bluuebunny/crossref_metadata_2025 huggingface dataset:
Structure and properties of oil extended styrene butadiene block copolymers
Authors: Polizzi S. , Stribeck N. , Zachmann H. G. , Bordeianu R.
Published: December 2004

The relationships between mechanical properties and structure of rapidly spin cast styrene‐butadiene‐styrene (SBS) block copolymers diluted with two different types of mineral oil are discussed. Small angle X‐ray scattering (SAXS) measurements indicate that both oils go mainly into the rubbery polybutadiene phase. While during rapid spin casting the paraffinic oil improves the phase separation of polystyrene (PS) and polybutadiene (PB), the opposite is true for the aromatic oil. The measurements also suggest that the oils cause a preferential swelling of the superstructure along the longitudinal direction of the cylindrical PS domains. From this, it can be concluded that a larger number of interconnecting PB chains is present in the cross direction than in the longitudinal one. With increasing oil content, a further decrease of interconnecting chains in the latter direction may be the reason for the decrease of the tensile modulus. Furthermore, above a critical value of dilution (33 wt percent for paraffinic oil, 50 wt percent for the aromatic one), more and more spherical domains are formed instead of the cylindrical ones. This morphological transition explains the step present in the double logarithmic plot of the large strain tensile modulus as a function of oil dilution.
Some studies on glass fiber‐reinforced polypropylene. Part II: Mechanical properties and their dependence on fiber length, interfacial adhesion, and fiber dispersion
Authors: Gupta V. B. , Mittal R. K. , Sharma P. K. , Mennig G. , Wolters J.
Published: December 2004

Two commercial grades of glass fiber‐reinforced polypropylene granules, one containing short fibers of average length around 0.5 mm with a coupling agent and the other containing relatively much longer fibers mostly around 9 mm, but no coupling agent, were injection‐molded into dumbbells and tested in tension between −43 and 90°C. There is considerable fiber attrition during injection molding; the fiber lengths are reduced to average values of 0.4 to 0.8 mm for thses two samples. Also during injection molding of the test sample, partial molecular alignment of the matrix (polypropylene) occurs which supplements the reinforcement of the matrix due to the aligned glass fibers (30 percent by weight) present in the composite sample. The stiffness and strength of these samples do not reflect the effects of fiber lengths since most of the fibers are of very small length in the molded specimen and also since the sample with longer fibers has a non‐uniform distribution of fibers. While the interfacial shear strength does not appear to play a significant role in determining stiffness, it turns out to be extremely important in controlling strength, particularly at the higher test temperatures. The room temperature impact strength is high for the sample containing relatively longer fibers of average length around 0.8 mm in which fiber dispersion is non‐uniform and fiber agglomerates are present. Acoustic emission data shows that debonding and fiber pull‐out are the main contributors to sample toughness; this observation is supported by scanning electron micrographs of the fracture surfaces.
A simple illustration of structure‐properties relationships for short fiber‐reinforced thermoplastics
Authors: Doshi S. R. , Charrier J.‐M.
Published: December 2004

In this paper, a simple theoretical model for elastic properties of short fiber‐reinforced thermoplastic (SFRTP) composite systems is described, which considers the effect of two important structural parameters, viz., fiber aspect ratio distribution and fiber orientation distribution. An experimental technique for producing SFRTP samples featuring a fairly uniform fiber orientation is described, and a broad set of experimental results on the mechanical properties of SFRTP systems based on polystyrene (PS) and polyethylene (PE) resins with several fiber loadings is presented. The use of the simple theoretical model to analyze the experimentally‐determined elastic properties is discussed.
Fracture of ultrafine calcium carbonate/polypropylene composites
Authors: Levita G. , Marchetti A. , Lazzeri A.
Published: December 2004

The strength and fracture properties of a polypropylene filled with ultrafine calcium carbonate (0.07 μm) have been studied in the composition range of 0 to 40 percent by volume. Untreated and surface treated (with stearic acid and a titanate coupling agent) grades have been considered. The untreated filler caused a decrease of toughness whereas a maximum, at ∼10 percent, was observed for the treated filler. The fracture energy was analyzed in terms of the crack pinning model. Due to the very small size of particles the pinning contribution proved to be negligible.
Simulating fiber distribution in thin sheets
Authors: Chalifoux J.‐P. , Sanschagrin B. , Roche F. , Benkirane A.
Published: December 2004

A model is presented that simulates on a microcomputer the fiber distribution in a planar arrangement applicable to thin sheets of composites. Although it does not take into account spatial fiber interference, the software allows one to investigate a number of important design parameters. The inputs used in the simulation are: thickness of the composite sheet, diameter and volume fraction of fibers along with fiber length, and angular position which can be specified in a number of ways. The program utilizes the internal random function to depict each fiber, as it is generated, in a “simulation window” of the monitor while the mathematics of the system are being done simultaneously within the smaller “valid window”. The procedure prescribes that the center of gravity of all fibers be uniformly distributed over the simulation window area. For that reason and because the 2D model is extended to simulate a three‐dimensional sheet, results will represent the ideal configuration or upper bonds of a corresponding practical case. Graphical representations and statistics are thus produced to quantify the geometric pattern of the system. One use of the model is, for instance, to see how the geometric patterns and their associated statistical parameters compare at a given volume fraction when the fiber length is fixed or when fiber length can be modeled as a normal, a uniform, or another specific distribution.
Room temperature aging of Narmco 5208 carbon‐epoxy prepreg. Part I: Physicochemical characterization
Authors: Cole K. C. , Noël D. , Hechler J.‐J. , Chouliotis A. , Overbury K. C.
Published: December 2004

