Design And Experimental Research Of EFPI-based Optical Fiber Lamb Wave Sensor Used In Plate Structure Health Monitoring

With the continuous development of society,various large-scale engineering structures in civil,mechanical and aerospace fields have become ubiquitous.To ensure the integrity of these structures and materials,there is an increasing need for structural health monitoring(SHM)to ensure safety and reduce maintenance costs and time.Ultrasonic guided wave(especially Lamb wave)technology is one of the most widely used SHM methods,and ultrasonic guided wave sensor is the key device for signal detection.Commonly used sensors for ultrasonic guided wave detection include piezoelectric,electromagnetic and pulsed lasers.The optical fiber sensor is one of the ideal choices to replace the traditional piezoelectric sensor because of its high sensitivity and anti-electromagnetic interference.At present,the research on fiber-optic ultrasonic guided wave sensor mainly focuses on the fiber grating sensor using contact coupling,while the research on fiber-optic ultrasonic guided wave sensor using air coupling is rarely reported.In this thesis,an ultrasonic guided wave sensor structure based on the fiber extrinsic Fabry-Pérot Interferometer(EFPI)is proposed,and the sensor is applied to the flaw detection experiment of flat plate structure based on Lamb wave.The main contents are as follows:First of all,the thesis focuses on the structure of the optical fiber EFPI ultrasonic guided wave sensor,through the theoretical analysis of the Lamb wave propagation characteristics and the finite element simulation of the damage detection structure,the excitation frequency of the ultrasonic guided wave detection is determined.According to the excitation frequency,the ultrasonic sensitive micro-mechanical(Micro-ElectroMechanical System,MEMS)diaphragm in the structure of the optical fiber EFPI sensor is studied,and the natural frequency and response characteristics of the diaphragm with different structural parameters were simulated and analyzed,and the optimal structural parameters suitable for ultrasonic guided wave sensing are obtained through optimization.Secondly,the thesis uses the MEMS processing technology to complete the preparation of the ultrasonic sensitive diaphragm,and combines the optical fiber EFPI structure to successfully realize the development and packaging of the optical fiber EFPI ultrasonic guided wave sensor.The performance of the sensor before and after the package was tested by air coupling and contact coupling,and the comparison experiment with the electrical ultrasonic guided wave sensor was carried out to verify the excellent performance of the optical fiber ultrasonic guided wave sensor.Finally,using the fiber EFPI ultrasonic guided wave sensor developed in this thesis,the experimental study of Lamb wave detection of plate structural damage is carried out.The contact and air-coupled Lamb wave damage detection tests were carried out on the plates of metal materials and acrylic materials respectively.The experimental results proved that the sensor has good detection performance for the Lamb signals formed by the structural damage of different plate materials.In conclusion,the EFPI-based optical fiber Lamb wave sensor developed in this thesis has high sensitivity and high signal-to-noise ratio,which can be used not only for the detection of contact Lamb wave and structural damage detection,but also for noncontact situations,and has a good application prospect in the field of structural health monitoring.