Through-thickness behavior of thick unidirectional composites

An investigation was conducted on the through-thickness compressive and tensile mechanical behavior and failure of thick unidirectional composites. The through-thickness characterization of composite materials is critical for the reliable and robust design of thick composite sections used in structural applications. Additionally, the out-of-plane failure modes and strength parameters of composite laminates are of great importance in the verification of three-dimensional failure theories. In characterizing the through-thickness mechanical properties of a composite, the objective is to produce a state of stress in the test specimen which is uniform and serves to measure the true properties of the material. Whereas in the case of multidirectional or textile laminates the failure is nearly always interlaminar, in the case of a unidirectional lamina the matrix-dominated failures and failure planes are not always easily predictable by theory. In this study, specimens were machined from a thick (-25 mm) unidirectional carbon/epoxy lamina and tested in compression and tension at various angles with respect to the lamina plane (1-2). Straight-sided and waisted specimens were used for direct loading in the through-thickness direction. The stress-strain behavior, failure modes, and strengths were recorded. Of special note was the failure mode in transverse compression, where the failure plane was not predictable by conventional composite failure theories. The orientation of the failure plane was more in line with predictions by a Mohr-Coulomb failure model.