Samples of Narmco Rigidite 5208/WC3000 carbon‐epoxy prepreg were exposed to ambient temperature (22°C) and 50% relative humidity for different periods up to 66 days. They were analyzed in depth using various techniques to determine the extent of the chemical changes occurring. The physicochemical techniques used were Fourier transform infrared spectroscopy (transmission, attenuated total reflection, diffuse reflection), liquid chromatography (reverse‐phase, high‐speed reverse‐phase, and high‐performance size‐exclusion), thermal analysis (differential scanning calorimetry and thermogravimetric analysis), and pyrolysis‐gas chromatography. All showed evidence of significant changes, the most sensitive being Fourier transform infrared spectroscopy (FT‐IR) and reversephase liquid chromatography (RPLC). FT‐IR showed that the number of unreacted epoxy groups decreases steadily at a rate of 0.34% per day, based on the initial amount. At the same time, the number of free amine‐hardener molecules, as monitored by RPLC, drops at a rate of 1.05% per day. RPLC also showed that the amount of initial epoxy‐amine reaction product increases significantly over the first 30 days, but then declines as it undergoes further reaction to give higher‐molecular‐weight products. The heat of polymerization of the resin, measured by thermal analysis, decreased by 0.26% per day from its initial value of 561 J/g.
Low temperature relaxation of DGEBA epoxy resins: A thermally stimulated discharge current (TSDC) study
Authors: Pangrle S. , Wu C. S. , Geil P. H.
Published: December 2004

The effect of curing conditions on the low temperature relaxation behavior of catalyst‐cured epoxy systems based on digliycidyl ether of bisphenol A (DGEBA) has been characterized by the thermally stimulated discharge current (TSDC) technique. In these chemically relatively simple epoxy systems, five relaxation processes, designated as γ, β, β′, βOH, and β″, have been observed: Their molecular origins are discussed in detail. The results are in agreement with prior suggestions of an inhomogeneous cross‐link density morphology.
Failure of composite laminae under biaxial loading
Authors: Neale K. W. , Labossière P.
Published: December 2004

The failure of thin, fiber‐reinforced composite laminae is considered. A parametric failure criterion applicable to plane stress loading conditions is presented. Both static and fatigue‐type loadings are treated. In general, the formulation of failure criteria for composite materials has closely followed the development of yield criteria in metal plasticity. The parametric failure criterion presented here was inspired by a general formulation recently proposed for the plastic yielding of sheet metals. The criterion satisfies the essential conditions that must be met in developing phenomenological strength criteria for composite materials. This entirely general formulation encompasses all previously proposed failure theories for fiber‐reinforced orthotropic laminae. The inherent advantages and drawbacks of the proposed parametric approach are discussed. An example of application for static loading is presented in detail. Possible extensions of the approach to fatigue‐type loading are also suggested.
Numerical and experimental evaluation of the Mode III interlaminar fracture toughness of composite materials
Authors: Becht G. J. , Gillespie J. W.
Published: December 2004

The Crack Rail Shear (CRS) specimen is a proposed test method to characterize the interlaminar Mode III critical strain energy release rate (GIIIc) of continuous fiber‐reinforced composite materials. The specimen utilizes the two rail shear test fixture and contains embedded Kapton film between designated plies to provide a starter crack for subsequent fracture testing. Analytical expressions for specimen compliance and GIII are based upon Strength of Materials (SM) principles. The model identifies important material and geometric parameters and provides a simple data reduction scheme. A quasi‐three‐dimensional, linear elastic finite element stress analysis verifies the purity of the Mode III fracture state and identifies admissible crack lengths to be used in the experimental study. A fully three‐dimensional linear elastic finite element analysis of the CRS is employed to investigate the influence of edge effects on the fracture state for the finite length sample. Results based upon a uniform crack extension indicate a small region of mixed mode behavior at traction free edges which decay to a pure Model III fracture state in the interior of the sample. Furthermore, the GIII distribution along the crack front decreases at the free edges from a maximum plateau region in the interior. The three‐dimensional analysis allows edge effects to be minimized by selecting appropriate specimen lengths. Compliance and strain energy release rates are in good agreement with the SM model. An experimental program was performed to measure GIIIc of two graphite epoxy systems. GIIIc results for AS4/3501‐6 were found to be 1.6 times the Mode II fracture toughness, while IM7/8551‐7 exhibited equivalent Mode II and Mode III fracture toughnesses. Mode III fracture surfaces revealed microstructural deformations characteristic of Mode II fracture.
The effect of processing variables on microstructure of injection molded short fiber reinforced polypropylene composites
Authors: Singh P. , Kamal M. R.
Published: December 2004

