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    Fatigue Thresholds of Delamination Crack Growth in Orthotropic Graphite/Epoxy Laminates

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    During manufacturing of fiber composite structures, cracks of small sizes are introduced during layup and drilling, and in the making of cutouts. The component itself sometimes has sites that are prone to delaminations, such as the junction of the web and the flange of a beam. Under cyclic loading these interlaminar defects can grow.

    One key for designing fiber composite structures that will not exhibit delamination crack growth is to make sure that the actual or anticipated interlaminar flaws produce strain-energy release rates below the fatigue threshold value Gth for the actual laminate and actual combination of opening mode and shearing mode energy release rates, GI and GII, respectively.

    Four types of delamination crack growth experiments were carried out to experimentally determine the critical curve inside which no cyclic crack growth will take place in the GI and GII planes. Pure Mode I crack growth and near pure forward shearing mode crack growth experiments were performed, as were experiments that possess a combination of the first two modes of crack growth. The laminates used in this investigation were (0/90), T300/914C layups. In the experimental work a unique automatic load reduction system was used, allowing for the determination of the threshold state with a constant stress ratio maintained during the load reduction.

    For all but one case, GIth and GIIth were determined numerically, using a two-dimensional finite-element method program, by employing a relaxation procedure. Finally, the results from the experiments are presented in the GIth and GIIth planes.


    graphite/epoxy, delamination, fatigue threshold, strain-energy release rate, test methods

    Author Information:

    Gustafson, C-G
    The Swedish Plastics and Rubber Institute, Sundsvall,

    Jilkèn, L
    The Swedish Plastics and Rubber Institute, Sundsvall,

    Gradin, PA
    Det norske Veritas, Hovik, Oslo,

    Committee/Subcommittee: E08.06

    DOI: 10.1520/STP36306S