The Study On Analyzing The Laser Generated Ultrasound

The inspecting method of Laser-generated Lamb waves combines the advantages ofLaser-generated ultrasound with ones of Lamb waves. It has a number of advantages, such asnon-contact, wideband, high efficiency and extensive application. But, due to the typical characters ofLamb waves and the complexity of the interaction between the laser and materials, It is hard toanalyze the Laser-generated Lamb waves. Based on this, in this thesis, the Lamb waves generated bythe Laser were studied by different time-frequency analysis methods from experiment and computersimulation, respectively.The thesis is organized as follows:(1)、By consulting the relevant documents, got the Lamb wave dispersion equations. Then,programming using MatLab software, the dispersion curves of Lamb waves in the aluminium platewere plotted.(2)、Two common time-frequency analysis methods were introduced. These methods included theshort-time Fourier transform (STFT) and Hilbert-Huang Transform (HHT). Then, throughprogramming using MatLab software, the two kinds of transforms were realized.(3)、Numerically simulated the Lamb waves generated by a round dot type pulsed laser beamincoming on a1mm thick aluminum plate which is homogeneous and isotropic by finite elementmethod (FEM). By simulating, the transient temperature field was known. Then, the transient stressfield induced by temperature grads was obtained in the plate, and it is found that the transient stressfield propagates in the form of Lamb waves in the aluminum plate. At last, the Lamb waves wereanalyzed by STFT. It was found that the generated Lamb waves were multimodal and wideband,furthermore, It was also shown that the low order modes, such as: A0, A1, S0and S1, had much morepower than other high order modes in the1mm thick aluminum plate.(4)、By using an experiment set-up including a Nd3+:YAG pulse laser, a digital oscilloscope, aphotodetector, lens, and some piezoelectricity transducers. Lamb waves were generated by a rounddot type pulsed laser beam in1.08mm thick aluminum plate. The Lamb waves received by differenttypes of piezoelectric probes were analyzed by STFT and HHT. It was found that the multimodeLamb waves could be recognized by STFT and HHT, and these were the low order modes of Lambwaves (A0, S0and A1), which is same as the results from simulation. In addition, the different types ofprobes received the different modes, indicating the mode of the laser-generated Lamb waves could bechosen by using the different probes.As a result, this paper provides a groundwork for the research of the application of the lasergenerated Lamb wave inspection.