Injection molded composites of polypropylene reinforced with short glass fibers were obtained under a variety of injection molding conditions. The microstructure of the moldings was determined using a variety of experimental techniques, including optical and scanning electron microscopy, differential scanning calorimetry, Fourier transform infra‐red spectroscopy, and thermogravimetry. Thus, it was possible to obtain a detailed characterization of the crystallinity, morphology, and orientation distribution in the matrix, in addition to the distribution of fibers and their orientation of the fibers in the composite. The influence of molding conditions on the above microstructural characteristics is summarized in an effort to explain the experimental observations.
Effect of60Co gamma radiation on the mechanical properties of epoxy blends and epoxy‐graphite fiber interface
Authors: Netravali A. N. , Manji A.
Published: December 2004

The effect of60Co gamma radiation of up to 100 Mrads on an IM6‐G graphite fiber‐epoxy interface was studied using the single‐fiber‐composite (SFC) technique. Flexible epoxy blends were formulated using diglycidylether of bisphenol‐A (DGEBA) based and polyglycol diepoxide epoxies which were cured with aliphatic and aromatic curing agents. Bulk epoxy specimens and graphite fibers were tension tested to obtain their tensile properties. The fragment length distribution from SFC tests, single fiber strength data, and a Monte Carlo simulation of Poisson/Weibull model for fiber strength and flaws were used to obtain the effective interfacial shear strength values. The results indicate that while graphite fiber strength is not affected by radiation, the tensile properties of the epoxies used are adversely affected by the radiation. The interfacial shear strength, however, increases significantly with the radiation dose. The study also supports the earlier results of many workers that the interfacial shear strength for flexible epoxies is much higher than the shear yield strength of the epoxies.
Kinetics of strain‐induced crystallization during injection molding of short fiber composites of poly(ether ether ketone)
Authors: Mishra Ajit Kumar , Schultz Jerold M.
Published: December 2004

Crystallization kinetics of short glass and carbon fiber composites of poly(ether ether ketone) (PEEK) under melt‐strain conditions have been obtained for the first time, using in‐situ wide angle X‐ray scattering, and have been correlated to a model based on the Avrami equation in order to enable minimization of the processing time for injection molding of these materials. It has been demonstrated that increased flow rate of the melt in the mold and, consequently, increased shear rate accelerates the crystallization process of PEEK composites, analogous to similar trends observed previously in PEEK resin. Short glass fiber composites of PEEK crystallize slower than the resin under identical processing conditions, while short carbon fiber composites crystallize faster than the resin, except at the highest mold temperatures and the lowest flow rates. A model based on the Avrami equation has been proposed to fit the kinetics data obtained experimentally. The Avrami coefficient has been calculated and Arrhenius plots have been used to predict the crystallization kinetics at temperatures lower than those at which experimental data have been obtained here. Fiber orientation, flexural elastic modulus, and flexural fracture toughness of the composites have also been evaluated.
On the dynamic mechanical behavior of poly(methyl methacrylate) reinforced with Kevlar fibers
Authors: Calleja R. Díaz , Ribelles J. L. Gómez , Pradas M. Monleón , Greus A. Ribes , Colomer F. Romero
Published: December 2004

The dynamic mechanical relaxation behavior of poly(methyl methacrylate) reinforced by continuous parallel Kevlar‐49 fibers is investigated here on samples with several fiber volume fractions. The shifts in the temperature of the main relaxation of the matrix are interpreted according to free volume considerations and with the help of a thermomechanical block model. The dependence of the storage and loss moduli both on temperature and fiber content found experimentally can be reproduced by the model. It is not necessary to rely on the existence of an interphase to account for the modifications evidenced by the spectrum of the matrix, which can be explained on the basis of the two phase model.
In‐situ acoustic emission monitoring during electron microscopy of a model composite system
Authors: Siegmann A. , Kander R. G.
Published: December 2004

Three‐point‐bend testing of a continuous unidirectional glass reinforced unsaturated polyester composite was chosen as a model to study the acoustic emission (AE) response and associated damage events in composite materials. Conventional three‐point‐bend samples were tested over a wide range of span‐to‐depth ratios, with failure modes established by post‐failure scanning electron microscope (SEM) analysis. In addition,in‐situAE monitoring with simultaneous SEM recording was undertaken during three‐point‐bend testing inside an SEM vacuum chamber. These two test methods, using different equipment and sample geometry, were found to yield similar mechanical and AE results. Thein‐situtests enabled the development of the various stages of damage to be observed while the AE response was simultaneously monitored. This enabled the AE signals to be unambiguously assigned to specific damage accumulation mechanisms. It was concluded that the AE amplitude distribution can be divided into three main ranges in this system: 60 to 65 dB, 75 to 85 dB, and larger than 90 dB corresponding to matrix damage, fiber‐matrix debonding, and fiber breakage, respectively. It has been demonstrated that the coupling of AE monitoring with SEM recording of a composite system undergoing mechanical loading is a very powerful technique in the study of damage accumulation in advanced composite materials.
The influence of processing on fiber orientation and creep in short carbon‐fiber reinforced low density polyethylene and polycarbonate
Authors: Bertilsson H. , Franzén B. , Klason C. , Kubát J. , Kitano T.
Published: December 2004

