Wireless Crack Monitoring by Stationary Phase Measurements from Coupled RFID Tags

The possibility to wirelessly monitor the state and the evolution of cracks is of increasing interest in emerging structural health monitoring systems. A simple and effective measurement method considers the placement of two passive radio frequency identification (RFID) antennas on top of the crack, so that the crack´s evolution will produce a change of the inter-antenna coupling and in turn of the phase of the backscattered field. An ad-hoc design technique, based onto the coupled-modes physics, permits to maximize the sensor´s sensitivity avoiding, or at least mitigating, the read range reduction during the evolution of the displacement that is instead typical of amplitude-oriented RFID displacement sensors. The proposed idea is demonstrated by numerical and experimental examples showing the possibility of sub-millimeter resolution with low-cost devices.