STP723: Compression Fatigue Behavior of Graphite/Epoxy in the Presence of Stress Raisers

    Rosenfeld, MS
    Aero research engineer and aerospace engineer, Aircraft and Crew Systems Technology Directorate, Naval Air Development Center, Warminster, Pa.

    Gause, LW
    Aero research engineer and aerospace engineer, Aircraft and Crew Systems Technology Directorate, Naval Air Development Center, Warminster, Pa.

    Pages: 23    Published: Jan 1981


    Abstract

    Graphite/epoxy composites are being applied to aircraft structures because of their demonstrated capability to reduce weight and increase life. Although composites have better fatigue properties than metals, their behavior differs significantly. Unlike metals, composites exhibit excellent tensile fatigue behavior—the constant amplitude fatigue strength at 107 cycles being close to static ultimate. Compression and reversed loading fatigue behavior in the presence of stress raisers, however, has not been well characterized. This program was therefore undertaken to determine the characteristics of composites under these conditions. Two stress raisers were considered; the first was an open hole and the second was damage resulting from low velocity, hard object impact.

    For the open hole specimen, constant and variable amplitude tests were performed to determine the significance of compression fatigue and to investigate the failure mechanism. These tests indicated that the fatigue life under compression and reversed loading is less than for tension-tension loading and will be an important design consideration in future composite applications. The failure mechanism appeared to be local progressive failure of the matrix near the stress raiser resulting in delamination, and final failure by fiber buckling. The variable-amplitude loading results also demonstrated the unconservativeness of Miner's rule for making analytical predictions.

    In order to determine the nature and significance of impact damage to generic composite structural elements, low velocity impact and residual properties tests were conducted on solid laminate specimens of 42 and 48 plies thick and on honeycomb sandwich specimens with 12-ply composite face sheets. Results of post impact properties tests indicate subvisual damage can degrade compression static and fatigue strength, although subvisual damage will not propagate under moderate (0.003) cyclic strain.

    Keywords:

    Composite materials, graphite/epoxy laminates, compression fatigue, impact damage, fatigue (materials)


    Paper ID: STP27620S

    Committee/Subcommittee: E08.05

    DOI: 10.1520/STP27620S


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