Full-Field strain analysis of compressively loaded flat composite laminates with undulated fibers

In filament wound composites, the existenceof undulated fibers introduces unique challenges in the prediction of compressive modulusand strength using traditional laminated composite theories. Undulations, consisting of fibers passing over and under each other,result from the interweaving process inherent in helical fiber winding patterns. In the current investigation, full field strain measurements were used to evaluate local strain distributions in the region ofa 0-deg. undulated ply in a [0_n/90_n]_s laminate(n=2,4,6) and a 30-deg. undulated ply in a [30_n/-60_n]_s laminate (n=2,4). Specimens were manufactured withcarbon fibers, various amplitudes of undulation, and matrix materials with three different moduli of elasticity. Full-field strains were measured on the free edge and across the width of the compressively loaded specimens using two-dimensional digital image correlation (DIC). The observed strains were highly influenced by the undulation geometry. The axial modulus of a [0_n/90_n]_s laminate was more sensitive to reinforcement undulation when the matrix was flexible ratherthan rigid.An undulation with an amplitude/length ratio of 0.1 reduces the average axial modulus of elasticity in the undulation region by approximately 43% and 3% in laminates with flexible and rigid matrices, respectively, relative to a similar material without undulation. Observations of strain on the free edge revealed that fiber undulation caused elevated out-of-plane shear and through-thickness normal strains in regions eventually involved in the fiber microbuckling failure process. This information could not be obtained by observing strains across the width of the specimen. Comparisons with filament wound tubes indicated agreement in the failure modes.