bluuebunny/crossref_metadata_2025 huggingface dataset:
Structure and properties of amorphous polyamide/liquid crystalline polyester blends
Authors: García M. , Eguizábal J. I. , Nazábal J.
Published: April 2004

Amorphous polyamide (AP)/liquid crystalline polyester (VA) blends were obtained by extrusion‐injection molding (EI) throughout the whole composition range. The phase behavior, chemical nature and morphology of the blends were studied, and the mechanical properties discussed and compared with those of the 10 and 30% VA blends obtained by direct injection molding (DI). The blends showed two almost pure slightly reacted amorphous phases. The apparently higher reaction level of the EI blends, although small, led to a more homogeneous, fine and fibrillated morphology, attributed to a lower interfacial tension. Significant synergisms in the modulus of elasticity (up to 25%) and in the tensile strength (up to 40%) were seen in EI blends. The similar values of both specific volume and orientation in the blends and in the pure components suggest that the contribution to the modulus of the dispersed VA rigid particles is greater than that due to the proportion of VA in the blend. The 10% VA DI blend showed a similar behavior in these two properties, indicating that the DI procedure is preferred, provided that only stress‐related properties are sought. At 30% VA content, the moduli of elasticity were similar by the two molding processes, but the clearly lower tensile strength and lower ductility of the easier DI procedure, means that the more complex, but more effective, EI procedure is the one of choice for high performance materials.
Mechanical property improvement of carbon fiber reinforced polybenzoxazine by rubber interlayer
Authors: Ishida Hatsuo , Chaisuwan Thanyalak
Published: April 2004

The rubber interlayer method was chosen in order to improve the properties of carbon fiber‐reinforced polybenzoxazine composites. The resin used is benzoxazine based on bisphenol‐A, formaldehyde and 3,5‐xylidine. The effect of rubber concentration on the flexural properties of the composites is investigated. Sized and desized carbon fiber woven fabrics are used to study the effect of the sizing materials on the mechanical properties. The delamination toughness of the composites is increased by the ATBN rubber interlayer with increasing ATBN concentration. The strength of the composite also increased, but an anomalous concentration effect has been observed.
Predicting molding forces during sheet molding compounds (SMC) compression molding. II: Effect of SMC composition
Authors: Boylan Sarah , Abrams Lisa M. , Castro Jose M.
Published: April 2004

One of the fastest‐growing applications of SMC compression molding is the manufacture of truck body panels. Because of their large size, the molding forces required are substantial and have a major influence on the molding cycle. Also, as SMC moves towards parts requiring higher strength, the fiber length and percentage by weight of fibers must increase. This will also contribute to larger molding forces. In this paper, a procedure is presented to evaluate the SMC rheological parameters needed to predict molding forces. In addition, the effect of SMC composition on the molding forces is investigated. In particular, we evaluate the effect of reinforcement type (glass versus carbon) and level, filler level and thickener level. It was found that the factors most affecting molding forces are the reinforcement length and level; and the filler level. In addition, it was discovered that for SMC thickened with magnesium oxide, the level of thickener does not affect the molding force.
Injection molding of glass fiber reinforced phenolic composites. 2: Study of the injection molding process
Authors: Singh R. , Chen F. , Jones F. R.
Published: April 2004

This study of injection molding of glass fiber reinforced phenolic molding compounds examines fiber breakage and fiber orientation with key material and processing variables, such as injection speed, fiber volume fraction, and the extent of resin pre‐cure. The fiber orientation, forming discrete skin‐core arrangements, is related to the divergent gate to mold geometrical transition, the extent of pre‐cure and injection speed functions of the melt viscosity. Transient modifications to the melt viscosity during mold filling produce variations in skin/core structure along the flow path, which are correlated to the mechanical properties of injection moldings. The melting characteristics of the phenolic resin during plasticization impose a severe environment of mechanical attrition on the glass fibers, which is sequentially monitored along the screw, and during subsequent flow through runners and gates of various sizes. Differences found between the processing characteristics of thermosets and thermoplastics raise questions concerning the applicability of thermoplastic injection molding concepts for thermosets.
Effect of CaCO3 on the mechanical and rheological properties of a ring‐opening phenolic resin: Polybenzoxazine
Authors: Suprapakorn Nimolrat , Dhamrongvaraporn Sujitra , Ishida Hatsuo
Published: April 2004

Tensile, flexural, and impact behaviors of CaCO3‐filled polybenzoxazine are studied in the 0–30 wt% filler content range. This work also investigates the role of the particle size of the filler and the effect of the interface on these properties. Dynamic mechanical measurement has been carried out to support the static mechanical testing. Moreover, the viscosity of neat benzoxazine monomer and the monomer with 18% oligomer has been measured to compare against the viscosity of a novolac resin.
Ionic interphase of glass fiber/polyamide 6,6 composites
Authors: Salehi‐Mobarakeh Hamid , Brisson Josée , Ait‐Kadi Abdellatif
Published: April 2004

