Multifunctional Soft Transducer for Electrical and Optical Sensing Applied to Fatigue Crack Monitoring

Abstract

The timely discovery and monitoring of fatigue cracks on steel bridges is critical in ensuring structural safety and continuous operations. Existing sensing solutions, for example foil gauges, can be used to monitor known cracks, yet cannot be used for crack discovery because of their highly localized nature. A solution is the development and deployment of large-area electronics capable of detecting local states over large surfaces. Polymer-based nano-structured materials that respond to external impacts, such as mechanical strain, with color changing properties have attracted significant attention in structural health monitoring (SHM) community, because their passive optical/visual properties can be used to assist inspectors at quickly localizing new fatigue cracks. Here, the authors proposed a multifunction skin sensor that combines optical and electrical sensing properties. The optical function is passive and engineered to visually assist in localizing fatigue cracks, and the electrical function is added to send timely warnings to infrastructure operators. This is achieved by modifying the nanoscale structures within a photo-elastomer to obtain a soft stretchable optically-active film that is sandwiched between transparent carbon nanotube electrodes (CNT) to form a capacitor structure for adding electrical functionality. The developed sensor has a stiffness of 460 kPa and withstands reversible strain levels of up to 40%. Additionally, it exhibits a reversible and repeatable color change from light blue to deep blue, and changes the reflected color in the Vis spectra from approximately 500 nm to 600 nm and insensitive to viewing angle. The performance of the sensor is characterized through a free-standing dynamic test and further extended to a free-vibration test conducted on a steel cantilever plate. A correlation- based image processing algorithm was developed to discriminate color change and further quantify strain. The measured shift in the material’s reflection center wavelength was merged with an RGB correlations matrix and an optical gauge factor of 0.52 was obtained. An electrical gauge factor of 0.48 was obtained by subjecting the sensor to a triangular load. It was found that the parallel capacitance measurements exhibited better performance by yielding higher accuracy for free vibration strain measurements.