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  • 1
    Electronic Resource
    Electronic Resource
    Brookfield, Conn. : Wiley-Blackwell
    Polymer Composites 16 (1995), S. 370-377 
    ISSN: 0272-8397
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Gas permeation and creep deformation of a commercial prepreg, which exhibits viscoelastic characteristics, were investigated as a function of time, temperature, and consolidation pressure. Experiments using a prepreg stack demonstrated that the material exhibited a linear viscoelastic bulk deformation under vacuum/autoclave pressure and furthermore, the in-plane gas flow exhibited non-Darcian flow behavior with a permeation hysteresis. This behavior was viewed and analyzed by two viscoelastic relaxation processes: (1) bulk dimensional relaxation, and (2) microscopic pore structure rearrangement. A modified standard linear solid (SLS) viscoelastic model was used to interpret the creep compliance and dynamic gas permeability utilizing two independent relaxation parameters. By visual investigation of pore sizes and their distribution, air permeation was found to take place mostly through the interlaminar porosity network for the prepreg system examined.
    Additional Material: 8 Ill.
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  • 2
    Electronic Resource
    Electronic Resource
    Brookfield, Conn. : Wiley-Blackwell
    Polymer Composites 16 (1995), S. 424-428 
    ISSN: 0272-8397
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: We have examined the micro indentation hardness of a series of carbon fiber reinforced epoxy and thermoplastic polyimide (TPI) composites. In the epoxy systems, the influence of Nylon particles was studied. The effect of crystallization of the thermoplastic polyimide upon the microhardness values of the resin was also investigated. The microstructure of the TPI-composites was characterized by X-ray diffraction. The results show that the addition of carbon fibers to the neat resins greatly increases the microhardness and thus the yield stress of the composite. The value of the microhardness technique is highlighted in emphasizing the heterogeneity of the CFRC.
    Additional Material: 3 Ill.
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  • 3
    ISSN: 0272-8397
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Dynamic mechanical analysis, stress relaxation, and creep experiments were performed to characterize the viscoelastic properties of a basic unmodified epoxy (Hercules 3501-6) and a modified multiphase epoxy (Hercules 8551-7), which are commonly used as matrices in high performance composites. The Arrhenius and WLF equations and a modified Standard Linear Solid (SLS) Model were used to quantify the viscoelastic behavior below, above, and at the glass transition temperatures, Tg. Both the modified and the unmodified epoxy systems exhibited a single relaxation peak above ambient temperature. The Tg of the modified epoxy at full cure was 170°C, which is 50°C lower than that of the basic epoxy. The glassy moduli of these two resin systems were found to be comparable, but the rubbery modulus of the modified epoxy was much lower than that of the basic epoxy system. However, the viscoelastic behavior of these two systems and their sensitivity to time-temperature may be considered to be quite similar if they are compared with respect to their corresponding glass transition temperatures.
    Additional Material: 15 Ill.
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  • 4
    Electronic Resource
    Electronic Resource
    Brookfield, Conn. : Wiley-Blackwell
    Polymer Composites 13 (1992), S. 197-206 
    ISSN: 0272-8397
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: As part of an engineering analysis and experimental methodology to characterize prepreg tack, a compression-to-tension test was optimized to enhance reproducibility and generate intrinsic property data. With the resulting stress-strain compression and tension data, a theoretical model was developed to describe tack as a bulk viscoelastic property of a prepreg laminate stack. Using the viscoelastic analysis, four intrinsic material parameters to characterize prepreg tack could be defined. These were 1) relaxed modulus, 2) unrelaxed modulus, 3) relaxation time, and 4) initial void content of the prepreg stack. Relaxed and unrelaxed moduli of the prepreg stack were independent of temperature, while the relaxation time was highly dependent on temperature and matrix viscosity. In addition, the relaxation time was found to be influenced by resin/fiber content and prepreg surface characteristics, which also influenced the void content of the prepreg stack. Using these measured parameters, good agreement was observed between theory and experimental data for both the stress-strain curve of the tack test and the simplified compression tack index (CTI*), defined as the ratio of output energy of the prepreg stack during tensile unloading to input energy during compressive loading.
    Additional Material: 11 Ill.
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  • 5
    Electronic Resource
    Electronic Resource
    Brookfield, Conn. : Wiley-Blackwell
    Polymer Composites 12 (1991), S. 146-152 
    ISSN: 0272-8397
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: A simple apparatus was designed and constructed capable of measuring the unsteady-state permeability and the capillary pressure simultaneously in a simulated composite impregnation experiment. It was found that the Kozeny-Carman equation used to describe the permeability of composites during impregnation adequately described experimental results for woven fabric preform up to porosity values of 0.5. Above this value, observed deviations were attributed to interfacial effects between adjacent woven fabric layers. For woven fabric preforms made of T-300 carbon fibers, a maximum capillary pressure of 3.7 × 104 Pa (=5.4 psi) was observed at low porosity values. Thus, the capillary pressure may compete with other pressure sources in low pressure processes, such as the prepregging process. The woven fabric preform used in this study is observed to have a permeability similar to a unidirectional fibrous preform along the transverse direction. Furthermore, an existing modeling methodology capable of predicting permeability and capillary pressure through different preforms was found to be valid for fibrous preforms of complex orientation.
