bluuebunny/crossref_metadata_2025_split huggingface dataset:
Practical guidelines for predicting steady state cure time during sheet molding compound (SMC) compression molding
Authors: Abrams Lisa M. , Castro Jose M. 
Published: April 2004

AbstractThe longest part of the molding cycle during SMC compression molding is the curing stage. Thus it is extremely important to be able to predict its duration to estimate the cost of manufacturing a new part. During an SMC molding cycle, the mold surface temperature drops suddenly when it contacts the cold charge. The surface temperature then gradually recovers as heat is conducted from the interior of the mold and the resin releases heat during curing. In general, this exchange of heat remains locally unbalanced, causing a gradual decrease in the local surface temperature. To avoid blistering, the cure time must be increased with consecutive moldings until a steady state value is achieved (tcss). In this paper, we present a series of charts that can be used to estimate the steady state cure time for new parts. These values can then be used to estimate the manufacturing cost.
Effect of resin crosslink density on the impact damage resistance of laminated composites
Authors: Lesser Alan J. 
Published: April 2004

AbstractIt is generally recognized that fiber‐reinforced laminated composites are susceptible to damage resulting from low‐velocity impacts. Over recent years, many strategies have been devised to increase the fracture toughness of resin matrix materials with the aim of improving the composite's overall resistance to impact damage. One popular strategy for enhancing the fracture toughness of thermosets involves increasing the molecular weight between crosslinks, which, in turn, enhances the resins ductility. In this paper, we investigate the efficiency of this toughening approach with regard to resisting damage in composite laminates subjected to low‐velocity impacts. A mechanistic study shows that at least two distinct processes occur during an impact event. First, the laminate experiences a local failure, which resembles a Hertzian fracture process followed by subsequent delamination between the plies. Hertzian fracture occurs once at a critical threshold level initiates laminate damage through the development of a spatially configured array of matrix microcracks, which resemble that of a Hertzian cone together with radial cracks. Further damage accumulates in the laminate by inter‐ply delamination with the size of delaminated area increasing coincident with the impact load. Systematic changes in resin crosslink density show that both damage initiation and accumulation are affected. However, the maximum resistance for damage initiation occurs at a much higher crosslink density than that measured for damage accumulation.
Influence of reinforcing continuous carbon fibers on the viscoelastic properties of the epoxy matrix
Authors: Raghavan J. , Meshii M. , Feng D. 
Published: April 2004

AbstractThe influence of reinforcing continuous carbon fibers (AS4) on the creep and fracture of the epoxy (3501–6) matrix was studied by dynamic mechanical thermal analysis test, creep test and constant strain rate test. While the glass transition temperature (Tg) of the epoxy matrix (512 K) was increased by 5–6°K by the reinforcing fibers, the activation energy for creep/relaxation was the same for both the epoxy and its composites except off‐axis composites, for which the activation energy was higher. An increase in the instantaneous and rubbery moduli and a reduction in tanδmax indicate that the reinforcing fibers stiffen the matrix and reduce the extent of viscous deformation in the epoxy matrix. This is corroborated by the creep results that recorded a reduction in the creep rate and the magnitude of creep of the pure epoxy due to the reinforcing fibers. A consequence of this reduction in energy dissipation by viscous deformation, in the epoxy matrix due to the reinforcing fibers, is a decrease in the toughness (i.e. the total fracture energy) of the epoxy matrix measured by constant strain rate test.
Modeling the imidization kinetics of AFR700B polyimide
Authors: Russell John D. , Kardos John L. 
Published: April 2004

AbstractAFR700B imidization was monitored using Fourier transform infrared spectroscopy (FTIR) by following the increase in the carbon‐nitrogen bond of imide groups displayed by the 1360 cm−1 band over several isotherms. Imidization occurred until ∼300°C (572°F). This was confirmed by the presence of water up to 300°C (572°F) in thermogravimetric analysis‐mass spectroscopy (TGA‐MS) data. First, reaction kinetics were described by first‐order kinetics with a fast and slow region of reaction. However, this model did not accurately describe the data in the transition from the fast to the slow reaction region. Imidization is kinetically controlled at short times. However, when the monomers rapidly react to form a high molecular weight polymer gel, imidization is controlled by diffusion. Therefore, a second‐order kinetic/diffusion model was used to describe the imidiation kinetics. This resulted in a substantial improvement in the fit of the data. The kinetic/diffusion model can also be used to describe imidization in polyimides other than AFR700B.
Mechanical properties of wood‐fiber/toughened isotactic polypropylene composites
Authors: Park Byung‐Dae , Balatinecz John J. 
Published: April 2004

