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  • 1
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    Thermal and ultrasonic evaluation of porosity in composite laminates (1992)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Description: The effects of porosity on damage incurred by low-velocity impact are investigated. Specimens of graphite/epoxy composite were fabricated with various volume fractions of voids. The void fraction was independently determined using optical examination and acid resin digestion methods. Thermal diffusivity and ultrasonic attenuation were measured, and these results were related to the void volume fraction. The relationship between diffusivity and fiber volume fraction was also considered. The slope of the ultrasonic attenuation coefficient was found to increase linearly with void content, and the diffusivity decreased linearly with void volume fraction, after compensation for an approximately linear dependence on the fiber volume fraction.
    Keywords: COMPOSITE MATERIALS
    Type: In: Review of progress in quantitative nondestructive evaluation. Vol. 11B; Proceedings of the 18th Annual Review, Brunswick, ME, July 28-Aug. 2, 1991 (A93-19582 06-38); p. 1555-1562.
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  • 2
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    Electron-radiation effects on the ac and dc electrical properties and unpaired electron densities of three aerospace polymers (1986)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: The effects of gigarad-level total absorbed doses from 1-MeV electrons on the post-irradiation alternating-current (ac) and direct-current (dc) electrical properties and the unpaired electron densities have been studied for Kapton, Ultem, and Mylar. The unpaired electron densities (determined from electron paramagnetic resonance spectroscopy) and the dc electrical conductivities of the irradiated materials were monitored as functions of time following the exposures to determine their decay characteristics at room temperature. The elevated-temperature ac electrical dissipations of the Ultem and Mylar were affected by the radiation. The dc conductivity of the Kapton increased by five orders of magnitude, while the dc conductivities of the Ultem and Mylar increased by less than an order of magnitude, due to the radiation. The observed radiation-generated changes in the ac electrical dissipations are explained in terms of known radiation-generated changes in the molecular structures of the three materials. A preliminary model relating the dc electrical conductivity and the unpaired electron density in the Kapton is proposed.
    Keywords: ELECTRONICS AND ELECTRICAL ENGINEERING
    Type: IEEE Transactions on Nuclear Science (ISSN 0018-9499); NS-33; 1390-139
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  • 3
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    Ultrasonic detection and identification of fabrication defects in composites (1991)
    In:  CASI
    Details
    Publication Date: 2013-08-31
    Description: Methods for deliberate fabrication of porosity into carbon/epoxy composite panels and the influence of three-dimensional stitching on the detection of porosity were investigated. Two methods of introducing porosity were investigated. Porosity was simulated by inclusion of glass microspheres, and a more realistic form of porosity was introduced by using low pressure during consolidation. The panels were ultrasonically scanned and the frequency slope of the ultrasonic attenuation coefficient was used to evaluate the two forms of porosity. The influence of stitching on the detection of porosity was studied using panels which were resin transfer molded from stitched plies of knitted carbon fabric and epoxy resin.
    Keywords: COMPOSITE MATERIALS
    Type: First NASA Advanced Composites Technology Conference, Part 2; p 705-720
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  • 4
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    Effects of postcuring on mechanical properties of pultruded fiber-reinforced epoxy composites and the neat resin (1989)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The effects of postcuring on mechanical properties of pultruded fiber-reinforced epoxy-resin composites have been investigated. Composites with carbon, glass, and aramid reinforcement fibers were individually studied. The epoxy was a commercially-available resin that was especially developed for pultrusion fabrication. The pultrusions were conducted at 400 F with postcures at 400, 450, 500, and 550 F. Measurements of the flexural, shear, and interlaminar fracture-toughness properties showed that significant postcuring can occur during the pultrusion process. All three mechanical properties were degraded by the higher (500 and 550 F) temperatures; photomicrographs suggest that the degradation was caused at the fiber-resin interface for all three fiber types.
    Keywords: COMPOSITE MATERIALS
    Type: SAMPE Quarterly (ISSN 0036-0821); 20; 9-16
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  • 5
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    Spectroscopic comparison of effects of electron radiation on mechanical properties of two polyimides (1987)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The differences in the radiation durabilities of two polyimide materials, Du Pont Kapton and General Electric Ultem, are compared. An explanation of the basic mechanisms which occur during exposure to electron radiation from analyses of infrared (IR) and electron paramagnetic resonance (EPR) spectroscopic data for each material is provided. The molecular model for Kapton was, in part, established from earlier modeling for Ultem (pp. 1293-1298 of IEEE Transactions on Nuclear Science, December 1984). Techniques for understanding the durability of one complex polymer based on the understanding of a different and equally complex polymer are demonstrated. The spectroscopic data showed that the primary radiation-generated change in the tensile properties of Ultem (a large reduction in tensile elongation) was due to crosslinking, which followed the capture by phenyl radicals of hydrogen atoms removed from gem-dimethyl groups. In contrast, the tensile properties of Kapton remained unchanged because radical-radical recombination, a self-mending process, took place.
