STP907

    Effect of Ply Thickness on Longitudinal Splitting and Delamination in Graphite/Epoxy Under Compressive Cyclic Load

    Published: Jan 1986


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    Abstract

    The initiation and growth of damage due to compressive cyclic loading was investigated in [±45n/0n]s graphite/epoxy laminates where the “effective ply thickness” was varied by allowing n to take on the values 1, 2, and 3. A total of 35 axially loaded sandwich specimens with 6.35-mm-diameter holes were cycled at 7 Hz at peak stress levels of 52% to 72% of the static ultimate compressive stress of 425 MPa (which was experimentally determined to be independent of the value of n). Out-of-plane moiré interferometry and pulse-echo ultrasound techniques were used to nondestructively inspect the specimens and showed that three distinct modes of damage growth occur in these laminates. Delamination which initiates at the hole and grows in a radial or transverse fashion occurred only in 60% of the [±45/0]s specimens. For these two types of damage, the growth was rapid and led to catastrophic failure of the specimen. The remainder of the [±45/0]s specimens and all the laminates where the effective ply thickness was doubled and tripled, [±452/02]s and [±453/03]s, exhibited delaminations which initiated at the hole edges and grew parallel to the load direction with the width of the delamination equal to the width of the hole. Catastrophic failure did not occur in these cases. There is a linear relationship between the delamination length and the logarithm of the number of applied load cycles in all these cases. However, the delamination initiated earlier and at lower stress levels for laminates with larger effective ply thicknesses. Specimen sectioning and microscopic examination show that this damage depends on the development of splitting in the 0-deg plies and subsequent delamination as a result of shear failure at the −45°/0° ply interface in the region between the splits. Several [0/±45]s and [02/±452]s coupon specimens were cycled in tension and this splitting and subsequent delamination also developed. Residual tensile strength tests conducted on graphite/epoxy coupons debonded from the honeycomb after cycling showed a 50% increase in tensile strength over undamaged specimens with 6.35-mm-diameter holes. This is attributed to the redistribution of stress around the hole due to the relieving of stress concentration in the 0-deg plies by the splitting and delamination.

    Keywords:

    composite materials, cyclic load, damage growth, delamination, splitting, ply thickness, residual strength


    Author Information:

    Lagace, PA
    Assistant Professor of Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, MA.

    Coauthor Nolet is currently 1st Lieutenant, USAF, McClellan AFB, Sacramento, CA

    Nolet, SC
    Assistant Professor of Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, MA.

    Coauthor Nolet is currently 1st Lieutenant, USAF, McClellan AFB, Sacramento, CA


    Paper ID: STP19994S

    Committee/Subcommittee: D30.04

    DOI: 10.1520/STP19994S


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