Interfacial polymerization to polyamide 6, 6 followed by introduction of ionic groups was performed on the surface of short glass fibers. The ionic interphase‐modified fibers were used with poly(ethylene‐co‐methacrylic acid) (DuPont Surlyn) to prepare composites with specific fiber‐matrix interactions. Fiber treatment increased composite tensile and bending properties. An increase in the average fiber length was observed, which was attributed to a decrease in the fiber attrition during mixing. The effect of increasing temperature on the composite mechanical properties was studied. Different behavior was observed before and after the glass transition temperature, Tg, of the matrix. The dynamic mechanical measurements showed an increase in the Tg of the matrix after the treatments, which is attributed to a decrease in chain mobility at the interface resulting from increased interactions of the treated fiber surface with the polymer. Scanning electron microscopy of fractured composites after tensile tests revealed a smooth fiber surface with no polymer at the surface for the untreated composites. Adhered polymer was clearly observed on the surface of treated fibers, indicating better fiber wetting by the matrix. This improved adhesion was attributed to the grafted nylon molecules at the glass fiber surface.
Manufacturing of all‐thermoplastic sandwich systems by a one‐step forming technique
Authors: Breuer U. , Ostgathe M. , Neitzel M.
Published: April 2004

Thermoplastic sandwich composite systems offer great potential for future automotive applications. High specific bending stiffness, good insulation properties, high specific energy absorption and impact resistance can be achieved by the use of fabric reinforced sheets as sandwich faces and thermoplastic foam as core material. Recycling is possible by chopping and injection molding. Fabric‐reinforced thermoplastic sheets consisting of 10 layers glass fabric were impregnated continuously with a polyamide‐12 matrix using isobaric double belt press technique to 2‐mm‐thick faces with a fiber content of 50 vol%. Sandwich angles and hemispheres were manufactured by heating the faces and the 13‐mm‐thick foam core above the melting temperature and high‐speed stamp forming with matched metal tools. Cycle times for forming and cooling were below 40 s/part. Different heating methods were matter of investigation. Best results could be achieved by conductive heating. This new material system and its processing technology are promising methods to reduce structural car weight and enhance passenger safety.
Modeling of heat transfer and crystallization kinetics in thermoplastic composites manufacturing: Pultrusion
Authors: Carlsson Andreas , Åström B. Tomas
Published: April 2004

Whereas pultrusion with thermoset resins has been widely analyzed, there is a scarcity of knowledge about pultrusion with thermoplastic resins. The aims of this work were to strive towards deeper knowledge of temperature distribution and degree of crystallinity in the composite during processing, to develop a tool for process simulations with a variety of processing parameters, and to provide input data to models for pressure and matrix flow. The heat transfer model presented herein describes the temperature distribution within the composite throughout the thermoplastic pultrusion process. The transient heat transfer situation is modeled one‐dimensionally through a transverse cross section far from the edges of a high aspect ratio composite. Temperature‐dependent thermal properties, partly non‐infinite contact conductance, and heat contribution from crystallization were taken into consideration. The degree of crystallinity within the composite was determined as well. The analysis also includes an experimental verification of the heat transfer model. Results show good agreement with experimental data.
The effect of long‐term ultraviolet light irradiation on polymer matrix composites
Authors: Liau W. B. , Tseng F. P.
Published: April 2004

This study reports the individual and combined effects of ultraviolet light and thermal shock on the physical properties of polymer matrix composites in air and in a “near‐vacuum system.” The longest exposure time was 180 days. It was found that the weight loss of composites increased with irradiation time. In the graphite/epoxy system, the weight loss in air was 2–3 times that in a “near‐vacuum system.” Similarly, the weight loss of the glass/epoxy system in air was 3–6 times that in a “near‐vacuum system.” The weight loss of the glass/epoxy system was always larger than that of the graphite/epoxy system. In all cases, the UV irradiation and 1000 times thermal shock did not change the fracture mechanism—it was always brittle fracture in addition to fiber pullout. Surface erosion was observed in the irradiated surfaces by scanning electron microscopy. For both graphite/epoxy and glass/epoxy composites, the tensile strength decreased with increasing irradiation time irrespective of the irradiation environment. However, the decrease was not significant. By SEM, cracks could be observed in the up‐surface and side‐surface of the glass/epoxy system that was irradiated more than 1 month in air and through 1000 times thermal shock. Obviously, UV light cannot deeply penetrate the sample, and only the surface of the sample will be influenced. The UV radiation initiated microcracks, which propagated through the thermal shock.
An interface model for the prediction of Young's modulus of layered silicate‐elastomer nanocomposites
Authors: Shia D. , Hui C. Y. , Burnside S. D. , Giannelis E. P.
Published: April 2004

