published: April 2004

Biocomposites are prepared from a cheap, renewable natural fiber, coir (coconut fiber) as reinforcement with a biodegradable polyester amide (BAK 1095) matrix. In order to have better fiber‐matrix interaction the fibers are surface modified through alkali treatment, cyanoethylation, bleaching and vinyl grafting. The effects of different fiber surface treatments and fiber amounts on the performance of resulting bio‐composites are investigated. Among all modifications, cyanoethylated coir‐BAK composites show better tensile strength (35.50 MPa) whereas 7% methyl methacrylate grafted coir‐BAK composites show significant improvement in flexural strength (87.36 MPa). The remarkable achievement of the present investigation is that a low strength coir fiber, through optimal surface modifications, on reinforcement with BAK show an encouraging level of mechanical properties. Moreover, the elongation at break of BAK polymer is considerably reduced by the incorporation of coir fibers from nearly 400% (percent elongation of pure BAK) to 16‐24% (coir‐BAK biocomposites). SEM investigations show that surface modifications improve the fiber‐matrix adhesion. From biodegradation studies we find that after 52 days of soil burial, alkali treated and bleached coir‐BAK composites show significant weight loss. More than 70% decrease in flexural strength is observed for alkali treated coir‐BAK composites after 35 days of soil burial. The loss of weight and the decrease of flexural strength of degraded composites are more or less directly related.
Measurement of in‐plane permeability of anisotropic fiber reinforcements
Authors: Gebart B. Rikard , Lidström Peder
Published: April 2004

Three of the most common methods (two with parallel flow and one with radial flow) for determination of the in‐plane permeability tensor are studied both theoretically and experimentally. An error analysis shows that the difference between the methods is negligible if the error levels are equal. However, the radial flow method is found to be susceptible to large errors from mold deflection in an experimental comparison between the methods. Additional experiments with the radial flow method in a stiffer mold show that the method gives the same values for the permeability tensor as the other two methods. A new method with multiple cavities in parallel is proposed that combines the simplicity of the radial flow method with the stiff mold of the parallel flow method. Only mass and time need to be measured in one experiment and it eliminates the need to measure fluid viscosity, temperature, and injection pressure. The method depends on the availability of a reference material with known permeability.
Polymer encapsulation of glass filler in ternary PP/PA‐6/glass blends
Authors: Benderly D. , Siegmann A. , Narkis M.
Published: April 2004

The morphology and rheology of ternary isotactic polypropylene (PP)/polyamide‐6 (PA‐6)/glass blends is investigated and contrasted with the behavior of two‐component (binary) PP/PA‐6 blends. Injection molded samples of binary blends exhibit an interlayer slip morphology for both PP and PA‐6 as the matrix and the blend shear viscosity is lower than expected from a rule of mixtures. The morphology of ternary blends is dependent on the choice of the matrix phase. In ternary blends with a PA‐6 matrix, the PP domains and glass fibers are separately dispersed within the matrix. In ternary blends with a PP matrix, the PA‐6 is mainly found surrounding (encapsulating) the glass fibers, and the extent of the interlayer slip morphology is reduced. Variations in glass surface treatment, blending time, and order of addition did not affect the rapid encapsulation of glass by the PA‐6. A reduction in blending temperature, below the peak PA‐6 melting temperature, hinders encapsulation.
Improvement of adhesion between polyethylene and regenerated cellulose fibers by surface fibrillation
Authors: Karlsson J. O. , Gatenholm P. , Blachot J.‐F. , Peguy A.
Published: April 2004

Regenerated cellulose fibers spun from straw pulp using the N‐methylmorpholine N‐oxide (NMMO) process were evaluated as a reinforcement for low‐density polyethylene (LDPE). Surface fibrillation was carried out by a mechanical treatment to improve interfacial adhesion. Surface fibrillation resulted in a gradual change in surface topography, as detected by SEM. Long and numerous twisted fibrils were observed on the surface of the treated fibers. The fiber perimeters, determined by the Wilhelmy plate method, increased with an extended degree of fibrillation, while the strength of the fiber was not affected by the surface treatment. Model composites were prepared by embedding untreated and surface‐fibrillated single fibers into an LDPE matrix, and the single fiber fragmentation (SEF) test was carried out to determine the critical fiber length. The interfacial shear strength (τ) was then calculated by applying a modified form of the Kelly‐Tyson equation. It was found that the interfacial shear strength increased significantly as a result of surface fibrillation. The proposed mechanism for the improvement of interfacial adhesion is a mechanical anchoring between the matrix and the fiber.
On the effects of modifiers on the viscoelastic behavior of composites
Authors: Tsotsis Thomas K.
Published: April 2004

