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Nonlinear tapered flexbeam laminates, with significant ply waviness, were cut from a full-size composite rotor hub flexbeam. The specimens were tested under combined axial tension and cyclic bending loads. All of the specimens had wavy plies through the center and near the surfaces (termed marcelled areas), although for some of the specimens the surface marcels were very obvious, and for others they were much smaller. The specimens failed by first developing cracks through the marcels at the surfaces, and then delaminations grew from those cracks, in both directions. Delamination failure occurred in these specimens at significantly shorter fatigue lives than similar specimens without waviness, tested in Ref 2.
A 2-D finite-element model was developed which closely approximated the flexbeam geometry, boundary conditions, and loading. In addition, the FE model duplicated the waviness observed in one of the test specimens. The model was analyzed using a geometrically nonlinear FE code. Modifications were made to the original model to reduce the amplitude of the marcels near the surfaces. The analysis was repeated for each modification. Comparisons of the interlaminar normal stresses, σn, in the various models showed that under combined axial-tension and cyclicbending loading, for marcels of the same aspect ratio, σn, stresses increased as the distance along the taper, from thick to thin end, increased. for marcels of the same aspect ratio and at the same X-location along the taper, σn, stresses decreased as the distance from the surface into the flexbeam interior increased. A technique was presented for determining the smallest acceptable marcel aspect ratio at various locations in the flexbeam.
marcel, ply waviness, delamination, finite element, flexbeam
Research engineer, U.S. Army Research Laboratory, Vehicle Technology Directorate, NASA Langley Research Center, Hampton, VA