This paper is concerned with the creep behavior of short fiber‐reinforced thermoplastics, especially with regard to the role of fiber orientation. Rectangular samples of low density polyethylene (LDPE) and polycarbonate (PC) containing varying amounts of carbon fibers were prepared by compression and injection molding. The materials were compounded using a technique producing a concentration independent fiber length distribution. The orientation distribution, on the other hand, was found to be strongly influenced by fiber concentration. The creep parameters were measured for both LDPE and PC. The contraction ratio was determined for the PC samples. In the case of LDPE reliable data could not be obtained due to the low modulus of the matrix. The creep properties of the PC and LDPE samples varied significantly with the orientation of the fibers. The creep strain was measured as a function of time for both polymers for different fiber concentrations and orientations. The predictions of the Halpin‐Tsai equation underestimated the experimental strain figures somewhat when the stress direction coincided with that of the fiber orientation. When the stress acted across the fibers the theoretical and experimental results showed satisfactory agreement. A plausible explanation is that incorporation of carbon fibers changes the morphology of the matrix material. We conclude that the contraction ratio is an important measure of the volume and its changes during deformation, especially with regard to its relation to the free volume and similar quantities. We have shown that the necessary data can be obtained, in spite of experimental difficulties even for anisotropic samples.
Mechanism of damage and failure in an orthotropic glass/polypropylene composite
Authors: Kander R. G. , Siegmann A.
Published: December 2004

The mechanism of damage accumulation and failure in thin sheets of an orthotropic long discontinued glass fiber reinforced polypropylene composite was studied using optical and electron microscopy coupled with acoustic emission (AE) monitoring. The effect of strain rate (0.1 to 100 percent) was investigated, and a close correspondence was established between AE response and microscopic observations. Furthermore, simultaneous AE monitoring and optical microscopy performed during sample deformation substantiated the assignment of certain AE amplitude ranges with specific failure events in the composite. Specifically, the failure process was found to be very inhomogeneous in nature, and could be divided into three main stages of damage accumulation. Finally, cumulative AE energy curves were found to follow an interesting powerlaw behavior. Results based on the coupling of AE methods with optical and electron microscopy were found to be self consistent and in agreement with previous reports on thicker‐section composites manufactured from the same materials.
Cure kinetics of dicyanate matrix polymers
Authors: Zeng Shangying , Ahn Kyujong , Seferis James C. , Kenny J. M. , Nicolais L.
Published: December 2004

The cure kinetics of two dicyanate resins were investigated by differential scanning calorimetry under isothermal and nonisothermal conditions. An autocatalytic kinetic model was proposed and a set of general kinetic parameters was calculated from the isothermal experiments. Good agreement between experimental data and the kinetic model has been obtained under different processing conditions, which demonstrated the validity and the usefulness of the analytical procedure and of the kinetic expression employed. Consequently, in its current form, the modeling methodology is capable of describing the degree of cure during a typical processing cycle and has the potential for comparing different resin formulations as well as the manifestations of cure in property development. Specifically, in this work the modeling methodology was tested using two distinct dicyanate resin systems while the development of the cure was also observed through dielectric analysis (DEA).
Friction and wear bahavior of short fiber‐reinforced poly(amide‐imide) composites
Authors: Lee Kee H. , Youn Jae R.
Published: December 2004

Tribological behavior of short fiber‐reinforced thermoplastic composites was investigated experimentally and theoretically. Short carbon fiber and glass fiber reinforced poly(amide‐imide) composites were tested. Titanium oxide powder‐filled composite was also tested for comparison with the fiber composites. Block‐on‐ring type wear testing was performed for 24 h at three different sliding conditions. Frictional force was measured and stored by a data acquisition system and wear was measured as weight loss after the test. Wear tracks on the specimen and the counterface were examined with an optical microscope to observe fiber damage and formation of wear film. The equivalent stress distribution around each fiber at the sliding surface was calculated by employing a finite element program. The lowest friction and wear was obtained for the carbon fiber composite, the highest friction for the glass fiber composite, and the highest wear for TiO2‐filled one. It was observed that the glass fibers are damaged and removed from the surface more easily than the carbon fibers, and the finite element analysis also suggests easier debonding of glass fibers.
Fiber mat deformation in liquid composite molding. II: Modeling
Authors: Han K. , Lee L. James , Liou Ming
Published: December 2004

Mathematical models are proposed to simulate the flow induced fiber mat deformation during liquid composite molding. The fiber bed is treated as an elastic beam and the load acting on the bed causes its deformation. The lubrication approximation is used to simplify the resin flow equation in the fiber free region, while Darcy's law is used to calculate the pressure and velocity fields in the deformed fiber bed. The governing equations are solved using the control volume/finite element method. The numerical results show reasonable agreement with the experimental results from Part I.
Electrical degradation of blends and laminar composites of polyethylene and polystyrene
Authors: Gustafsson A. , Salot R. , Gedde U. W.
Published: December 2004