Recent experiments on layered silicate‐elastomer nancomposites by Burnside and Giannelis have shown that there is a discrepancy between theoretical modulus predictions and experimental modulus measurements. A theory is proposed to explain this discrepancy. We hypothesize that the discrepancy is due to imperfect bonding between the matrix/inclusion interface which effectively reduces the aspect ratio and the volume fraction of the inclusion. We use a simple interface model to quantify the imperfect interfacial bonding. From this model, we introduce the concept of the effective aspect ratio and effective volume fraction of the inclusions. These effective quantities depends on a single material parameter, namely, the constant interfacial shear stress, τ. The interfacial shear stress for the elastomer‐silicate nanocomposites is found by fitting the theory to the experimentally measured modulus of Burnside and Giannelis. The interfacial shear stress is in the range of thousands of Pascals. For the elastomer‐silicate nanocomposite systems considered here, the interfacial shear stress can be decomposed into two parts; intrinsic shear stress τi and frictional shear stress τf. The intrinsic interfacial shear stress τi depends only on the volume fraction of inclusions and decreases with increasing volume fraction of inclusions. On the other hand, the frictional shear stress τf is found to increase linearly with the applied strain. Since the mean stress is also proportional to the applied strain, this gives rise to an effective coefficient of friction, which is found to be 0.0932 for the nanocomposite system considered here.
On the effect of the fiber orientation on the flexural stiffness of injection molded short fiber reinforced polycarbonate plates
Authors: Neves N. M. , Isdell G. , Pouzada A. S. , Powell P. C.
Published: April 2004

The through‐thickness fiber orientation distribution of injection molded polycarbonate plates was experimentally determined by light reflection microscopy and manual digitization of polished cross sections. Fiber length distribution was determined by pyrolysis tests followed by image analysis. A statistical analysis was done to determine the confidence limits of the fiber orientation results. The fiber orientation distribution was described by using second‐order orientation tensors. The through‐thickness stiffness variations were determined by the orientation averaging approach. This layer stiffness distribution was used to simulate the behavior of beams subjected to three point bending with a FEM Ansys model. The results were compared with experimentally determined flexural stiffness both in the flow direction and in the transverse flow direction. The effect of flow rate and melt‐temperature on stiffness and fiber orientation is discussed.
The influence of post‐cure on properties of carbon/phenolic resin cured composites and their final carbon/carbon composites
Authors: Ko Tse‐Hao , Kuo Wen‐Shyong , Lu Yng‐Ren
Published: April 2004

Two‐dimensional (2D) carbon/carbon (C/C) composites were prepared with phenol‐formaldehyde resin and graphite fabric. After curing, polymer composites were post‐cured in air at 160°C and 230°C for several hours and then all polymer composites were carbonized up to 1500°C. The effect of post‐cure on the microstructure and fracture behavior of the resultant carbon/carbon composites was studied. The post‐cure process was characterized by weight loss. This process promoted the crosslinking and condensation reactions and led to the formation of long‐chain, cross‐linked polymeric structures in the matrix. The post‐cured composites had a greater density than the unpost‐cured composite. This study indicates that a longer post‐curing time and higher post‐curing temperature would limit the shrinkage for the post‐cured composites during carbonization. The improvement in linear shrinkage was 22% to 44%. This process also limited the formation of open pores and decreased the weight loss of the resultant C/C composites. The resultant C/C composites developed from post‐cured composites had a greater flexural strength by 7 to 26% over that developed from unpost‐cured composite.
New nanocomposite materials made of an insulating matrix and conducting fillers: Processing and properties
Authors: Flandin L. , Cavaillé J. Y. , Bidan G. , Brechet Y.
Published: April 2004

The work described in this paper deals with the preparation and the characterization of an homogeneous composite material composed of electrically conductive fillers dispersed in a thermoplastic insulating matrix. These fillers were chosen to have either a spherical shape or a high aspect ratio. Processing of these conductive polypyrrole particles, which were obtained either through a classical polymerization of pyrrole in the presence of stabilizer, or polymerization onto the surface of cellulose monocrystals is detailed. The relationship of filler content in the composite to electrical and mechanical properties was investigated. In order to characterize the connectivity of the fillers in the matrix, electrical measurements were performed and these results have been compared with the predictions of the statistical percolation theory. Further analysis was undertaken by considering the influence of the network of rigid particles on the viscoelastic properties of the composites. These results were compared with two models; first, with a mechanical percolation approach which was based on strong interactions between fillers, and second, with a mean field model, which in no way accounted for interactions bewteen fillers. In recent developments, it has been shown that these materials used as coating on various substrate could be good sensors, allowing to determine some specific features of the substrate deformation.
Modulus recovery kinetics and other insights into the payne effect for filled elastomers
Authors: Chazeau L. , Brown J. D. , Yanyo L. C. , Sternstein S. S.
Published: April 2004