AbstractThis study investigates the mechanical properties of wood‐fiber/toughened PP composite modified by physical blending with an EPDM rubber to improve impact toughness. Wood‐fiber thermoplastic composites were prepared with a modified PP matrix resin, employing high shear thermokinetic compounding aided with maleated PP for the fiber dispersion. The addition of EPDM improved the impact toughness, while it reduced stiffness and strength properties. To compensate the non‐plane strain fracture toughness, the specimen strength ratio (Rsb) was adopted as a comparative measure of fracture toughness. The strength ratio increased with the addition of EPDM, while it decreased with increasing wood‐fiber concentration. The work of fracture increased with EPDM level except at large wood‐fiber concentration. The effectiveness of the impact modification was assessed with the balance between tensile modulus and unnotched impact energy as a function of wood‐fiber concentration. EPDM rubber modification was moderately effective for wood‐fiber PP composites. The examination of fracture surfaces showed twisted fibers, fiber breakage, and fiber pull‐out from the matrix resin.
Flow near the permeable boundary of an aligned fiber preform: An experimental investigation using laser doppler anemometry
Authors: Gupte Sunil K. , Advani Suresh G. 
Published: April 2004

AbstractThe flow of fluid at the interface of an aligned fiber bed and an open flow is the governing phenomenon in a number of processes of industrial importance. Traditionally, this has been modeled by applying Brinkman's modification of Darcy's law to obtain the velocity profile in planar geometries in terms of an additional parameter called “apparent viscosity.” to test this ad hoc approach, a detailed experimental investigation of the flow was conducted using Laser Doppler Anemometry (LDA) in the close vicinity of the permeable boundary of aligned fiber preforms. The performs used in the experiments consisted of cylindrical fibers aligned in one direction. Two cases were investigated. In the first case, the axis of alignment was in the direction of flow and in the second case the axis of alignment was perpendicular to the flow direction. A Hele‐Shaw cell is partially filled with a fibrous preform such that an open channel flow is coupled to the Darcy flow inside the fiber bed through the permeable interface of the bed. The unfilled portion of the Hele‐Shaw cell acts as an ideal porous medium of known in‐plane permeability, which is much higher than the permeability of the fibrous/porous bed. Modeling this flow situation using a Hele‐Shaw cell is appropriate because most composite parts are long and wide in comparison with their thickness. When a viscous fluid is injected at a constant flow rate through the above arrangement, a steady state coupled flow is created. This coupling of the open flow and the Darcy flow through the fibrous bed occurs through the boundary layer zone inside the fibrous bed. Using LDA, steady state velocity profiles are accurately measured in the boundary layer zone by traversing the fibrous bed at a suitable location. For aligned fiber beds, the depth of the boundary layer zone inside the bed was found to be of the order of the mold depth, which is much larger as compared to the Brinkman's prediction. This finding indicates the presence of a length scale that is much larger than the known length scale \documentclass{article}\pagestyle{empty}\begin{document}$ \sqrt K $\end{document}, where K is the permeability of the bed made up of aligned cylindrical fibers. Based on this finding, the depth of the boundary layer thickness is incorporated in the Brinkman's solution through a boundary condition. This results in a model that compares well with the experimental data for the planar geometry and the fibrous beds considered here.
Cure monitoring of the liquid composite molding process using fiber optic sensors
Authors: Woerdeman Dara L. , Spoerre Julie K. , Flynn Kathleen M. , Parnas Richard S. 
Published: April 2004

