Optimizing the damping properties of unidirectional composites by incorporating carbon fibers with a thin viscoelastic coating

Two distinct techniques were used to predict, and then optimize, the viscoelastic properties of unidirectional carbon fiber/epoxy composites where the carbon fibers were coated with a thin lossy coating. First, finite element models with hexagonal and random microstructures were used to predict the effect of various parameters on the five independent stiffness constants of such unidirectional composites, most importantly that of the ratio of the coating thickness to the fiber diameter. Second, these predictions were compared to those obtained from the n-layered micromechanical model of Hervé and Zaoui and it was shown that the modelling predictions were remarkably accurate. It was found that while the longitudinal moduli were little affected by the presence of the coating, both the transverse and shear moduli showed a maximum loss (and a significant enhancement compared to uncoated fibers) for a coating thickness ratio of 0.001 corresponding to a coating thickness of ∼10 nm for a typical industrial carbon fiber. The enhancement of the shear loss moduli was shown to be particularly important, as a study of the eigenfrequencies of a simply supported viscoelastic plate strip showed that the shear deformation mode was activated and hence led to a significant increase in the effective vibration damping.