The nonlinear viscoelastic behavior of filled elastomers is examined in detail using a variety of samples including carbon‐black filled natural rubbers and fumed silica filled silicone elastomers. New insights into the Payne effect are provided by examining the generic results of sinusoidal dynamic and constant strain rate tests conducted in true simple shear both with and without static strain offsets. The effect of deformation history is explored by probing the low amplitude modulus recovery kinetics resulting from a perturbation by a large strain deformation such as a sinusoidal pulse or the application or removal of a static strain. It is found that a static strain has no effect on either the fully equilibrated dynamic (storage and loss) moduli or the incremental stress‐strain curves taken at constant strain rate. The reduction in low amplitude dynamic modulus and subsequent recovery kinetics due to a perturbation is found to be independent of the type of perturbation. Modulus recovery is complete but requires thousands of seconds, and is independent of the static strain. The results suggest that deformation sequence is as critical as strain amplitude in determining the properties, and that currently available theories are inadequate to describe these phenomena. The distinction between fully equilibrated dynamic response and transitory response is critical and must be considered in the formulation of any constitutive equation to be used for design purposes with filled elastomers.
Modeling compression molding of all‐thermoplastic honeycomb core sandwich components. Part B: Model verification
Authors: åkermo Malin , Tomas åström B.
Published: April 2004

The compression molding process for manufacturing of all‐thermoplastic honeycomb core sandwich components is studied with the aim of verifying a previously proposed model describing the instantaneous face‐core bonding during molding. Experiments are performed with glass/polypropylene (PP) faces and PP honeycomb core and experimental data are compared to model predictions in terms of temperature and degree of face‐core interfacial contact with reasonable good agreement. Model predictions show that the face‐core interface bond strength may increase with up to 200% when molding at an interface temperature slightly above the crystal melting temperature of the matrix for only a few seconds. The influence of the major process parameters, including molding temperature, molding pressure and core viscosity, on the predicted bond strength is investigated using the model.
Polymer bonded magnets. II. Effect of liquid crystal polymer and surface modification on magneto‐mechanical properties
Authors: Xiao Jun , Otaigbe Joshua U.
Published: April 2004

We studied the effect of liquid crystal polymer (LCP) and surface modification of neodymium‐iron‐boron (Nd‐Fe‐B) magnetic alloy on the magneto‐mechanical behavior of poly (phenylene sulfide) (PPS) bonded Nd‐Fe‐B magnets to accelerate efforts to develop useful thermoplastic magnets with optimal performance. The results indicate that blending the LCP with PPS provides the required balance of properties for the targeted applications. These properties include superior magneto‐mechanical performance at elevated temperatures, minimal melt viscosity at optimal LCP volume fraction, high stiffness, and improved dimensional stability, making the thermoplastic magnets suitable for use at elevated temperatures and in chemically corrosive environments where commercial rare earth alloy magnets are not useable. Enhanced wetting of the magnetic Nd‐Fe‐B powders by the polymers, formation of reinforcing LCP domains, and interactions between the polymers and the magnetic powders are thought to be responsible for the beneficial function of the LCP and Nd‐Fe‐B surface modifier in the PPS bonded Nd‐Fe‐B magnets.
Properties of films from polypropylene and thermotropic liquid crystalline polymer blends
Authors: Incarnato L. , Scarfato P. , Motta O. , Acierno D.
Published: April 2004

In this paper the effect of the inclusion of two different thermotropic liquid crystalline polymers, namely Rodrun 3000 and Vectra A950, in a PP matrix is analyzed with particular attention to the gas transport and mechanical properties of the extruded blend films. The experiments, conducted on PP/Rodrun 3000 and PP/Vectra A950 films, have shown that the presence of TLCPs, also at low percentages, modify the properties of the thermoplastic matrix in a manner depending on the degree of compability and interfacial adhesion between the two components of the blends. Moreover, the effect of a maleic anhydride grafted PP (MAP), used as compatibilizing agent, on the properties and morphology of the PP/Rodrun 3000 system was examined. It was found that the addition of the MAP determines an increase in the barrier properties and in toughness of the films compared to those without MAP.
The need to re‐evaluate laminate design criteria
Authors: Piggott Michael R.
Published: April 2004

The aerospace industry uses carbon‐fiber epoxy laminates for structures to reduce weight and increase payload. The “standard” design criterion for strength is that proposed by Tsai‐Wu. For stiffness, which is generally more critical than strength, classical laminated plate theory (LPT) is used. The normal lay‐ups considered for commercial aircraft are made up from 0°, 90° and ± 45° orientations. Angle ply laminates, [± ϕ]ns, with ϕ fixed to some angle such as 20°, are not normally used (although this type of structure is employed with great success in the pressure vessel industry). According to the Tsai‐Wu criterion, such a structure should be extremely weak, which probably accounts for the absence of simple angle ply structures in aerospace designs. However if short and wide samples (aspect. ratio 0.5 or less) are tested, higher values are obtained for modulus and much higher values for strength than the long narrow samples used to develop the Tsai‐Wu criterion. The short and wide sample test results are in agreement with results from tests on tubes. These observations show that there is an “edge softening” effect: long narrow samples have a relatively large amount of this soft edge. Since design software normally uses Tsai‐Wu and LPT, large errors in strength and significant errors in stiffness are possible at this stage, and better lay‐up designs may be totally missed. The experimental work leading to these conclusions is described and innovative designs are discussed.
A fiber optic thermoset cure monitoring sensor
Authors: Giordano Michele , Nicolais Luigi , Calabrò Antonio Maria , Cantoni Stefania , Cusano Andrea , Breglio Giovanni , Cutolo Antonello
Published: April 2004