Physical blends and laminar composites of polystyrene (PS) and polyethylene (PE) were prepared with and without a tri‐block copolymer compatibilizer (poly(styrene‐block‐(ethylene‐co‐1‐butene)‐block‐styrene)), denoted SEBS. Dielectric strength and breakdown time of samples exposed to external partial discharges (PD) were measured and relationships were sought with composition, phase morphology, and phase bonding. The polymers were selected because they are incompatible and because the dielectric strength and stability towards PD are greater for PS than for PE. It was shown by adding small quantities of SEBS that a minimum amount of cohesive forces between the components was required to achieve good electrical properties. The major morphological factor controlling the electrical stability was which polymer constituted the continuous phase. Laminar composites with continuous sheets of stable PS as a top layer exhibited a stability equal to or better than that predicted from the upper bound rule of mixing.
Micromechanics prediction of the transverse tensilc strength of carobn fiber/epoxy composites: The influence of the matrix and interface
Authors: Blackketter D. M. , Upadhyaya D. , King T. R.
Published: December 2004

Currently, there is great interest in understanding and improving the bond between the fibers and matrix in high performcance composite materials. In many recently developed systems, fiber surface treatments have been developed to improve poor bonding. These treatments are often evaluated by measureing their effect on a composite property sensitive to the interfacial bond strength, typically the composite shear strength. This paper presents an evaluation of the influence of the matrix and interface properties on the transverse tensile strength. These effects were quantified by compring transverse flexural experimental data with results from a finite element micromechanics model. The results indicate that the transverse tensile strength is significantly more dependent upon sizing than is the shear strength. Finally, the transvere flexure test appears to provide an additional and complementary test for evaluating interface bond characteristics.
The effect of pre‐carbonization on the properties of PAN‐based carbon fibers
Authors: Ko Tse‐Hao , Day Tzyy‐Chin
Published: December 2004

A continuous stabilization and two‐stage carbonization process was used to prepare polyacrylonitrile (PAN)‐based carbon fibers, The effect of pre‐carbonization (300 to 550°C) on the final properties and microstructure of carbon fibers was measured. Experimental results using an X‐ray diffractometer indicated the presence of a less ordered structure at 2Θ from 5 to 18° in the pre‐carbonized fibers and the final carbon fibers. This study found that the pre‐carbonization process strongly affects the microstructure of the resulting carbon fibers. The results also showed that a suitable pre‐carbonization was very conducive to improvement in tensile strength or in Young's modulus of the final carbon fibers. When the final carbon fiber was pre‐carbonized at 300 and 550°C, respectively, these fibers had a higher tensile strength and higher Young's modulus than carbon fibers pre‐carbonized at other conditions.
Physical modeling of the interphase in amorphous thermoplastic/glass bead composites
Authors: Alberola N. , Bergeret A.
Published: December 2004

Mechanical spectrometry was performed on polystyrene/glass bead and poly (styrene‐co‐methacrylic acid)/glass bead composites to test for the influence of the interphase on molecular mobility. To quantify these motion changes, a physical model was developed to predict the deformation of amorphous materials near Tg. Before applying this model, the dynamic mechanical behavior of the two phases (matrix and inclusions) must be separated to remove the reinforcement effect of the fillers and to keep only the physical crosslinking effect of the amorphous phase by the inclusions. It was shown that (i) the fillers can induce the nation of specific interactions, which act as physical ties, reducing molecular mobility and that (ii) the decrease in molecular motion with increasing filler content is enhanced for fillers coated by a silane coupling agent.
The application of fracture mechanics to short fiber composites
Authors: Huang Donald D.
Published: December 2004

The fracture performances of three short glass fiber‐reinforced polymer composite systems, PET with 30 wt% glass, nylon 6/6 with 33% glass fibers, and a nylon 6 and 6/6 copolymer with 33% glass, have been characterized using both standard mechanical characterization and fracture mechanics. These results have been compared to fracture tests of an experimental chair base. None of the characterizing techniques was successful in predicting the chair base fracture performances of the materials when the tests were conducted on standard 3‐mm‐thick test specimens. When larger specimens with comparable morphologies to the chair base were tested, the fracture mechanics tests compared favorably to the chair base fracture tests while the tensile test results were inconsistent in their predictive ability. The findings emphasize the importance of testing laboratory specimens that are similar in morphology to the final part and suggest that fracture mechanics methodologies can be used in materials selection of glass reinforced systems for structural applications. However, for quantitative design, it is necessary to address issues regarding the nature of the inherent flaw in the finished parts.
The effects of stress level on steady crack growth in an epoxy reinforced with long aligned glass fibers
Authors: Botsis J. , Caliskan A. , Beldica C.
Published: December 2004

