Study On Active And Passive Damage Detection Methods Based On Ultrasonic Fiber Optic Sensing Technique

Ultrasonic Fiber optic sensor(UFOS) is one of the focus issues in the field of structural health monitoring and smart structures. Compared with traditional wave sensors generally based on the electronic sensitive components, Fiber optic sensors are more sensitive, smaller, anti-interference, more adaptive and flexible and combined sensing and transferring of signals. Combining guided waves theory or acoustic emission, optic fiber sensors can be used more conveniently and widely in damage inspection and monitoring for real structures. In this work, active and passive structural damage detection methods using UFOS based on Mach-Zehnder(M-Z) interferometer are investigated mainly.At first, the principle of Mach-Zehnder interferometer was introduced. The principle of ultrasonic measurement using fiber optic sensors was analyzed. For obtaining the best measurement results, the effects of the environment temperature, random vibration and the birefringence was analyzed.Subsequently, the UFOS was used to conduct active damage detection of the pipe structures. For increasing the sensitivity of the UFOS, it was wrapped around the pipe to increase the length of the sensing arm. Through the analysis of guided wave propagating along the axial direction in the pipe, L(0,2) modes was selected to carry out the inspection in this work. The 31 d type piezoelectric wafers array was used to excite guided waves in the pipe. The characteristics of guided wave signals received by UFOS was investigated, and compared with results from the conventional PZT.Finally, a reusable UFOS was proposed. Its characteristic of frequency response was studied on the plate for different sizes. Furthermore, it was used to detect the acoustic emission signals generated from a concrete beam under loading. Comparative analysis was conducted with results from the conventional AE sensors.