Laser-ultrasonic Transmission In Elastic Plates With Periodic Structures

In this paper,a set of laser ultrasonic detection system is established.Compared with the traditional detection methods,the system uses the fizeau optical fiber interferometer to detect the laser ultrasound,and this method has the characteristics of non-contact,wide frequency band and anti-electromagnetic interference.Then we used the system to study the laser ultrasonic surface wave and the Lamb wave.The main content of the thesis include the following aspects:1?In this paper,the propagation characteristics of Rayleigh wave and Lamb wave in metal plate are analyzed theoretically.The relationship between the velocity of the transverse wave,the longitudinal wave and the surface wave is derived from the Navier-stokes equation.The potential function of the surface wave indicates that the amplitude of the surface wave propagates in the medium and its amplitude decays rapidly within a wavelength depth below the surface of the medium.Through the Lamb wave equation we obtain the Lamb wave with both symmetric and antisymmetric modes,and it also has the dispersion characteristics.Finally,the excitation method of laser ultrasound and several commonly used detection methods are introduced.2?A laser ultrasonic excitation detection device based on the Fizeau interferometer was constructed and the propagation of Rayleigh wave was detected by using the device.Firstly,the influence of the laser of different power density on Rayleigh wave is studied.It is found that the laser power density does not affect the frequency domain distribution of Rayleigh wave.Then we scan the aluminum plate with the single groove in the time domain and the frequency domain,and use the scanning result to calculate the structural parameters of the measured aluminum plate and the velocity of the different waves.Finally,we study the influence of periodic structure on Rayleigh wave and we find that the attenuation and absorption bands appear in the spectrum after adding the periodic structure.3?The propagation of Lamb waves in metal plates was studied in both theoretical and experimental aspects.First,we use the Lamb wave equation to calculate the Lamb wave dispersion curve of the metal aluminum used in the experiment.Then we experimentally detect the propagation of Lamb waves in pure plate.So we obtained the time-domain scan and frequency-domain scan of the Lamb wave.And then the signal is subjected to short-time Fourier transform.It is found that the time-frequency spectrum of the signal can correspond to the calculated dispersion curve.Finally,we study on the propagation characteristics of the Lamb waves in the periodic structure.It is found that the periodic structure will attenuate some frequency components of the Lamb wave,and the periodic structural parameters will also affect the spectral characteristics of the Lamb wave.In conclusion,the propagation characteristics of laser ultrasound in metal plates with different parameters are studied theoretically and experimentally.Which provides a reference for the application of laser ultrasound in nondestructive testing and acoustic device design.