Theory And Application Of Detection Technology Based On Ultrasonic Excitation And Fiber Bragg Gratings Sensing

The fiber Bragg gratings (FBG) detection technique under ultrasonic excitation is a new technology being developed in recent years. Its basic idea is to use the FBG as an ultrasonic detector instead of piezoelectric ceramics and analyze the injury condition of the construction based on FBG signals. By combining the unique advantages of FBG in electromagnetic interference immunity, distributed sensing with the ultrasonic features such as good directivity, high penetrating power, it has great development potentiality in structural damage detection field. Some foreign universities and research institutes have studied on this technology, however it hasn’t been reported in china.This dissertation aims to set up a new theory for mechanical structure damage detection by using ultrasonic wave to excite mechanical structures and FBGs to detect the ultrasonic strain wave with damage information and structural stress signals. It mainly studies the FBG high speed demodulation technology and glued FBG multiple structure under ultrasonic excitation; it reveals the strain sensing characteristics from host material to fiber core of the glued FBG under ultrasonic excitation; it presents a new principle and method on structural damage detection and location based on ultrasonic excitation and distributed FBG sensing in order to solve the foundation problem toward the on-line detection identification and location of the mechanical crack damage. The detailed content of this paper shows as follows:1. It gives a review of the FBG high speed demodulation method, FBG strain transferring characteristic and sensitivity property as well as the application of this technique in composite and metallic materials health monitoring.2. It analyses the acoustic field distribution in mechanical structure and proposes that the FBG should be arranged at the closer far-field region from the ultrasonic source. The dispersion curves of the alloy aluminum as well as the relationship between the ultrasonic incidence angle and the frequency-thickness product are studied, to provide a theoretical basis for the reasonable selection of acoustic frequency and incidence angle according to different test materials. The ultrasonic stress field of the test mechanical structure under ultrasonic excitation is simulated with Comsol software, offering references for the layout and optimization of FBGs.3. Two high-speed demodulation systems for fiber Bragg grating (FBG) sensor respectively based on matched fiber Bragg grating and tunable laser light are studied. Their measuring principles are presented and the output curves are given by theoretical deduction. Then the vibration signals of an ultrasonic cleaner were measured using these two systems. The wavelength measurement sensitivity and demodulation speed of these two systems were compared with experiments. Finally, the experimental results demonstrated the feasibility to detect Lamb waves in metal plates with the FBG demodulation system based on a tunable laser light.4. A four-cylinder sensing model for glued FBG sensor is established in this paper. According to the shear-lag theory, an improved strain sensing function is derived from this model. Then, based on above function, the strain sensing characteristics and the effects of ultrasonic frequency, adhesive layer thickness and host material on strain transfer efficiency from host material to fiber core are studied with simulation. Finally, the system with an ultrasonic transducer to excite aluminum alloy thin plates and surface-bounded FBGs with AB glue to detect the Lamb wave propagating through the plate was established to validate the theoretical analysis.5. The optical spectrum of FBG under ultrasonic excitation is analyzed with the Transmission Matrix method. The influences of FBG length and ultrasonic power on the FBG spectrum distribution are discussed. Numerical results prove that the FBG can be used to detect ultrasonic signal in structure detection when the length of the FBG is shorter than a half of ultrasound wavelength with proper energy6. The detection system based on ultrasonic excitation and fiber Bragg gratings sensing was established. The ultrasonic fault detector was employed to stimulate the ultrasonic probe, thus generating ultrasonic waves in the thin plate. The glued FBG was used to measure the ultrasonic waves and the demodulating system based on tunable laser source was applied to achieve the center wavelength demodulation of FGB. On these bases, the experiment set was used to study the character of acoustic intensity distribution along the acoustic axis in the aluminum alloy (5052) plates, proving the feasibility to detect the acoustic waves with the system. Then, the experiment device was used to detect the two hole defects with diameters of6mm in the aluminum alloy (5052) plates, discovering new wave packets in the FGB measured ultrasonic waveform when there was hole damage in the plate, whose arriving time before and after the damage can be used to determine the damage localization. The positioning errors of the two holes were3.3mm and0.8mm, respectively. The datas obtained from distributed FBGs with this experiment set were used to study the position and size of the damage in a plate.