Results are reported of fatigue crack propagation experiments on an epoxy reinforced with long aligned glass fibers. The composite was prepared in such a way that fiber spacing was approximately the same in each specimen and the reinforcing fibers were sufficiently stronger than matrix. For a number of experiments with the same fiber spacing, fatigued under various stress levels, the crack speed as well as the rate of debonding reached steady values, i.e., independent of the crack length. In addition, the evolution of debonding followed a self‐similar growth pattern. The data implied that the applied load was a controlling parameter of the steady growth. Within the resolution of observations, no fiber fracture was observed in the bridging zone. Fiber debonding seemed to be the dominant mechanism of energy dissipation. Moreover, the crack front was not straight. Instead, it consisted of two branches growing on the specimen's surfaces first and then through the thickness of the specimen with a highly curved front. Stress intensity factor calculations showed that when the fibers in the bridging zone were under a uniform load, the total stress intensity factor was proportional to \documentclass{article}\pagestyle{empty}\begin{document}$ \sigma \sqrt \lambda $\end{document} (where σ is the applied stress and λ is the fiber spacing) and constant within the regime of steady crack growth. The steady values of crack growth and rate of debonding were correlated with the stress level, spacing, and fiber radius. The resulting power equations were found to have the same exponent.
Anisotropy of electrical conductivity and structure in polymer‐carbon fiber composite materials
Authors: Nascimento J. Fernandes , Ezquerra T. A. , Baltà‐Calleja F. J. , Seferis J. C.
Published: December 2004

Electrical conductivity for polyether and polyimide‐carbon fiber composites was measured using different fiber types. The microstructure was characterized by X‐ray diffraction methods. D.C. conductivity measurements were carried out in directions perpendicular, parallel, and at varying angles to the fiber axis. The influence of the distance between electrodes on the conductivity was examined. The dependence of the frequency and the temperature upon conductivity has been, likewise, investigated. Experimental results support the concept of a conduction mechanism in the composites occurring in the carbon fibers, which built up a highly interconnected conducting network. It is also shown that the high degree of fiber orientation is consistent with the conspicuous anisotropic behavior of the electrical conductivity.
An approximate method for the determination of poisson's complex ratio in harmonic viscoelastic behavior
Authors: Agbossou Amen
Published: December 2004

To improve the simulation of viscoelastic behavior of composites, an approximate and incremental method for the determination of Poisson's complex ratio of the polymer matrix is proposed. This method is based on the fact that many polymers exhibit a slight variation of their bulk modulus throughout, at their main relaxation temperature. We examine different parameters that affect the proposed method. It is shown that (i) the Poisson's complex ratio of the matrix and the complex modulus of the composite depend significantly on the initial values of the incremental method and (ii) the Poisson's complex ratio of the matrix allows for the effects of filler content on the magnitude of the mechanical relaxation.
Modification of pitch‐based carbon fibers using a nickel‐catalyzed oxidation treatment: Effect of treatment on fiber‐matrix interfacial shear strength
Authors: Rearick B. K. , Harrison I. R.
Published: December 2004

Pitch‐based carbon fiber surfaces were modified using a nickel‐catalyzed, dry oxygen etch (NCDO) and a dry oxygen etch (DO), both of which were applied using a continuous process. The treatments differed in that DCDO produced preferential pitting on fiber surfaces at nickel particle sites. Tensile strengths of all etched fibers were slightly lower than values for untreated, or “as received” (AR), fibers. Reductions in tensile strength were independent of the type of treatment administered, indicating that most degradation resulted from damage produced by the mechanics of the continuous process, not the etching treatments. Fiber‐matrix interfacial shear strengths (IFSS) of AR and etched fibers were evaluated using the Microbond test. At first glance, Microbond test data indicated that NCDO produced large increases in IFSS. However, inconsistencies in the data raised questions regarding the validity and usefulness of the Microbond test when applied to this system. Only after detailed analysis of the data for this series of fibers was it concluded that NCDO did indeed produce an increase in IFSS over AR fibers and DO fibers.
The influence of pre‐carbonization on the properties of PAN‐based carbon fibers developed by two‐stage continuous carbonization and air oxidation
Authors: Ko Tse‐Hao , Li Chien‐Hung
Published: December 2004

The Properties of three kinds of carbon fibers, which were pre‐carbonized at 500, 550, and 600°C during two‐stage continuous carbonization, were measured after being air oxidized for periods of 1 to 6 min at 550°C. The effects of precarbonization temperatures on mechanical properties, density, morphology, elemental compositions, and microstructure of the carbon fibers are discussed. The pre‐carbonization process affected strongly the surface properties and mechanical properties of the final carbon fibers, as measured after air oxidation. Carbon fibers measured one to six min after air oxidation showed a different oxidation behavior in the surface morphology for each pre‐carbonization temperature. Optimum conditions not only improved the tensile strength and modulus by over 50%, but also increased the density and oxygen content.
Resin flow analysis in the consolidation of multi‐directional laminated composites
Authors: Young Wen‐Bin
Published: December 2004

