First review of capacitance-based self-sensing in structural materials

Abstract

This is the first review of the emerging field of capacitance-based self-sensing in structural materials. Capacitance-based self-sensing is rich in both science and applications. It is applicable to materials ranging from insulators to conductors, whereas the longstanding resistance-based self-sensing is only applicable to conductors. The attributes sensed include the stress/strain, temperature and damage. The stress/strain sensing exploits piezopermittivity, which is typically positive. The temperature sensing exploits pyropermittivity, which is typically positive. The damage sensing exploits the fact that defects reduce the permittivity. The dielectric behavior of highly conductive materials is consistent with the 1999 theoretical prediction by Jonscher concerning charge-carrier polarization and the consequent “giant polarizability”. The materials for which capacitance-based self-sensing has been shown include 3D-printed polymer, asphalt (bitumen-based material), cement-based materials (without or with conductive admixture), continuous carbon fiber polymer-matrix composite, carbon-carbon composite, steel and aluminum. For the conductors, the polarization hinges on the carrier-atom interaction. For the electronic conductors (carbons and metals), the carriers are electrons. For the ionic conductors (cement-based material without conductive admixture), the carriers are ions. For the nonconductors (3D-printed polymer and asphalt), the molecules present align to cause polarization. This work is focused on the ordinary frequencies (such as kHz). For the conductors, the capacitance measurement using an LCR meter needs to be performed with a dielectric film between the specimen and an electrode. For determining the permittivity, the capacitance should be measured for three or more different values of the inter-electrode distance, so that the specimen volumetric capacitance and the interfacial capacitance of the specimen-electrode interface are decoupled.