Supersensitive wireless sensing - how cracks can help composites get smarter?

Abstract

Non-intrusive embedded sensors are a deeply needed technology for monitoring of large scale composite structures. Composite structures such as pipelines, tanks, aircraft, ships, and ground vehicles confront some challenges with embedding strain sensing systems incorporating strain gauges or optical fibers that can introduce delamination, cracking and structural failure of the host in addition to the need for dedicated and expensive equipment. We present the evolution of the research in our lab, that led to our latest RF sensing technology with high sensing sensitivity. This work started with the quest for supersensitive piezoresistive sensors, in which the unique response of cracked structures is leveraged. We illustrate, in the field of stretchable electronics, how the sensors properties can be controlled by tailoring the crack networks. While interesting, super piezoresistive sensors are however not practical, as difficult to integrate in wireless systems, and with poor reproducibility. However, this initial work paved the way for a new generation of sensors based on piezo capacitance. These leverage the unique properties of capacitive sensors with superpiezoresistive electrodes. We develop a flexible and thin sensor based on LC circuit where the capacitance is considered a sensing unit. We introduce a controlled network of cracks in the parallel electrodes and benefit from the way these cracks modify the electromagnetic wave penetration inside the parallel plate capacitor. The tailored network of cracks creates a piezoresistive effect that leads to a transmission line behavior of the capacitance resulting in a tremendous increase in sensitivity. This unconventional change in capacitance of the LC oscillator allows a large shifting in resonance frequency of the flexible circuit, producing a sensitive wireless strain sensor with a Gauge factor of 50 for less than 1% strain. Eliminating wires, power source, and electronic chip from the sensor body allow the sensor to be integrated easily inside the composites materials while maintaining the materials' mechanical performance. The experimental results show the ability of our cracked wireless strain sensor to detect small strain signals through the composites structure with high accuracy. The developed sensor is intended to be a part of a wireless sensor network (WSN ) for monitoring large composites structures.