AbstractFluorescence has been demonstrated to be an accurate tool for monitoring resin cure. It is measured using an evanescent wave fiber‐optic sensor. An economical optical fiber sensor has been developed with a refractive index greater than 1.6, permitting evanescent wave monitoring of epoxy resins. The fluorescence wave‐length‐shift, which has been correlated with monomer conversion, is monitored during the liquid molding process. Unidirectional glass fabrics with volume fractions from 40% to 60% were injected with epoxy resin at a variety of driving pressures and cured at several temperatures. Several composite parts were fabricated to test the effects of vacuum pressure, injection rate, cure temperature, and fiber fraction on the performance of the sensor. The sensitivity of the evanescent wave fluorescence sensor to the condition of the resin system was also examined. The sets of resin/hardener samples were subjected to rigorous chemical analysis to determine the extent of their differences.
A new bismaleimide system for resin transfer molding
Authors: Aijuan Gu , Guozheng Liang , Zhongming Li 
Published: April 2004

AbstractA new bismaleimide (BMI) resin system, designated 4503A, suitable for resin transfer molding (RTM) has been developed. It is prepared by employing allyl methyl phenol and diallyl bisphenol A as reactive diluents for BMI. The processing properties of 4503A were investigated by time‐temperature‐viscosity curves, gel characteristics, and differential scanning calorimetry (DSC). Data show that the injection temperature of 4503A prepolymer can be as low as 75°C, while its viscosity is only 0.34Pa · s. In addition, after being maintained at 75°C for 12h, the viscosity is <0.95Pa · s. The cured 4503A resin has a glass transition temperature (Tg) and a heat deflection temperature (HDT) of 266 and 232°C, respectively; properties include tensile strength of 81MPa, flexural strength of 108MPa, and G1C of 213J/m2. Other properties of a composite based on 4503A system and woven glass cloth are also discussed. Regarding short beam shear (SBS) strength, for tests at 150 and 180°C, 80 and 61% of the original room temperature (RT) strength is retained. A similar strength retention (86 and 68%) is noted for flexural properties.
Morphological effects on the transverse permeability of arrays of aligned fibers
Authors: Papathanasiou T. D. , Lee P. D. 
Published: April 2004

AbstractThe effect of micro‐structure on the effective transverse permeability (K) of unidirectional arrays of cylindrical fibers is investigated computationally using the Boundary Element Method (BEM). By converting the original problem into an integral equation on the boundaries of the domain of interest, the BEM allows the straightforward discretization of complex, multi‐connected domains with a fraction of the effort required by domain methods. The equations of Stokes flow are solved in cells containing up to 36 individual fibers located either in uniform arrays (square and hexagonal) or in random locations and the effective permeability is calculated from the corresponding pressure drop and flow rate. Random placement of particles results in the generation of individual microstructures whose permeability shows a scatter around a mean value. This scatter increases significantly as the porosity (ϕ) of the cell is reduced. Statistical averages are compared to analytical and numerical predictions for the permeability of a perfect square array. It is found that a fully random structure exhibits a permeability slightly higher than a perfect square array for very high porosity values (ϕ > 0.8), with this trend disappearing for ϕ ≤ 0.8 and the averages coinciding with the result of the perfect square array. Additionally, the effect of random perturbations in the location and size of fibers around mean values corresponding to perfect square packing of mono‐sized fibers is investigated. It is found that such variations have little effect on K at the high‐porosity end (ϕ > 0.8), but they result in a noticeable increase in the effective permeability (as compared to a perfect square array) at the limit of low porosity (ϕ > 0.5).
Analysis of two‐regional flow in liquid composite molding
Authors: Ni Jun , Zhao Yang , James Lee L. , Nakamura Sho 
Published: April 2004

AbstractThis study investigates the two‐regional flow in Liquid Composite Molding (LCM) with emphasis on the race tracking phenomenon. An equivalent permeability is introduced to describe the flow capacity in the fiber free region. A lumped permeability is also used to further simplify the flow modeling by averaging the flow across the flow direction. Both the equivalent permeability approach and the lumped permeability approach were verified with experiments. It is found that they are capable of modeling the race tracking effects in LCM.
Experimental characterization of autoclave‐cured glass‐epoxy composite laminates: Cure cycle effects upon thickness, void content, and related phenomena
Authors: Thomas Matthew M. , Joseph Babu , Kardos John L. 
Published: April 2004

