Alignment of carbon nanotubes in glass fiber composites using ac electric field

The ability to detect delamination damage in fiber reinforced composites by electrical means can be enhanced by reducing the through-thickness electrical resistivity in comparison with the normally higher in-plane resistivity. Conductive nanofillers such as carbon nanotubes (CNTs) offer the potential to tailor the conductivity of otherwise highly resistive composites, such as glass fiber reinforced polymers (GFRPs), on account of their small size and high aspect ratio. The current investigation aims to assess the efficacy of aligning CNTs in the through-thickness direction of unidirectional GFRP using an alternating electric field applied during the curing process, thus decreasing the through-thickness resistivity. For a fixed alignment field strength of 1,000 V/cm, it was found by experimentation that a 100 Hz alignment field provides slightly better electrical tailoring than a 1,000 Hz field. The through-thickness resistivity could be reduced by one or two orders of magnitude, resulting in a composite with in-plane and through-thickness resistivities of nearly the same order of magnitude. A finite element analysis showed that improved alignment results can be expected when the alignment electrodes are in direct contact with the composite rather than separated by a release film of low dielectric constant, although the presence of glass fibers causes significant disturbances in the orientation of the electric field within the composite in either case.