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    Observations of the Load Redistribution in Polymeric Composite Laminates Under Tension-Tension Fatigue Loading

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    A series of detailed measurements were obtained of the edge displacements at different stages in the fatigue life of a composite laminate subject to tension-tension fatigue loading. The objective of these measurements was to understand the nature of load redistribution in a composite laminate as it accumulates damage and fails. The measurements were performed using a computer-based stereoimaging system, which provides two-dimensional displacement maps by comparing photographs of the specimens taken at different load levels. Using this approach, a record of the edge displacements within a [0/452/-452/90]s graphite-epoxy laminate, subject to tension-tension fatigue loading, was obtained as a function of the cyclic life. These maps were used to provide a chronology of the strains and displacements along the specimen edge as the specimen failed. Very early in the fatigue life, large shear deformation occurred at the interface of the 45° and -45° layers. This deformation produced a delamination that eventually resulted in separation of the laminate into three sublaminates, consisting of the two edge [0/452] layers, together with the central [-452/902/-452] layers. No load was carried by the central off-axes layers, and final failure coincided with the failure of the outer 0° layers. The strains in each of the layers were obtained at different numbers of cycles from these displacement maps, and the results showed the redistribution of strains which occurs as the laminate accumulates damage and fails. These results are discussed in light of a recent proposed model based on a damage-dependent lamination equation to predict the stiffness degradation in composite materials subject to fatigue.


    composite materials, fatigue, load redistribution, displacement measurements

    Author Information:

    Connolly, MP
    Senior research engineers, Southwest Research Institute, San Antonio, TX

    Davidson, DL
    Senior research engineers, Southwest Research Institute, San Antonio, TX

    Committee/Subcommittee: D30.04

    DOI: 10.1520/STP14021S