Advanced fiber reinforced polymer composites have been increasingly used in various structural components. One of the important processes to fabricate high performance laminated composites is an autoclave assisted prepreg lay‐up. Since the quality of laminated composites is largely affected by the cure cycle, selection of the cure cycle for each application is important and must be optimized. Thus, some fundamental model of the consolidation and cure processes is necessary to properly select the suitable parameters for each application. This study applied the theory of consolidation and flow in a porous medium to provide a general model for the three‐dimensional consolidation process of the laminates with fibers reinforced in multi‐directions. Based on the model analysis, one can predict the pressure, velocity, and laminate thickness during consolidation process, which, as coupled with the curing analysis, can be used to properly select the cure cycle for applications of laminated composites.
Molecular structure effect of thermotropic liquid crystalline polymers on interfacial adhesion with polycarbonate
Authors: Kwon Soon Kil , Chung In Jae
Published: December 2004

The effect of molecular structure of thermotropic liquid crystalline polymers (TLCP) on the interfacial adhesion with polycarbonate (PC) was investigated by varying the flexible spacer length in the main chain of the TLCP. The interfacial adhesion was estimated by two methods: (1) the debonding stress from the melt‐contacted TLCP/PC plates was measured by a modified pin‐pull test; (2) the mechanical response of the melt‐mixed blend was analyzed by the composite theory. Both methods indicated that the interfacial adhesion of the TLCPs with flexible spacer was improved over that of a rigid TLCP (i.e., Vectra A950), and that it was further enhanced by an increase in the length of a flexible unit.
Composites based on drawn strands of thermotropic liquid crystalline polymer reinforced polypropylene
Authors: Sabol E. A. , Handlos A. A. , Baird D. G.
Published: December 2004

This paper is concerned with the use of two thermotropic liquid crystalline polymers (TLCPs), HX1000 and Vectra B950, to reinforce a thermoplastic matrix of polypropylene (PP). The goal was to pregenerate the optimal TLCP reinforcement in PP and then process the material at a lower temperature than the melting point of the TLCP to form a composite structure. Specifically, strands of the blend were produced using a dual extrusion process, which resulted in the formation of axially continuous TLCP fibrils within the PP matrix. It was found that the mechanical properties of the strands were greatly improved by increased draw ratio and that optimal reinforcement, as predicted by the rule of mixtures, could be achieved. Initial studies indicated that injection molding and sheet extrusion of the pelletized strands caused the TLCP phase to agglomerate and deform, which resulted in a reduction of the mechanical property enhancement. However, the TLCP fibrillar morphology in the pregenerated strands was maintained during compression molding, which resulted in uniaxial composites with properties equal to or greater than properties of the strands. In addition, composites were made using compression molding in which strands were randomly oriented prior to consolidation to show the limits of properties possible in composites produced from the pregenerated strands. It was found that this process could be used to produce composites in which the mechanical properties were isotropic in the plane of the sample and approached the properly limits predicted by composite theory. Additionally, it was found that many of the mechanical properties of the VB/PP materials were greatly enhanced by the addition of a maleated PP throughout the composite forming process.
Statistical aspects of infinite network formations
Authors: Saito Osamu
Published: December 2004

Infinite network formation is discussed using the theory of molecular size distribution functions. The necessary and sufficient conditions of infinite network formation in simultaneous crosslinking and main chain scissions are reviewed. Also, the condition of endlinking associated with main chain scissions is outlined; the condition of endlinking of special terminal groups is given. Infinite network. formation in condensation as well as addition polymerizations is studied. It is shown that the existence of branch units in condensation polymerization is only the necessary condition of infinite network formations, whereas the sufficient condition is related to the form of branch units; the necessary condition in addition polymerizations is the simultaneous occurrence of chain transfer to polymer and chain termination by recombination.
Barrier and surface properties of polyurethane‐epoxy interpenetrating polymer networks. II
Authors: Frisch H. L. , Foreman R. , Schwartz R. , Yoon H. , Klempner D. , Frisch K. C.
Published: December 2004

We have measured the advancing contact angles of drops of methanol‐ethylene glycol mixtures on films of previously studied polyurethane‐epoxy interpenetrating polymer networks. The extrapolated critical surface tensions were in excellent agreement with those obtained from advancing contact angles of drops of water‐methanol mixtures. A sharp minimum is observed in the critical surface tension at network compositions where we have previously found maxima in ultimate mechanical properties. We advance a physical explanation based on unrelieved surface strains. We have also measured the toluene vapor transmission (permeability, diffusion and sorption coefficients) in these films. These results, together with the previously obtained water vapor permeabilities, are in complete accord with the expected morphologies of these networks.
Origin of moisture effects on crack propagation in composites
Authors: Mandell J. F.
Published: December 2004

A study has been made of the origin of unexpected moisture effects on crack extension in fiberglass laminates. Water immersion has been found to greatly reduce the rate of crack growth under constant loading while increasing the rate under cyclic loading, the latter effect being the expected one. Observations were made of the extension of the stable damage zone at the tip of precut notches in wet and dry environments. The damage zone size is postulated as a critical element in the relaxation of high stress concentrations in composites, such as those at notch or crack tips. Under constant load, moisture is shown to greatly expand the interply delamination region in the damage zone, thus reducing the local fiber stresses and increasing crack resistance. Under cyclic loading moisture has little effect on the delamination region, which is large even for dry environments, and the only effect is weakening of the material and acceleration of cracks. Severe hygrothermal conditions can so weaken the material that the crack resistance is reduced under constant loading as well.
Strain‐induced crystallization, Part III: Theory
Authors: Yeh G. S. Y. , Hong K. Z.
Published: December 2004

