Detection and localization of fatigue cracks is a difficult task. Existing technologies enabling nondestructive evaluation and structural health monitoring are expensive to utilize, time consuming, and/or have a complex link signal-to-damage. Recent developments in large area electronics (LAE) have facilitated the deployment of large distributed networks of transducers onto structural surfaces. We have recently developed a soft elastomeric capacitor (SEC), a type of LAE, which transduces strain into a measurable change in capacitance. Deployed in a dense network configuration, the SEC would have the capacity to detect and localize damage over a global surface, analogous to biological skin. A particular network application of the SEC is for detection and localization of fatigue cracks, which has been demonstrated in prior work. However, such network demonstration used a limited number of sensors due to limitations in the off-the-shelf data acquisition system used in the experiment. To enable dense network applications, we propose a dedicated readout circuit. The circuit consists of a relaxation oscillator that produces a square wave based on a differential capacitance input. The period of the square wave is directly related to the change in capacitance, which in turn can be related to a change in strain, also linearly. Preliminary tests were conducted on ceramic capacitors, which simulated eight strain levels from 0% to 3.94% strain. Results show that the readout circuit was linear as a function of strain, stable as a function of time, and that the experimental data closely fit the electrical model.
doi: 10.12783/SHM2015/376