Sensing of transverse pressure in structural composite by impedance spectroscopy on embedded carbon nanotube sensing structures

Abstract Next-generation aerospace composite structures are expected to evolve from purely mechanical systems into multifunctional structures by integrating additional functionalities through the embedment of functional filler materials. One promising approach is the incorporation of carbon nanotubes (CNTs) to add sensing capabilities. In this paper, two CNT-based sensing structures are evaluated for transverse pressure sensing up to 20 MPa when embedded in an aerospace-grade glass fibre/epoxy laminate. In pursuit of higher sensing sensitivity, a shift from direct current (DC) to alternating current (AC) based sensing is implemented, enabling the exploration of frequency-dependent sensing behaviour. With this transition, a characterisation and measurement procedure is presented and justified, determining resistive and polarisation effects present in the sensing configurations and evaluating their susceptibility to stray capacitance prior to pressure sensing. The first sensing structure, using embedded Vertically Aligned CNT (VACNT) forests, exhibits pressure sensitivity with the resistive sensitivity increasing at frequencies above the critical frequency of the system, justifying the shift from DC to AC. The reactance shows similar pressure sensitivity except for in a region near 1 MHz, where it becomes pressure insensitive. The second sensing structure, consisting of two embedded VACNT forests separated by a Kapton film, emulates a capacitor. Its impedance shows a Kapton-dominated frequency range and a CNT-dominated range, with a transition region in between. Consequently, the pressure response becomes frequency-dependent, as the two constituents not only dominate different frequency ranges but also exhibit different sensitivities to pressure.