AbstractThis article presents results from over 100 experimental autoclave curing fiber‐glass‐epoxy composite laminate curing runs. The primary objective was to verify shrinking horizon model predictive control—SHMPC—for thickness and void content control, using readily available secondary measurements. The secondary objective was to present and analyze the extensive experimental results obtained through this verification. Seven series of curing runs (16 per series) were performed, with cure settings governed by partial‐ or full‐factorial orthogonal array based design of experiments. Through t‐tests and two‐way analysis of variance, it was found that pressure magnitude had the largest influence on laminate thickness and void content, while first hold duration/temperature, pressure application duration, and run delay influenced void content more than thickness. Thinner laminates with lesser void contents resulted from pressure application before the second temperature ramp. Prepreg age also affected thickness and void content. Photomicrographs revealed not one large void, but void clusters. Interrupted autoclave cure cycles revealed that significant laminate thickness reduction occurred during all curing cycle stages. The percentage of resin weight loss through laminate sides increased with pressure magnitude and application duration. Ten test curing runs indicated that SHMPC met difficult thickness targets while minimizing void content.
Recycled content in polymer matrix composites through the use of A‐glass fibers
Authors: Steenkamer D. A. , Sullivan J. L. 
Published: April 2004

AbstractThe utility of recycled A‐glass (primarily composed of soda‐lime‐silicate) fibers as reinforcement for structural composites has been studied. A series of plaques of unsaturated polyster composite were resin transfer molded with an A‐glass continuous strand mat (CSM) and a control with E‐glass CSM. The influence of fiber volume fraction on the physical and thermo‐mechanical properties of the resultant composites were investigated, both before and after environmental exposure. At the maximum fiber fraction considered (nominally 29 vol%), the use of A‐glass reinforcement lowered the warp direction tensile modulus from 8.6 to 7.6 GPa and strength from 139 to 100 MPa, relative to the control. Similar results were observed for both the flexural and the tensile properties, irrespective of fiber fraction and test direction (warp vs. weft), for the A‐glass reinforcement. Environmental exposure was found to affect equally the properties of A‐glass and E‐glass fiber reinforced composites. Based upon microscopic analyses and constituent properties, the lower mechanical properties of the A‐glass fibers composites have been linked to the lower properties of A‐glass fibers relative to E‐glass fibers. The experimental results were also used to test a micro‐mechanics models for random fiber reinforced composites. Reasonable correlation was found between the experimental results and the theoretical predictions. To offset their lower mechanical properties, A‐glass fibers could be used as a reinforcement in composite applications by simply increasing the fiber fraction relative to their E‐glass counterpart.
The concentrated emulsion approach to toughened polymer composites: A review
Authors: Ruckenstein Eli , Li Hangquan 
Published: April 2004

AbstractA series of tough polymers were prepared by combining flexible with rigid chains, using the method of concentrated emulsion polymerization. The tough materials obtained can be classified into four types: [a] those prepared via the polymerization of a monomer containing a dissolved elastomer, starting from its concentrated emulsion in water; [b] those prepared via heterogeneous (interfacial) crosslinking of two partially polymerized concentrated emulsion in water containing incipient latexes; [c] those prepared as semi‐interpenetrating or AB network latexes, starting from a concentrated emulsion in water and [d] those prepared by mixing two partially polymerized concentrated emulsions and completing the polymerization. A concentrated emulsion differs from the conventional emulsion in that the volume fraction of the dispersed phase is higher than that of the most compact arrangement of monosize spheres (0.74) and can be as high as 0.99. The cells of a highly concentrated emulsion are no longer spherical, but polyhedral in shape, compactly packed and separated by thin films of continuous phase. As a result of such structure, a high polymerization rate and a high molecular weight can be achieved, the size (in the colloidal range) of the flexible phase can be controlled and the latter phase can be uniformly dispersed in the rigid one. Consequently, the concentrated emulsion method constitutes a suitable pathway to toughened composites. Owing to the compact packing of cells, the concentrated emulsion polymerization method is particularly suitable for cases in which reactions occur at the cell interface. For the materials of type [d], these reactions generate quasi‐block copolymers, which compatibilize (via “auto‐compatibilization”) the components of blends.
Cationic polymerization of diglycidly ether of bisphenol a resins initiated by benzylsulfonium salts
Authors: McGowen John A. , Mathias Lon J. 
Published: April 2004