Some of the details concerning the reasons for the difference in behavior between untoughened and toughened composites are presented. This work demonstrates the influence of localized plastic deformation on the viscoelastic behavior of polymer composites. As an example, the presence of a thermoplastic modifier and its potential influence on the microscale deformation is presented by using two micromechanical models to determine the conditions under which the observed behavior may occur. Results suggest that the effect of thermoplastic tougheners is to increase the amount of time‐dependent plastic deformation in loadings that produce sufficiently large local stress concentrations. The tougheners, however, do not seem to change the functional form of the time‐dependent response, rather they change the magnitude of this response. The two models discussed are used to qualitatively characterize these phenomena by relating model parameters to constituent properties. Differences in observed behavior with respect to plastic deformation in creep tests vs. tests such as dynamic mechanical analysis are attributed to the small deformations in the latter test types, which do create insufficient localized zones of plastic deformation.
Effects of hygrothermal exposure on the mechanical properties of injection molded glass‐fiber reinforced poly(phenylene sulfide)
Authors: Munson‐McGee Stuart H.
Published: April 2004

An experimental program was used as the foundation for long‐term property prediction for a glass‐fiber reinforced poly(phenylene sulfide). Samples were aged up to 226 days at 95% RH at both 104°F (40°C) and 158°F (70°C) (based on MIL‐STD‐810E and MIL‐STD‐210C), and their mechanical properties measured. Data analysis indicated that property changes were more pronounced at the higher temperature with statistically significant changes, at the 99% confidence level, occurring after 100 days. At the lower temperature, property changes were observed, but their significance was not established. Further analysis of the data indicated that the moisture content did not affect the property changes. Analysis of the data using a viscoelastic description of the property changes resulted in property maps that permit estimation of the mechanical properties for up to a 20‐year exposure as a function of exposure temperature.
Short term isothermal aging of epoxy resin and epoxy‐carbon fiber composites
Authors: Chifu A. F. , Iroh J. O.
Published: April 2004

Short term isothermal aging of a neat epoxy resin and one ply epoxy‐carbon fiber composite has been performed. The glass transition temperature, Tg of the neat epoxy resin aged at 204°C increased with aging time. The weight loss of the neat epoxy resin due to aging increased with aging temperature and aging time. The weight loss of the epoxy–carbon fiber composite during aging was slightly less than that of the neat resin. SEM microscopy showed the presence of voids and pores on the surface of the composite due to loss of low molecular weight volatiles. The amount and the size of the voids formed during aging increased with aging temperature and time.
Anisotropic degradation of polymeric composites: From neat resin to composite
Authors: Salin Irene , Seferis James C.
Published: April 2004

The degradation of polymer matrix composites was investigated through weight loss measurements. A methodology originally developed based on the shrinking core model was extended to account for matrix orientation, lay‐up sequence, and differences in neat resin and composite degradation. This methodology was derived based on mass transfer and kinetic considerations taking into account the inherent anisotropy and heterogeneity of polymer composites. The model systems used were a bismaleimide (X5260) and a condensation polyimide (Avimid N). Bismaleimide composites were aged at 250, 260, and 270°C in air, whereas both neat resin and composite Avimid N samples were aged at 371°C. Based on the experimentally observed time exponents, the degradation was modeled as diffusion and reaction controlled. The weight losses of both unidirectional and quasi‐isotropic samples were successfully predicted. In addition, correlation between the degradation of neat resin and composite samples was established for the Avimid N system. Finally, this methodology can be used in scaling up weight loss data, providing an important tool in evaluating materials for long‐term use at elevated temperatures.
Braided thermoplastic composites from powder‐coated towpregs. Part III: Consolidation and mechanical properties
Authors: Ramasamy A. , Wang Youjiang , Muzzy John
Published: April 2004

Carbon fiber/nylon 6 powder coated and commingled tows were braided into biaxial braids with a ±45° braid angle. Consolidation experiments were carried out to study the influence of consolidation conditions on the properties of the laminates using a 23 full factorial experimental design. The consolidation studies reveal that commingled tows require more severe consolidation conditions than powder coated tows to obtain comparable properties. The dependence of powder‐coated laminate qualities like void content and fiber volume fraction on the consolidation conditions is reported. The ±45° braided laminates are tested for tensile properties. Although the dominant mode of failure of the laminates is shear failure between the fibers and the resin, the failure is non‐brittle because of fiber interlacement in the braided preforms.