Curing determines the chemical and physical properties of a reacting resin. Among these, the optical properties strongly correlate with the structural features of the developing polymeric network. By monitoring changes of the refractive index, it is possible to analyze the polymerization of thermoset resin. In this work, a fiber optic sensor system has been designed and developed (based on the optical time domain reflectometry principle) to measure the reflection coefficient at the interface between the fiber optic and the resin during a curing process. Correlation between the sensor output and conversion has been proposed, following the Lorentz‐Lorenz law. Isothermal data from the sensor have been compared with calorimetric analysis of an epoxy‐based resin.
The influence of fiber wetting in resin transfer molding: Scale effects
Authors: Binetruy C. , Pabiot J. , Hilaire B.
Published: April 2004

Resin transfer molding (RTM) has become a popular technique for fabrication of composite parts. During the mold filling stage, the resin is forced to flow into pore spaces between tow filaments and the between the tows themselves. Voids trapped in the composite during the filling stage are believed to be the consequence of the non‐uniform micro‐flows inside fiber bundles and macro‐flows between them. Thus, to minimize the void formation, the processing parameters should be determined to generate a uniform front. In this case, the flow inside tows is driven by both the injection pressure and the capillarity forces and matches the flow outside the tows, which is driven only by pressure. To control such a flow, both viscous and capillarity flows have to be known at all scales of the fibrous structure. For this purpose, the wetting effects at the different scales (ranging from the individual tow to the multi‐layers reinforcements) are investigated. Original experiments are carried out to visualize micro‐flows and to measure the capillarity flow rates. Wicking effects are quantified in terms of the overall and local effective permeabilities. A new parameter that accounts for interactions between flows splitting in macro‐ and micro‐flows is introduced. Practical ways to reduce voids entrapment are also discussed.
Applications of fiber‐metal laminates
Authors: Straznicky P. V. , Laliberté J. F. , Poon C. , Fahr A.
Published: April 2004

Advanced composite materials and fiber‐metal laminates (FMLs) have the potential to offer significant improvements in weight savings and durability in airframe structures. FMLs are an advanced hybrid material system consisting of metal layers bonded with fiber‐reinforced polymer layers. This paper presents an overview of the history of fibre‐metal‐laminates, describes several common types and also discusses the results of impact durability experiments conducted at the Structures, Materials and Propulsion Laboratory of the Institute for Aerospace Research (SMPL‐IAR) of the National Research Council Canada (NRCC). An impact fixture was developed specifically for FMLs and is also described. Numerous low velocity impact tests have been carried out that demonstrate the improved impact response of FMLs over traditional composite materials. This research builds upon earlier impact testing on carbon‐fiber‐reinforced polymers conducted by NRCC and Carleton University.
Glass fiber reinforced polyamide‐6 nanocomposites
Authors: Akkapeddi M. K.
Published: April 2004

Polyamide 6 nanocomposites (PA6/NC) are novel type of composite materials comprising extremely thin, “nanometer scale” dispersions of smectitic silicate platelets. We prepared such PA6/NC materials via a melt compounding technique using suitably organomodified montmorillonite or hectorite type clays. The high surface to volume ratio of such thin silicate dispersions in PA6/NC leads to a high reinforcement efficiency even at 2 to 5 wt% of clay, achieving high specific modulus, strength and heat distortion temperature under load (DTUL). In addition, the platelet orientation and their surface nucleation effects seem to promote a faster crystallization and higher crystallinity in PA‐6/NC (particularly at the surface and in thin‐wall injection moldings), as compared to the standard PA‐6. Such morphological features in PA‐6/NC also result in improved moisture resistance. Recognizing these benefits, we investigated the use of PA‐6/NC as a matrix for making both short and long glass fiber (GF) reinforced composites. Significant improvements in modulus were achievable in both the dry and the moisture conditioned state for PA‐6/NC compared to standard PA‐6, at any given level of glass fiber reinforcement. In general a small amount (3‐4 wt%) of the nanometer scale dispersed clay is capable of replacing up to 40 wt% of a standard mineral filler or 10‐15 wt% of glass fiber in PA‐6 composites to give equivalent stiffness at a lower density or a lower part weight advantage. In addition, improved moisture resistance, permeation barrier and fast crystallization/mold cycle time contribute to the usefulness of such composites.
Characterization of interfacial properties of composite materials by acoustic emission
Authors: Ndiaye I. , Maslouhi A. , Denault J.
Published: April 2004

