RESISTIVITY MEASUREMENT TECHNIQUES FOR COMPOSITE MATERIALS

Abstract

In this article, the theoretical concepts and the experimental techniques for measurements of the volume and surface resistivity of carbon fibre reinforced polymer (CFRP) composite materials have been explained. The purpose of these measurements is to investigate the potential of composite materials as lightning arrestors for an aircraft. In particular, the AC resistivity method and the Delta mode of measurement have been described in detail. A few measurements were carried out using an indigenously designed and fabricated collinear four-probe resistivity measurement tool, for which factors dependent on the dimensions of the samples have to be incorporated, as per the ASTM F390-11 standards. The validity of the two-probe and four-probe methods is also analyzed, along with techniques to minimize errors in measurements. Conductivity measurements on uni-directional laminates have been made in the longitudinal (fibre) direction as well as perpendicular to it, indicating a marked difference in the relative magnitudes. When investigating the potential of materials as lightning arrestors, a very high surface conductivity is desired so that lightning currents (even of the order of kA) pass along the exterior of the aircraft, without penetrating the underlying structure. The effects of the addition of copper nanotubes (CNT) and copper mesh have also been studied. The instruments involved in these experiments have been automated based on the IEEE – 488.2 protocols. The software to program them was developed using Labview. Measurements have also been performed on standard samples to test the accuracy of the data. Currents ranging from 0.1 to 100mA were passed, to deduce the current-dependency of the surface resistivity of these materials.