Modulus Measurement for Prepreg-based Discontinuous Carbon Fiber/Epoxy Systems

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 (R). Although the average elastic modulus of this material has been shown to be as high as that of the continuous fiber quasi-isotropic 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.