IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308 _______________________________________________________________________________________ Volume: 04 Special Issue: 02 | NCIRET-2015 | Apr-2015, Available @ http://www.ijret.org 59 RESISTIVITY MEASUREMENT TECHNIQUES FOR COMPOSITE MATERIALS Florita D’Sa 1 , Shubha V 2 , Anil Kumar A 3 1 Scientist, Airport and Thermophysical Instrumentation Group, National Aerospace Laboratories, Bangalore, India 2 Chief Scientist, Airport and Thermophysical Instrumentation Group, National Aerospace Laboratories, Bangalore, India 3 Senior Scientist, Advanced Composites Division, National Aerospace Laboratories, Bangalore, India 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. Keywords: CNT, Labview, Delta, GPIB --------------------------------------------------------------------***------------------------------------------------------------------ 1. INTRODUCTION Measurements of surface resistivity are required when a conductive coating is present on an insulator, in which case the magnitude of the resistivity of the surface is distinctly different from that of the bulk of the material. The volume resistivity of a material is also referred to as its bulk resistivity since it is determined by the entire volume of the material. The two-probe method for the measurement of resistivity is adequate if the resistance of the contacts (or leads) is much lower than the specimen’s resistance, but if it is commensurate with that of the specimen, this method would lead to errors in measurements. Errors may also arise due to thermoelectric voltages, when conductors made of dissimilar metals are joined together. In order to avoid these problems, the four-probe method is employed, using either alternating current or a rapidly reversed direct current. To ensure that the current density in the sample is uniform, the current and the voltage contacts should not be very closely spaced [1]. If connecting surfaces are not kept clean and free of oxides, a copper-to-copper oxide junction could result in a thermo- emf as high as 1mV/ºC. Methodology: The three different techniques employed for the measurements are explained in the following section. Four-probe collinear measurements: A few measurements were carried out using an indigenously designed and fabricated measurement tool. This tool ensures that the four collinear probes touch the sample only at their tips and the points of measurement are in the middle of the sample. Delta measurements: For such measurements, the applied current is repeatedly reversed to nullify noise due to thermoelectric effects. With sophisticated data acquisition instruments, a high speed of reversal is possible and moving averages are calculated using three values at a time, instead of only two, to enhance the quality of the data, as illustrated in Figure 1 [2]. AC Measurements (lock-in technique): This is another technique employed for the measurement of resistivity, whereby a signal from the sample is locked to a reference signal, derived from the same current source. The operation of a lock-in amplifier (also referred to as a phase-sensitive detector) relies on the orthogonality of sinusoidal functions. Accurate measurements can be made even when the signal of interest is obscured by much larger noise sources. Measurements are usually made at a frequency of 400 Hz, to avoid line-frequency interference.