A Crack Mouth Opening Displacement Gauge Made With Passive UHF RFID Technology

In structural health monitoring crack detection and its width measurement are important monitored parameters. The Crack Mouth Opening Displacement (CMOD) is a good indicator of fracture properties. A new kind of CMOD gauge is shown in the paper, it is a wireless sensor tag based on off-the-shelf Ultra High Frequency (UHF) Radio Frequency Identification (RFID) technology. The sensor measures the opening of the crack as a variation of the phase of the signal backscattered from an RFID tag connected to a load sensible to the change of the crack width. The novelty of the sensor design relies on an electromagnetic method that permits to enhance both the communication stability and the sensitivity of the RFID tag. The paper shows the electromagnetic theory underlying the design procedure, the method for dimensioning the sensor tag, the realization of a prototype and its experimental measurements. Theoretical and experimental results show that the sensor has a tunable sensitivity with minimum value 16°/mm, a maximum range of 5 mm, a nominal resolution better than $187 ~\mu \text{m}$ and a measurement uncertainty less than $\pm 250 ~\mu m$ while it allows a reading distance longer than 1.5 meters.