EVALUATION OF 3-D REINFORCEMENTS IN COMMINGLED, THERMOPLASTIC STRUCTURAL ELEMENTS

Through-the-thickness reinforcement of biaxial woven composite laminates has always been an attractive alternative to 2-D construction. The obvious advantage lies in the improved resistance to delamination caused by the presence of fiber in the out-of-plane direction. Two methods which provide z-direction reinforcement in composites are under investigation at Boeing Helicopters. Both programs used commingled graphite/thermoplastic (polyetheretherketone - PEEK) yarns as the composite material. The first method is to simply stitch a 2-D woven commingled fabric with graphite thread. The stitched laminates were compression-after-impact (CAI) tested in a cooperative effort with the Air Force Wright Aeronautical Laboratories. After instrumented impact, pulsed-echo non-destructive evaluations (NDE) were made on both the stitched and unstitched coupons prior to compression testing. Data on the after-impact compression strengths of both stitched and unstitched specimens are included in the paper. The second method to provide z-direction reinforcement in a woven laminate is to 3-D braid the commingled yarns into a net-shape preform. The goals of this program are to measure the effects of processing on strength and to determine the improvement in damage tolerance caused by fiber placement in the z-direction. The 3-D preform can be processed by autoclave or press operations. Some specimens were evaluated using x-rays and by computer-aided tomography (CAT) scan at Ford Research. These and other evaluations to determine the material and structural properties of 3-D braided net-shape composites are discussed.