This paper quantifies the elastic behavior and failure response of discontinuous carbon fiber/ epoxy laminates produced by compression molding of randomly oriented preimpregnated unidirectional tape. Flat plates have been successfully molded using manually prepared prepreg charges, and showed a satisfactory degree of randomization. Complex relationships between reinforcement aspect ratio and tensile, compressive, and flexural moduli and strengths are observed. For this particular material system, failure is a matrix-dominated event, with little or no fiber breakage, and it promotes relatively high variation in the measured properties. The high-volume carbon fiber content, combined with an aerospace-qualified epoxy resin, opens up opportunities for more aircraft parts to be made of composite materials, particularly for stiffness-critical components where discontinuous fibers offer performance similar to the continuous quasi-isotropic value.

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The elastic modulus of discontinuous carbon fiber/epoxy laminates produced by compression molding of chopped unidirectional prepreg tape is measured by several means. Commercial applications for this type of material form already exist, such as Hexcel HexMC Õ . Although the average elastic modulus of this material has been shown to be as high as that of the continuous fiber quasiisotropic benchmark, its non-homogenous nature gives rise to variations as high as 19% in the measurement by means of strain gage or extensometer. This phenomenon would be attributed to the variability of the manufacturing process, were it not for the fact that strength variation is much lower, with a maximum of 9%. In order to assess whether the variation observed is a result of the measurement technique and not an actual variation in material properties, a series of tensile tests is conducted while systematically varying strain gage length and location. The measurements are then compared relative to each other as well as to multiple extensometer readings along the length and opposite sides of the specimen. Digital image correlation (DIC) technique is used to gain further insight on the observed phenomena, and it has shown to be fundamental to obtain a full-field strain measurement, which is more repeatable than any of the traditional measurement techniques. Furthermore, DIC shows that complex strain distributions exist on the surface of the specimen, varying greatly along the width and across the length of the specimen, and these are associated to the non-homogeneous nature of the sub-structure.