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    Test Methods for Fiber Tensile Strength, Composite Flexural Modulus, and Properties of Fabric-Reinforced Laminates

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    This paper examines several aspects of fiber and composite testing, including: fiber tension test methods, the three-point loading flexural test, and special problems related to test methods and properties of fabric-reinforced composites. Some major cause of property variability in hand lay-up laminates are studied. Tensile strength properties of the important reinforcing fibers—glass, graphite, aramid, and boron—are reviewed. Filament, yarn, and resin-impregnated strand test methods are discussed. Tensile strength theories for untwisted yarns (fiber bundles) and strands are summarized. A new theory for twisted fibrous structures, such as yarns and ropes, is presented. Predictions of the various theories are compared with test data for Kevlar®3 49 aramid. Standard flexural test procedures presented in ASTM Tests for Flexural Properties of Plastics and Electrical Insulating Materials (D 790) are reviewed, and it is shown analytically and experimentally that use of the shorter recommended span-to-depth ratios can give artificially low modulus values. The importance of considering characteristic dimensions of material internal structure in selection of test coupon geometry, a topic ignored in most test methods, is discussed. This criterion is particularly important for fabric composites because yarn bundle size, an important characteristic dimension, can be large. The effects on mechanical properties of two types of common nonuniformities found in hand lay-up fabric laminates—resin surface layers and unequal layer thickness—are examined and found to be very significant. Finally, it is shown that there is an apparent effect of number of plies on flexural modulus which can be significantly reduced by modifying test data to account for resin surface layer.


    composite materials, test methods, laminates, filaments, fibers, yarns

    Author Information:

    Zweben, C
    Staff engineer, Phila., Pa.

    Smith, WS
    Senior research chemist and research physicist, E. I. du Pont de Nemours & Co., Inc., Wilmington, Del.

    Wardle, MW
    Senior research chemist and research physicist, E. I. du Pont de Nemours & Co., Inc., Wilmington, Del.

    Committee/Subcommittee: D30.04

    DOI: 10.1520/STP36912S