A nucleation theory for strain‐induced crystallization is formulated to explain and to predict the effects of molecular strain on crystallization kinetics and crystallite size. Unlike any current theories that have based their formulations on some assumed extended‐chain line nuclei or folded‐chain crystals, the present theory avoids all assumptions concerning the crystal morphology. It is based on experimental findings which indicate limited crystal growth in the strain direction, following a reciprocal dependence of crystal thickness on supercooling ΔT. (ΔT = T, − T, where the equilibrium melting temperature, T, is a variable dependent on degree of molecular strain prior to strain‐induced crystallization.) It is predicted that the logarithm of the nucleation rate, No, is dependent on (T)2/T(ΔT) or T/T(ΔT), and that the critical nucleus thickness l*o is shown to be proportional to T/ΔT. In addition, expressions are also presented, including examples, to show the dependence of No, l*o and Tom on degree of molecular strain, ϵ, or melt entropy reduction, Δs′. Our analysis predicts that, on comparing a polyethylene crystallized in the presence of strain to one crystallized in the absence of strain at 130°C, an increase in “coil” dimension of less than about 50 percent can bring about a 104 fold increase in heterogeneous nucleation rate, a 30–40 percent reduction in critical nucleus thickness and a 10°C increase in equilibrium melting temperature. These results will be discussed and compared with available experimental evidence.
Ultradrawing of thermoplastics by solid state coextrusion illustrated with high density polyethylene
Authors: Zachariades Anagnostis E. , Griswold Phillip D. , Porter Roger S.
Published: December 2004

Ultra‐thin films of high density polyethylene of high orientation have been produced by the recently developed technique of solid state coextrusion. The films were prepared under moderate conditions, without lubricant in continuous lengths by extruding through conical dies of extrusion draw ratio up to 36. This is a draw ratio higher than achievable by conventional solid state extrusion at comparable processing conditions through slit dies. The ultra‐thin films of high orientation were transparent and exhibited dead bend. The physical and mechanical properties were evaluated and compared with the properties of the same high density polyethylene extruded through a slit die. The increase in the melt point, crystallinity, tensile modulus, and birefringence indicates that the method is very efficient for the production of ultra‐thin and highly oriented films. An experimental technique is also presented for preparing billets of controlled and uniform initial morphology and free of voids.
Application of the ‘dual sorption’ model to drug transport through skin
Authors: Chandrasekaran S. K. , Campbell P. S. , Watanabe T.
Published: December 2004

The dual sorption theory has been extended to the transport of drug molecules through human skin in vitro. By assuming that sorption of drug molecules occurs by both dissolution and binding of drug to immobile sites in the skin, the experimental sorption isotherm can be predicted, and the disparity between steady state and time lag diffusivities can be reconciled. Furthermore, the dual sorption model has been used to develop techniques for controlling these sorption transport processes in order to rapidly achieve predictable transdermal drug delivery in vivo.
A comparative study of the tensile and charpy impact tests from a fracture mechanics viewpoint
Authors: Newmann L. V. , Williams J. G.
Published: December 2004

Charpy and tensile impact tests have been conducted on two materials poly(methyl methaerylate) (PMMA) and polycarbonate. It is shown that by adopting a fracture mechanics approach, it is possible to measure meaningful values of material toughness, and that the results obtained in bending are consistent with those in tension. A discussion of the assessment of “kinetic energy” errors is also presented.
Sensitivity of diazoquinone resists to optical and electron‐beam exposure
Authors: Kaplan Michael , Meyerhofer Dietrich
Published: December 2004

This paper compares the response of resists consisting of novolak resin and diazoquinone sensitizers (inhibitors) to optical and electron‐beam radiation. In both cases, irradiation destroys the sensitizer leading to an increased rate of solubility in alkaline developer. For optical exposure in the near‐uv region, the light is absorbed only by the chromophore sites and a unique quantum efficiency of 0.30 ± 0.03 can be measured for a range of wavelengths and sensitizers (10.5 ± 1.0 eV/bleached site). In the case of electron‐beam exposure, the initial rate of bleaching in pure sensitizer implies that 7.6 ± 0.6 sites are destroyed per 100 eV absorbed (13 ± 1 eV/molecule). In mixtures of sensitizer and resin (i.e., resists), this inherent sensitivity, or G‐value, based on the calculated absorption by the sensitizer only, increases by almost a factor of 2 as the proportion of resin is increased. While the electron and optical exposure both destroy the sensitizer center, absorption spectra and solubility properties show that the end products of the chemical reactions are not the same in the two cases. In addition, high doses of electrons produce cross‐linking and insolubilization of the resin.