Capacitance-based self-sensing of flaws and stress in carbon-carbon composites, with reports of the electric permittivity, piezoelectricity and piezoresistivity

Abstract

Capacitance-based nondestructive flaw evaluation (NDE), electric permittivity, piezoelectricity (capacitance-based stress/strain self-sensing) and piezoresistivity (resistance-based stress/strain self-sensing) of carbon-carbon composite (C/C, density 1.5 g/cm3, PAN-based carbon fiber) are unprecedentedly reported. The in-plane capacitance measured using coplanar electrodes decreases monotonically with increasing cumulative artificially inflicted damage (1–62 blind/through holes, diameter 1.00 mm), indicating NDE feasibility. The in-plane relative permittivity is 1.1 × 104 (2 kHz); the in-plane DC resistivity is 2.4 × 10−3 Ω .cm. These values are similar to those previously reported for PAN-based carbon fiber. The permittivity increases reversibly by up to 16% with increasing elastic tensile strain (up to 0.0081%). However, the contribution of the stress-dependent permittivity to the piezoelectric effect is negligible. The observed in-plane direct piezoelectric effect is primarily due to the reversible monotonic decrease of the electric field output with the increasing tensile strain. The piezoelectric coupling coefficient d33 is negative, -(8.1 ± 0.2) × 10−9 pC/N, in contrast to the positive value of +(1.7 ± 0.3) × 10−8 pC/N for PAN-based carbon fiber. The in-plane piezoresistivity gage factor is negative, −7804 ± 429, which is much higher in magnitude than −1830 ± 47 previously reported for PAN-based carbon fiber. The differences in d33 and gage factor between C/C and PAN-based carbon fiber are attributed mainly to the carbon matrix in the C/C.