    Keywords: CHEMISTRY AND MATERIALS (GENERAL)
    Type: NASA-TP-2663 , L-16200 , NAS 1.60:2663
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  • 6
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    Infiltration/cure modeling of resin transfer molded composite materials using advanced fiber architectures (1991)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: A model was developed which can be used to simulate infiltration and cure of textile composites by resin transfer molding. Fabric preforms were resin infiltrated and cured using model generated optimized one-step infiltration/cure protocols. Frequency dependent electromagnetic sensing (FDEMS) was used to monitor in situ resin infiltration and cure during processing. FDEMS measurements of infiltration time, resin viscosity, and resin degree of cure agreed well with values predicted by the simulation model. Textile composites fabricated using a one-step infiltration/cure procedure were uniformly resin impregnated and void free. Fiber volume fraction measurements by the resin digestion method compared well with values predicted using the model.
    Keywords: COMPOSITE MATERIALS
    Type: First NASA Advanced Composites Technology Conference, Part 2; p 425-441
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  • 7
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    Absorbed dose thresholds and absorbed dose rate limitations for studies of electron radiation effects on polyetherimides (1989)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The threshold values of total absorbed dose for causing changes in tensile properties of a polyetherimide film and the limitations of the absorbed dose rate for accelerated-exposure evaluation of the effects of electron radiation in geosynchronous orbit were studied. Total absorbed doses from 1 kGy to 100 MGy and absorbed dose rates from 0.01 MGy/hr to 100 MGy/hr were investigated, where 1 Gy equals 100 rads. Total doses less than 2.5 MGy did not significantly change the tensile properties of the film whereas doses higher than 2.5 MGy significantly reduced elongation-to-failure. There was no measurable effect of the dose rate on the tensile properties for accelerated electron exposures.
    Keywords: NONMETALLIC MATERIALS
    Type: NASA-TP-2928 , L-16585 , NAS 1.60:2928
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  • 8
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    Studies of molecular properties of polymeric materials (1990)
    In:  CASI
    Details
    Publication Date: 2019-07-20
    Description: Aerospace environment effects (high energy electrons, thermal cycling, atomic oxygen, and aircraft fluids) on polymeric and composite materials considered for structural use in spacecraft and advanced aircraft are examined. These materials include Mylar, Ultem, and Kapton. In addition to providing information on the behavior of the materials, attempts are made to relate the measurements to the molecular processes occurring in the material. A summary and overview of the technical aspects are given along with a list of the papers that resulted from the studies. The actual papers are included in the appendices and a glossary of technical terms and definitions is included in the front matter.
    Keywords: NONMETALLIC MATERIALS
    Type: NASA-TM-101854 , NAS 1.15:101854 , PTR-90-3
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  • 9
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    Effects Of Irradiation By Electrons On Two Polyimides (1992)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: Polyimides of new class, known as polyetherimides, used for composites and castings. Study undertaken to compare effects of electron radiation, in doses expected for 30-year missions in geosynchronous orbit, on molecular structures and tensile properties of two commercially available polyimides: polypyromellitimide and polyetherimide. Findings helpful in development of new polymers that endure radiation, leading to synthesis of polymers not having methyl groups or other moieties that are sources of hydrogen, inhibiting self-mending. Significance of study for polymer industry relating to structures in outer space considerable.
    Keywords: MATERIALS
    Type: LAR-14309 , NASA Tech Briefs (ISSN 0145-319X); 16; 10; P. 72
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  • 10
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    Analytical modeling and sensor monitoring for optimal processing of polymeric composite material systems (1991)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: Process simulation models and cure monitoring sensors are discussed for use in optimal processing of fiber-reinforced composites. Analytical models relate the specified temperature and pressure cure cycle to the thermal, chemical, and physical processes occurring in the composite during consolidation and cure. Frequency-dependent electromagnetic sensing (FDEMS) is described as an in situ sensor for monitoring the composite curing process and for verification of process simulation models. A model for resin transfer molding of textile composites is used to illustrate the predictive capabilities of a process simulation model. The model is used to calculate the resin infiltration time, fiber volume fraction, resin viscosity, and resin degree of cure. Results of the model are compared with in situ FDEMS measurements.
    Keywords: COMPOSITE MATERIALS
    Type: International Conference on Composite Materials (ICCM/8); Jul 15, 1991 - Jul 19, 1991; Honolulu, HI; United States
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