Research On Ultrasonic Nondestructive Detection Based On Fiber Grating Sensing

As an efficient and fast detection technology,ultrasonic nondestructive testing is widely used in the field of structural health monitoring.The conventional ultrasonic nondestructive testing method is to detect ultrasonic waves propagating in a structure by piezoelectric sensors,and then identifies damage in the structure based on the detected ultrasonic signals.However,piezoelectric sensors that detect ultrasonic signals are susceptible to electromagnetic interference and corrosion resistance,resulting in unreliability in some special applications.Therefore,this thesis attempts to use the fiber grating instead of the piezoelectric sensor for ultrasonic detection,the ultrasonic sensing characteristics of the fiber grating and the ultrasonic nondestructive testing based on the fiber grating sensing are studied.The specific work is as follows:?1?The dispersion characteristics of the Lamb wave is studied.the dispersion curve of the 6061 aluminum alloy plate was drawn.And the finite element method is used to simulate the propagation of Lamb waves in aluminum plates,which proves the feasibility of using Lamb waves for damage identification.At the same time,the theoretical relationship between the wavelength of the reflection spectrum of the fiber grating and the ultrasonic modulation is derived.?2?The ultrasonic sensing characteristics of fiber gratings in different arrangement positions,different lengths and different bonding modes are studied.The experimental results show that in order to improve the signal-to-noise ratio of the detected ultrasonic signal,the choice of fiber grating should follow the principle that the ratio of fiber grating length to ultrasonic wavelength is small,and it is arranged along the axis of the ultrasonic sound field.At the same time,it was found that the remotely bonded fiber grating has the same ultrasonic detecting capability as the directly bonded fiber grating.Finally,in order to solve the problems of limited placement of fiber grating in ultrasonic detection,low reusability,low safety and stability of bare fiber grating,a high sensitivity fiber grating ultrasonic sensor based on coupling cone structure is designed.The performance of the sensor is verified by experiments,which proves that the sensor has good ultrasonic detection capability and can distinguish the A₀ and S₀ modes in the Lamb wave signal.?3?An ultrasonic non-destructive testing system based on fiber Bragg grating sensing is build The fiber grating ultrasonic sensor designed in this thesis is used to detect the Lamb wave signals carrying circular hole type defects and slit type defect information.The experimental results show that defects in the aluminum plate can be judged by detecting the attenuation of the wave packet amplitude of the Lamb wave in time domain;The identification of the defect type is realized by detecting whether the newly generated frequency component of the Lamb wave in frequency domain is 0.5 kHz or greater than 0.5kHz;The defect is expanded or not can be judged by comparing the attenuation of the wave packet amplitude in the two measured Lamb wave in time domain or the difference between the front and back signals in frequency domain.The specific features are as follows:?1?After the Lamb wave interact with the circular hole type defect,a new frequency component of 0.5 kHz is generated and the frequency is independent of the aperture size;and after the Lamb wave interact with the slit type defect,a new frequency greater than 0.5 kHz is generated.The frequency is related to the size of the slit.Therefore,the defect type identification can be realized by detecting whether there is 0.5kHz frequency component of Lamb wave in frequency domain.?2?After the Lamb wave interact with defect,the Lamb wave in time domain will has different degrees of attenuation,and the larger the defect,the greater the attenuation.Therefore,the defect is expanded or not can be judged by the attenuation of the wave packet amplitude of the Lamb wave in time domain.Further,the extension of the defect can also be characterized by the change of the Lamb wave in frequency domain.When the circular hole type defect is expanded in the aluminum plate,the frequency component of the Lamb wave does not change,but the energy of the S₀ mode decreases;When the slit type defect is expanded,the frequency components of the Lamb wave have changed.