Characterization of the Effect of Fiber Undulation on Strength and Stiffness of Composite Laminates

Report No. ARL-TR-7249
Authors: Todd C Henry; Jaret C Riddick; Ryan P Emerson; Charles E Bakis
Date/Pages: March 2015; 28 pages
Abstract: Stiffness and strength predictions of undulated composites (filament-wound cylinders, braids, plain weaves, etc.) using traditional laminated composite theories are complicated by complex fiber architecture. Undulations consisting of fibers passing over and under each other result from the interweaving process. In the current investigation, full-field strain measurements were used to evaluate local strain distributions in the region of a 0° undulated ply in a [0n/90n]s laminate (n = 2,4,6) and a 30° undulated ply in a [30n/60n]s laminate (n = 2,4). Specimens were manufactured with carbon fibers, various amplitudes of undulation, and matrix materials with moduli ranging from 300 to 3,000 MPa. Two-dimensional digital image correlation was used on the free edges of the compressively loaded specimens. The observed strain fields were highly influenced by the undulation geometry. The axial modulus (Ex) of a [0n/90n]s laminate was more sensitive to reinforcement undulation for flexible matrixes (300 MPa) rather than rigid (3,000 MPa). Fiber undulation was observed to elevate out-of-plane shear and through-thickness normal strains in regions eventually involved in the fiber microbuckling failure process.
Distribution: Approved for public release
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Last Update / Reviewed: March 1, 2015