EFFECT OF DROPPED PLIES ON THE STRENGTH OF GRAPHITE-EPOXY LAMINATES

The reduction in the compressive and tensile strength of graphite-epoxy laminates with thickness discontinuities due to dropped plies was studied by experiment and analysis. The specimens were fabricated with all the dropped plies lumped together in the center of a 16-ply quasi-isotropic layup, such that one surface was flat and the slope of the opposite surface changed abruptly at the dropped ply location to accomodate the thickness change. Even though the thick and thin sections are symmetrically laminated, there exists bending-extension coupling due to the geometric eccentricity of the middle planes of the thick and thin sections. Results from tests on 37 specimens are reported that differed in the configuration of the dropped plies only. The axial strength of a laminate with dropped plies is less than its tensile strength. The reduction in axial strength is directly related to the axial stiffness change between the thick and thin sections. To examine the mechanism of failure, the three-dimensional state of stress in the dropped ply region was evaluated by the finite element method. A tensile interlaminar criterion predicted the correct location of failure, but underestimated the failure load, and therefore is a conservative analysis procedure for design.