Wireless interrogation of antenna sensor to detect hidden cracks

Recently, we have validated that microstrip antenna can detect sub-millimeter crack growth. The wireless interrogation of the antenna sensor under line-of-sight illumination of an interrogation antenna has also been demonstrated. In this paper, we present a wireless interrogation technique that is out of the line-of-sight of the interrogation antenna. This technique enables detecting and monitoring crack propagation between a lap joint or in any other case where a crack is hidden between two parts of a structure. Implementation of the sensor and the wireless interrogation unit using commercially available microwave components is presented in detail. The experimental results confirmed the capability of this technique to wirelessly interrogate the antenna sensor. In addition to crack propagation detection, this technique can also be used to monitor compressive forces between two faying surfaces of a lap joint structure.