The global mechanical properties of composite structures in service depend on fiber/matrix interface and interlaminar strength. The paper proposes to use Acoustic Emission (AE) and advanced signal processing to evaluate the interlaminar performance of polymeric composites. A delaminating process simulated with a Double Cantilever Beam (DCB) in opening mode (Mode I) coupled with an Acoustic Emission (AE) technique has been employed. Different samples were analyzed to observe the damage evolution and to evaluate the interlaminar decohesion processes. The resistance to delamination growth is expressed in terms of the interlaminar DCB mode (mode I) fracture toughness, measured by strain energy release rate, GI, dissipated per unit area of delamination growth in composite. Three categories of samples were used: two unidirectional carbon fiber/epoxy resins with one degraded by heat and one with a commercially used resin. It was found that sample that was exposed at a temperature greater than the glass transition temperature Tg of the epoxy had a higher cumulative energy release rate than the two other samples types. The original type having the lowest release rate. Acoustic emission parameters have been found to be powerful indicators of the intensity of the damage. Multivariate analysis of up to 49 parameters was performed in order to group classes of AE signals with matching characteristics. A correlation was established between the energy release rate and the acoustic emission energy.
Fatigue behavior of continuous glass fiber composites: Effect of the matrix nature
Authors: Bureau M. N. , Denault J.
Published: April 2004

The effect of the matrix morphology on the fatigue behavior of a continuous glass fiber/polypropylene (GF/PP) composite system was studied by means of stress‐life and mode II cyclic delamination tests. The stress‐life behavior of a GF composite is considerably affected by the nature of the matrix. A two‐stage fatigue damage curve was observed in the composite made with a PP matrix, whereas a three‐stage curve was observed in the composite made with a thermoset polyester matrix. For a fatigue stress higher than 50% of the yield stress, the PP matrix composite showed a considerably longer fatigue life than the thermoset polyester matrix composite. Mode II cyclic delamination tests showed that the morphology itself of the PP matrix also played an important role. Higher fatigue delamination growth rates, at given strain energy release rates, and lower strain energy release rates at failure were obtained for a composite showing a coarse spherulitic morphology and well‐marked interspherulitic regions than for a composite showing a finer spherulitic morphology and less‐marked interspherulitic regions. While the fatigue mode of the composite with a coarse spherulitic morphology was interspherulitic, that of the composite with a finer spherulitic morphology was transpherulitic.
Raman study of the microstructure changes of phenolic resin during pyrolysis
Authors: Ko Tse‐Hao , Kuo Wen‐Shyong , Chang Ying‐Huang
Published: April 2004

After curing, phenol‐formaldehyde resins were post‐cured at 160°C, and then carbonized and graphitized from 300°C to 2400°C. The structure of the resulting carbonized and graphitized resins were studied using X‐ray diffraction and Raman spectroscopy. Thermal fragmentation and condensation of the polymer structure occurred above 300°C. The crystal size of the cured phenolic resins increased with an increase in temperature. The crystal size increased from 0.997 nm to 1.085 nm when the heat‐treatment temperature rose from 160°C to 500°C. Above 600°C, the original resin structures disappeared completely. Below 1000°C, the stack size (Lc) increased very slowly. The values increased from 0.992 to 1.192 nm when the heattreatment temperature rose from 600°C to 1000°C. Above 1000°C, the stack size showed an increase with the increase in heat‐treatment temperature. The values increased from 1.192 to 2.366 nm when the temperature rose from 1000°C to 2400°C. The carbonized and graphitized resins were characterized using Raman spectroscopy. The Raman spectrra were recorded between 700 and 2000 cm−1. Below 400°C, there were no carbon structures in the Raman spectra analysis. Above 500°C, G and D bands appeared. Raman spectra confirmed progressive structure ordering as heat‐treatment temperature increased. The frequency of the G band of all carbonized and graphitized samples shifted to 1600 cm−1 from the 1582 cm−1 of graphite. At the same temperature, the D band shifted to 1330 cm−1 from the 1357 cm−1 of the imperfect carbon. In the curve fitting analysis of the Raman spectra, a Gaussian shaped band centered at 1165 cm−1 was included. This band has not been described before in the literature and is attributed to disordered structures, which are formed from the original polymeric structures. These polymeric structures formed unknown disordered structures and remained in the carbonized phenolic resins. Above 1800°C, this band disappeared completely. But, a weak peak is present near 1620 cm−1. This indicated that those disoriented molecules and some disordered carbons were removed as volatiles or repacked into the glassy carbon structures during graphitization. The carbonized and graphitized phenolic resins were found to correspond to low order sp2 bonded carbon, but cannot be considered as truly glassy or amorphous carbon materials since they have some degree of order in the basal plane.
Determination of the rheological properties of thermoplastic composites for compression flow molding
Authors: Törnqvist Ronnie , Sunderland Paul , Månson Jan‐Anders E.
Published: April 2004