AbstractBenzylsulfonium salts are latent thermal cationic initiators that dissociate on heating to form benzyl cations that can initiate polymerizations. This paper describes the cure behavior of commercial epoxy resins using 1 ‐(p‐methoxybenzyl)tetrahydrothiophenium hexafluoroantimonate [2]. The thermal cationic cure of bisphenol A diglycidylether (DGEBA) resins was investigated using differential scanning calorimetry (DSC). DSC revealed complex cure behavior as indicated by multiple exotherms. The resins cured rapidly at low initiator concentrations with gelation occuring in 3.5 and 1.5 min at 75 and 85 °C, respectively, at conversions of epoxy groups α = 0.2–0.3. Cure kinetics were evaluated from both dynamic and isothermal DSC measurements. The effects of initiator concentration, isothermal cure temperature and heating rate on the cure behavior and mechanisms, especially involving potential termination pathways, are discussed.
Effect of isothermal aging on post‐imidization and glass transition temperature of LaRC‐IA polyimide resin
Authors: Jordan Kelvin M. , Iroh Jude O. 
Published: April 2004

AbstractThe isothermal aging of partilly imidized NASA Langley Research Center, LaRC‐IA polymide resin containing 70 wt% N‐methyl pyrrolidone, NMP was performed in a vacuum oven at 65, 95, 135, 165 and 200°C for 0.5, 1.0, 1.5 and 2 h. The weight loss and chemical changes that occurred during aging was determined gravimetrically and by FTIR spectroscopy, respectively. The imide absorption peak at 1778 cm−1, increased in intensity as the aging temperature was increased from 65 to 200°C. The expulsion of NMP (70 wt%) was completed after ∼2.5 h of aging at 135°C. Additional weight loss ≤4 wt%, after the expulsion of NMP, was attributed to post‐imidization. The imide carbonyl peak absorption at 1721 and 1778 cm−1, respectively, were broadened after aging at 200°C. The broadening of the imide absorption peaks was marked by the disappearance of the amide peak near 1660 cm−1 and is attributable to post‐imidization of the partially imidized polyamic‐acid. Dissolution of the polyimide aged at T ≥ 165°C in dimethyl formamide, DMF, was unsuccessful even after long times of stirring (∼12 h) at elevated temperature (T ∼ 85°C). The DSC thermogram for the LaRC‐IA resin showed a series of broad endothermic peaks between 150–180°C and narrow endothermic peaks at 210°C. The low temperature endotherm disappeared after aging at T ≥ 135°C for t ≥ 1 h. The high temperature endotherm decreased with increased aging temperature and time. The glass transition temperature of the polyimide increased with increased aging temperature and time.
Processing models and characterization of thermoplastic composite wound parts
Authors: Lin Hong‐Ru , Advani Suresh G. 
Published: April 2004

AbstractSqueezing flow was adopted to model the processing of thermoplastic composites in filament winding. It is found that the resin flow is facilitated as a result of the slip condition, thus reducing the process times. An apparatus with a constant dead weight was employed to characterize the rheology of the thermoplastic composite in filament winding. The system is carbon fiber (AS4) reinforced with the polyetherketone‐ketone (PEKK) thermoplastic matrix. It is found that resin flow is dominant in the transverse fiber direction and there is insignificant flow along the fiber direction. An empirical equation of viscosity for AS4/PEKK as a function of temperature was obtained based on the model. For mechanical properties, either in a 4‐point bending test or a short beam shear test, most of the hoop wound rings failed at the inside radius or on the compression side. Insufficiently applied heat and a cold mandrel in the beginning of the winding process may result in low degree of consolidation on the compression side. The average void content of the hoop wound ring is lower than that of the preimpregnated tow because of void compression through resin flow during the winding process.