STP636

    Effect of Compression on Fatigue Properties of a Quasi-Isotropic Graphite/Epoxy Composite

    Published: Jan 1977


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    Abstract

    The effect of compressive loading on the fatigue properties of a quasi-isotropic graphite/epoxy composite is discussed. Tests were conducted using parallel-sided, unnotched T300/934 graphite/epoxy coupons cured at 177°C (350°F). Results are based on a large sample size (number of coupons tested at a particular test condition) which allowed a statistically significant evaluation of data scatter. All tests were conducted at room temperature. Fatigue tests were conducted at 10 Hz under constant-amplitude loading. Static tensile and compressive properties were obtained for coupons without prior fatigue damage. Tension-tension and tension-compression fatigue stress-life curves were qualitatively evaluated using a small sample set at each stress level. Three specific stress levels were chosen and the extent of data scatter defined for each type of fatigue loading. Additional coupons were subjected to the same amount of fatigue damage and half were tested for tensile residual strength degradation and half for compressive. Such residual strength tests were conducted at the same two fatigue maximum stress levels for both tension-tension and tension-compression fatigue. Data scatter was larger for static compression tests than for static tension tests. Static and fatigue failure modes were dissimilar. Compressive load excursions reduced fatigue life especially at lower stress levels. At stress levels where fatigue failures occurred, degradation in residual strength due to fatigue also occurred. Data analysis was performed using three-parameter Weibull asymptotic fits, which proved to be superior in predicting failure rates than two-parameter fits. The effect of sample size on descriptions of data scatter and accuracy of extrapolation is discussed.

    Keywords:

    composite materials, fatigue, compression, residual strength


    Author Information:

    Ryder, JT
    Research scientist and research development engineer, Lockheed-California Company, Rye Canyon Research Laboratory, Valencia, Calif

    Walker, EK
    Research scientist and research development engineer, Lockheed-California Company, Rye Canyon Research Laboratory, Valencia, Calif


    Paper ID: STP27967S

    Committee/Subcommittee: D30.04

    DOI: 10.1520/STP27967S


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