The influence of the fiber mat structure and the matrix rheology on the squeeze flow behavior of thermoplastic composites for compression flow molding has been investigated. An analytical model for isothermal squeeze flow has been developed and experimentally confirmed. The model is based on the role of the matrix at local fiber interaction points. The squeeze flow rheology of the composite can be evaluated from the shear flow of the pure matrix as expressed by a Carreau equation, using two micromechanical‐based shift factors. It was found that the fiber mat structure influences the yield stress of the composite, but has a minor influence on the squeeze flow in comparison to the influence of the matrix.
Phthalonitrile‐glass fabric composites
Authors: Sastri S. B. , Armistead J. P. , Keller T. M. , Sorathia Usman
Published: April 2004

Phthalonitrile polymers, under development at the Naval Research Laboratory, offer promise as high temperature, high performance composite matrix materials. A fully cured resin shows outstanding thermal stability with no evidence of a glass transition temperature or Tg up to 450°C, good mechanical properties, and is easily processed into void‐free components. Phthalonitrile/glass fabric composite panels have been successfully fabricated by conventional consolidation of prepregged glass and by a more recently developed simplified process, resin infusion molding. Both processes can be used to produce panels with comparable mechanical properties. More important, flammability performance of these composites, evaluated in terms of specific optical density, combustion gases, heat release, and ignitability, excels over other state‐of‐the‐art polymer/glass composites. This finding is significant given that overcoming flammability obstacles has been the main limiting factor for use of composites in marine applications.
The effect of microwave resin preheating on the quality of laminates produced by resin transfer molding
Authors: Johnson M. S. , Rudd C. D. , Hill D. J.
Published: April 2004

Cycle times in resin transfer molding (RTM) of an unsaturated polyester have been reduced significantly using an in‐line microwave resin preheating system. Microwave preheating lowers the resin viscosity during injection and modifies the thermal age of the resin, potentially influencing the quality of RTM laminates. The tensile properties of RTM laminates have been measured with regard to improved fiber wet‐out by the lower viscosity resin. Degree of cure measurements have been included to establish the effect of microwave preheating on resin conversion within the laminate. Local pressure that develops within the mold during the cure phase can lead to mold deflections. Variations in the laminate thickness associated with these deflections are presented, and the use of microwave resin preheating to reduce these variations is discussed.
Thermoplastic nanocomposites filled with wheat straw cellulose whiskers. Part II: Effect of processing and modeling
Authors: Dufresne Alain , Cavaillé Jean‐Yves , Helbert William
Published: April 2004

The reinforcing effect of cellulose microcrystals, or whiskers, dispersed in a thermoplastic matrix is analyzed. In Part I [Polym. Compos., 17, 604 (1996)], the preparation and the characterization of cellulose whiskers obtained from wheat straw were detailed. Composite films were processed by freeze‐drying and molding a mixture of aqueous suspension of these microcrystals and a film forming latex of poly(Styrene‐co‐ButylAcrylate). Using the same filler and latex, materials were processed by casting and evaporating the mixture of aqueous suspensions. It was found that the reinforcing effect is greater in the latter case. This behavior is ascribed to the sedimentation of the filler during evaporation, evidenced by scanning electron microscopy, wide angle X‐ray scattering, and dynamic mechanical analysis, and to the formation of a network of whiskers governed by a percolation mechanism. The behavior of the composite is modeled by subdividing the sample into layers with different whisker contents lying parallel to the film surface.
Uniaxial, shear, and poisson relaxation and their conversion to bulk relaxation: Studies on poly(methyl methacrylate)
Authors: Lu H. , Zhang X. , Knauss W. G.
Published: April 2004

Multiple viscoelastic properties were determined for poly(methyl methacrylate) in the context of examining experimental limitations on deriving other properties from these measurements, specifically time dependent bulk modulus behavior. By means of master curves for uniaxial extension on plate and cylinder specimens, as well as shear (torsion) and Poisson behavior from measurements in the temperature range from −40°C to 125°C, it is established that excessive precision is needed for reliable interconversion of some properties to others. While normal inverse relations (modulus‐compliance) are readily obtained from one set of measurements, bulk (relaxation or creep) properties cannot be derived reliably from other functions and must be determined directly.
Effects of flange/web on the buckling behavior of fiber‐reinforced composite channel section column under axial compression
Authors: Rhee K. Y. , O J.
Published: April 2004

The flange height (bf) to web length (bw) effect on the buckling behavior of a composite column with a channel section was studied. The channel section collumns of bf /bw values 0.3, 0.5, and 0.7 were employed. A theoretical analysis was conducted to decide the dimensions of the columns so that the columns produce as many buckling modes as possible. The buckling mode and critical buckling load for each column were determined from the experiments. Results showed the columns exhibited local buckling and overall buckling (flexural and torsional‐flexural) modes depending on the bf /bw and column length. That is, the flexural buckling occurred only for columns having a bf/bw value of 0.3, while the torsional‐flexural buckling occurred for columns with bf /bw values of 0.5 and 0.7. The results also showed that the local buckling load decreased as bf /bw increased.
Hydrostatically extruded glass fiber reinforced polyoxymethylene. II: Modeling the elastic properties
Authors: Hine P. J. , Wire S. , Duckett R. A. , Ward I. M.
Published: April 2004

