This sensor is to detect cracks, delamination, and corrosion damage based on electrical impedance. A prototype CNT-based sensor has been built and cracking and electrolyte representing corrosion have been detected based on the impedance of the sensor. CNT film sensors, highly distributed on the surface or embedded within a composite, are used to form a continuous sensor for crack and corrosion monitoring. An example of nanotube neurons on a simple panel is explained. A highly distributed network of nanotube continuous sensors, which sense along their entire length and have a biomimetic architecture, allows coverage of large structures. An example of the change in resistance as a crack passes through a single neuron is shown in Figure 16(a). There is a small change in resistance as the crack begins propagating through the neuron. The change becomes larger and approaches infinity as the crack passes through the neuron. An example of the change in capacitance as electrolyte is put on a single neuron is shown in Figure 16(b). Up to a factor of 50 increase in capacitance occurs because of the double layer supercapacitance property of nanotubes. The change in resistance owing to the electrolyte is only 6% as shown in Figure 16(b). Therefore, crack and corrosion sensing are mostly decoupled and can be measured using the same neuron. Signal processing using the nanotube neurons is greatly simplified because the electrical properties of the neurons, rather than structural waves, are used to characterize damage.
A prognostic method for damage modeling can be developed based on the changes in electrical parameters of the CNT neurons. The modeling of the neuron is based on the Randal Warburg circuit. The CNT is a strain sensor (piezoresistive effect) and a highly sensitive corrosion sensor (electrochemical impedance spectroscopy (EIS) effect). A model should be developed to predict the crack length based on the EIS spectra of the nanotube composite. An electrode configuration can also be used on a nanocomposite plate where the nanotubes are dispersed in the polymer matrix. Low-cost carbon nanofibers can replace high-cost CNT for large applications of an SNS.
An important application of the CNT neuron in WTB is health management of polymer matrix composites (PMCs). The CNT neuron can be developed for service life monitoring of PMCs used WTB structural applications. Two of the primary life-limiting mechanisms in polymer composites are hygrothermal degradation and oxidative degradation. Hygrothermal and oxidative degradation can lead to chemical changes in the resin system causing cracking and embrittlement in the surface layers of the composites. Within the oxidized layer of the composite, the tensile strength, strain to failure, flexural strength, density, and toughness decrease while the modulus increases. Surface cracks provide pathways for the transport of moisture and oxidants to the fiber/matrix interfaces that act as high-diffusion paths thereby increasing the degradation rate. The CNT neuron is a passive continuous sensor that can monitor the electrical impedance of composite materials to indicate degradation of PMCs used in WT structures. Future work should investigate the scientific, technical, and commercial feasibility of the CNT neuron for PMC health management including monitoring oxidative degradation for a neat resin aged in low temperature and oxidizing environments.