An Experimental Exploration of Deformation-Dependent AC Conductivity in Carbon Nanofiber-Modified Epoxy

Conductive nanofiller-modified materials have received a significant amount of interest for use in self-sensing, nondestructive evaluation (NDE) and structural health monitoring (SHM) owing to their piezoresistive properties (i.e. having deformation-dependent electrical transport). To date, the majority of the work related to piezoresistivity has focused on the relation between direct current (DC) conductivity or resistivity and strains. However, DC-based methods of self-sensing have important limitations such as poor sensitivity to spatially distributed damage and high resistivity. Alternating current (AC)-based methods of electrical interrogation have potential to address these limitations. Unfortunately, much less work exists on the effect of strain on AC conductivity. Therefore, we herein explore the effect of strain on AC conductivity in piezoresistive polymer nanocomposites. Specifically, epoxy is modified with carbon nanofibers (CNFs) at 1 wt.% and tested under uniaxial loading as AC conductivity is measured as a function of interrogation frequency. The AC conductivity-frequency relation is then fit to a universal power law for a range of compressive and tensile strains such that power-law fitting constants can be expressed as a function of normal strain. The basic insights revealed from this work are an important step toward transitioning piezoresistive-based self-sensing from prevailing DC approaches to potentially much more powerful AC methods.