Electrical Impedance Characterization of Discontinuous Conductive Fiber Composites

Northwestern University, Evanston IL

Investigators

Abstract

The premise of this work is that the electrical impedance response of composites with discontinuous, highly-conductive fibers can be employed to characterize their microstructure (fiber bonding, orientation, number density, etc.) and to monitor important changes upon loading, including strain and permanent damage effects (e.g., matrix cracking and fiber debonding, fracture and pullout). Such capabilities are needed for the controlled fabrication and deployment of 'smart', self-monitoring structural composites. Since the necessary requirements for such an impedance response-highly conductive fibers, a moderately conductive matrix, and a frequency-switchable interfacial impedance-have been identified, this project will study the impedance response of cementitious matrix (steel or carbon fibers in cement) and ceramic matrix (SiC in silicon nitride) composites. A combination of laboratory 'physical' simulations (e.g., single wire/fiber in a dielectric medium), pixel-based computer modeling (in collaboration with NIST), and actual composite studies (single fiber pullout, bulk compressive, and notched-beam tensile tests) will be carried out, with the latter appropriately instrumented for simultaneous impedance measurements. %%% This project will study the microstructure and microstructural changes during loading of composite materials using impedance spectroscopy. The results obtained should be applicable to a broad range of composites, including cement-matrix, ceramic-matrix, and even polymer-matrix systems (if the above-mentioned conditions are met) and are an important first step in the development of novel self-monitoring composite materials and methodology. As well as improving our understanding of these composites, the impedance spectroscopy technique will be developed into a reliable characterization tool for microstructural analysis. In addition to the training of two graduate students and several undergraduates in materials science and engineering, the materials and experiences from the project will be incorporated in a course teaching science and engineering principles to non-science undergraduates. ***