This paper describes an investigation into modeling the elastic properties of hydrostatically extruded short glass fiber reinforced polyoxymethylene (POM). The starting material for the extrusion was randomly arranged short glass fibers (25 wt%, average length 150 μm) in an isotropic POM matrix. Extrusion was carried out through a reducing conical die at 15°C below the melting point of the matrix phase (hence the composite was extruded in the solid state), such that after extrusion, preferential alignment along the extrusion direction was developed for both the fibers and for the crystalline fraction of the polymer matrix. The elastic properties of samples, made over a range of extrusion ratios, were measured using the ultrasonic immersion method, a technique that allows a complete set of elastic constants to be determined for a composite. Theoretical predictions for the elastic properties of the oriented extrudates were generated by combining a modification to the theory of Wilczynski to allow for the fibers' being surrounded by an oriented matrix phase, together with the aggregate model of Ward to model the effects of partial orientation of the fiber oriented matrix units.
Model resin permeation of fiber reinforcements after shear deformation
Authors: Lai Chyi‐Lang , Young Wen‐Bin
Published: April 2004

In liquid composite molding processes such as resin transfer molding and structural reaction injection molding, fiber reinforcements are formed with automated processes to conform to the complex shape of the mold cavity. Deformation of the fiber reinforcement during the forming operation can be characterized by factors such as the local surface curvature of the mold and the type of reinforcement. For bidirectional fiber fabrics, simple shear is the major deformation mode in the forming process. Deformation of the fiber reinforcement after being formed to the mold cavity shape results in variations of local fiber content. In addition, the network structure of the fiber reinforcement is also rearranged. This may cause some significant effects on the fiber permeability and result in a mold filling pattern quite different from that expected. Therefore, a good understanding and measurement of the permeabilities for the deformed fiber reinforcements is of great importance. In the flow simulation of the filling process, the success of the prediction depends greatly on the correct values of in‐plane permeabilities. A change of the in‐plane permeability of the fabric after shear deformation must be well understood before an accurate flow simulation can be obtained. This article investigates the permeability of fiber reinforcements in relation to different shear angles. Several flow experiments were conducted on bidirectional woven roving fabrics at different shear angles. Two relevant factors—the ratio of principal permeabilities and the direction of principal axes with respect to the orientation of the fabric—are studied to investigate their variations with respect to shear deformation of the fiber reinforcements. It is found that the angle shift of the principal axes increases with the shear angle. At the same time, the in‐plane permeability ration may decrease with the shear angle.
Effect of surface coverage of silane treated CaCO3 on the tensile properties of polypropylene composites
Authors: Demjén Zoltán , Pukánszky Béla
Published: April 2004

Polypropylene (PP) composites were prepared with a constant volume fraction of a CaCO3 filler, which was treated with eight trialkoxy functional silane coupling agents and the routinely used stearic acid, as comparison, The tensile properties of the composites were determined as a function of surface coverage and the chemical composition of the coupling agent. Significant differences were detected in the effect of the silane coupling agents on the tensile properties of the composites studied. They could be classified into groups of reactive, nonreactive and inactive compounds, in accordance with an earlier study. Moreover, tensile properties proved to be very sensitive to surface treatment. Reactive coupling leads to a maximum in tensile strength, while the use of nonreactive agents decreases strength rapidly with increasing surface coverage. The rate of strength decrease slows down above the concentration necessary for the formation of a monolayer coverage. Treatment influences deformability accordingly, it decreases on reactive coupling, while agents with a surfactant effect increase elongation. Amino functional silanes show a strong reactive coupling effect. A trialkoxy derivative of stearic acid, (3‐stearyloxy‐propyl‐triethoxy‐silane), synthesized the first time in our laboratory, has an outstanding surfactant effect; its use increases both the elongation and the tensile strength of the composites. Both characteristics exceed the corresponding properties of the reference composite containing the filler treated with stearic acid.
In‐plane permeability measurements on fiber reinforcements by the multi‐cavity parallel flow technique
Authors: Lundström T. Staffan , Gebart B. Rikard , Sandlund Erik
Published: April 2004

This report discusses the advantages and drawbacks of the multi‐cavity parallel flow technique for permeability measurements. An experimental series with repeated measurements on material from the same roll shows that the repeatability of the technique is very good considering the manufacturing variability of the fabric. The measured standard deviation in the repeatability study is about 10%. It is, however, shown that the permeability can vary considerably‐ between reinforcements of similar geometry. Furthermore, computer simulations were used to estimate the errors when highly anisotropic materials are oriented at an angle to the material principal direction in the parallel flow technique. The conclusion based on the simulations is that the length to width ratio of the cavity should be larger than the anisotropy of the reinforcement for an acceptable error.