    Additional Material: 8 Ill.
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  • 6
    Electronic Resource
    Electronic Resource
    Brookfield, Conn. : Wiley-Blackwell
    Polymer Composites 16 (1995), S. 109-113 
    ISSN: 0272-8397
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: 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.
    Additional Material: 8 Ill.
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  • 7
    Electronic Resource
    Electronic Resource
    Brookfield, Conn. : Wiley-Blackwell
    Polymer Composites 3 (1982), S. 118-124 
    ISSN: 0272-8397
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: This work forms a basis for relating moisture solubilities and dynamic mechanical properties to the hygrothermal history of epoxy systems. Two different classes of epoxy systems were investigated; a low-performance epoxy (DGEBA-TETA) and a high-performance system (TGDDM-DDS) commonly used in carbon fiber composites. An increase in the equilibrium moisture content from TGDDM-DDS epoxy samples having various DDS compositions was observed as a result of thermal cycling in a liquid environment. Interpretation of the experimental results suggest that hygrothermal interactions produce changes in the epoxy network structure and result in the observed moisture sorption behavior.
    Additional Material: 8 Ill.
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  • 8
    Electronic Resource
    Electronic Resource
    Brookfield, Conn. : Wiley-Blackwell
    Polymer Composites 14 (1993), S. 349-360 
    ISSN: 0272-8397
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: A description of the hot-melt prepreg process was developed by coupling resin matrix flow at the coating and impregnation zones with heat transfer considerations. Both bismaleimide (BMI) and epoxy resin systems were used as matrices. With continuous carbon fiber reinforcement. For the laboratory scale prepregger utilized in this study, the thickness of the resin film produced in a doctor blade was approximately 63% of the blade distance, showing good agreement with experimental data. In addition, the measured heat transfer coefficient between prepreg and atmosphere indicated that surface convective resistance is large compared to the internal conductive resistance and thus the chill plate in the prepregging operation plays on important role in determining prepreg quality. By formulating an analytical methodology in terms of the “Prepreg Flow Number” (PFN) that combined important operating parameter (e.g., temperature, pressure, and production rate), a temperature-pressure-velocity superposition was confirmed for the prepregging process. In addition, the model could easily be extended to different scales and types of prepregging processes with the aid of dimension-less parameters that are quite descriptive of the operating conditions employed in prepreg manufacturing.
    Additional Material: 15 Ill.
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  • 9
    Electronic Resource
    Electronic Resource
    Stamford, Conn. [u.a.] : Wiley-Blackwell
    Polymer Engineering and Science 31 (1991), S. 253-257 
    ISSN: 0032-3888
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Dynamic mechanical properties of polymer systems immersed in liquid environments have the potential of providing a wealth of characterization information relating to durability and performance characteristics of the material. However, when oscillatory experiments are performed in liquids, the effect of fluid drag on the sample and/or clamps must be subtracted from the data if the true material properties of the polymer are to be measured. Using classic dimensional analysis, a correlation was developed for calculating the friction of a sample/clamps combination in different liquids In relation to a Reynolds number defined for the immersed system. The methodology was adapted to the DuPont DMA using a poly(tetrafluoroethylene) (PTFE) polymer sheet to avoid any liquid sorption by the sample. It was found that storage and loss friction parameters could be determined and subtracted from the measured data to provide fluid-independent dynamic mechanical properties for the sample.
    Additional Material: 3 Ill.
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  • 10
    Electronic Resource
    Electronic Resource
    Stamford, Conn. [u.a.] : Wiley-Blackwell
    Polymer Engineering and Science 34 (1994), S. 1664-1673 
    ISSN: 0032-3888
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: A novel approach for toughening thermosetting epoxy matrices using both thermoplastics and liquid reactive rubbers as modifiers has been investigated. The network structure of the modified epoxy systems was characterized using dynamic mechanical analysis, and the morphology of the multiphase structure was examined using scanning electron microscopy (SEM). To investigate the continuity of the phase domains, the constituents in the phase domains were positively identified using solving etching and RuO4 staining techniques for transmission electron microscopy (TEM). The fracture toughness of the modified and basic epoxy samples was measured using compact tension (CT) specimens. Quite limited toughness improvement was achieved for the epoxy modified with only the PSu thermoplastic, or the liquid rubber by itself. However, the fracture toughness was found to increase dramatically when a proper combination of both the liquid reactive rubber and thermoplastic was simultaneously incorporated into the epoxy. Toughening by using dual modifiers resulted in maximum improvement of fracture toughness with minimal compromises in processability and Tg depression by rubbers.
    Additional Material: 12 Ill.
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