Research On Ultrasonic Characteristics Of Metals Based On Fiber Fizeau Interferometer

In this dissertation,a combination of laser ultrasound and optical fiber sensing technology is used to build an all-fiber Fizeau interferometer ultrasonic detection system.Compared with traditional detection methods,the system has the characteristics of non-contact,wide frequency band and anti-electromagnetic interference.Based on this system,we studied the ultrasonic transmission characteristics of the metal and the elastic wave mode conversion problems,and performed ultrasonic scanning on the metal surface and internal defects.The contents of the dissertation are as follows:At first,the research background,significance and development status of the subject are introduced.It mainly summarizes the laser ultrasonic testing technology and the related knowledge of optical fiber sensing,and gives a brief description of the work content of this dissertation.After that,the classification characteristics,excitation principle,detection methods and ultrasonic detection methods of laser ultrasonic technology are introduced in detail,which provides reference for the following ultrasonic detection and signal analysis.Then we introduces the related background of optical fiber sensing technology,and focuses on the classification,characteristics and detection mechanism of common optical fiber interferometers.After referring to the knowledge of fiber optic interferometer,the fiber Fizeau interferometer system used in this experiment is built,and the composition,construction and debugging of the system are introduced in detail,which lay the foundation for the next part of the experiment.Next,the problem of elastic wave mode conversion in metal is studied.Firstly,the classification and characteristics of elastic waves are discussed,and the phenomenon of mode conversion and its principle are analyzed.Then the fiber Fizeau interferometer detection system is used to detect the mode transition phenomenon in the aluminum plate.Combined with experimental data and theoretical calculations,the various types of converted waves are identified,and the relationship between the transmission distance and the receiving time is obtained by linear fitting.Finally,ultrasonic scanning inspection is performed on groove defects and internal cylindrical defects on the metal surface.By analyzing the surface Rayleigh wave of the sample,the transmission mode of the ultrasonic wave and its interaction with the surface defect are studied.Then,the position,width and depth of the surface defect are quantitatively measured by theoretical derivation;In the method,the longitudinal and shear waves in the sample are ultrasonically examined to study the interaction between internal defects and ultrasonic waves.Through data analysis,the C-scan image of the sample is finally obtained,and the imaging detection of the internal defect size,position,shape and the like is realized.In summary,this dissertation combines theoretical analysis and experimental measurements to study the propagation characteristics of laser ultrasound and the interaction between ultrasonic and defects in metal plates.At the same time,it also provides a high-resolution optical detection method,which provides an important reference for the application of optical fiber sensing in non-destructive testing and ultrasonic characteristics research.