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Study on detecting S-wave reflection metal material internal defect based on laser ultrasonic

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Tutor: ShenZhongHua
School: Nanjing University of Technology and Engineering
Course: Optics
Keywords: Laser ultrasonic,aluminum,internal defect,shear wave,Laser Dopplervibrometer
CLC: TN249
Type: Master's thesis
Year:  2014
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In this paper, we mainly used experiments and finite element method(FEM) to research the applications of the laser ultrasonic shear wave in the metal material internal defect detection. The propagation characteristics of the shear wave were mainly discussed, and to assess the location, size and other information of the defect by analyzing the interaction between the shear wave with the defects.On the basis of internal defects detection studies by other scholars, a "Secondary Blocking Method" use the shear wave and the echo-wave from the bottom was put forward for the detection of inside defects in this paper. The distance between the laser excitation and the detection point was fixed in this method, scanning to detect through moving the sample. when there were defects in the propagation path of the shear wave, the defect would weaken the amplitude of the shear wave echo from the bottom twice, under the shear wave amplitude influenced by defects acquiring the position and the size of defects.FEM was used to simulate the laser source excite ultrasound on the aluminum material surface containing an internal hole defect, simulated the same size different burial depth and the same burial depth different size internal defects of the sample in both cases. The numerical results showed that the defects would weaken the amplitude of the shear wave echo from the bottom, in the case of a certain defect size, different burial depth would make a little effect to the amplitude of the echo-wave; in the case of a certain defect burial depth, the amplitude of the echo-wave weakened by the larger defects was more obvious.Building corresponding experimental setup, using the laser line source excite ultrasonic and the laser Doppler vibrometer to receive ultrasonic signals on the same side. First fixed the position of the signal reception point, moving the laser line source to take scanning detection, getting the best detection distance met the biggest amplitude of the shear wave. Then keep the distance between the laser excitation and the detection point a constant, moving the sample along a straight path through the internal defects to achieve the aluminum samples with different size and different burial depth hole defects scanning detection. On the basis of the scanning detection distance, the distance between the two weaken position, and the thickness of the sample to calculate the detection of the burial depth, the error of the results were less than ten percent. Through analyze the delay of the shear echo-wave to make a qualitative estimate of the size of the defect. Both of the experiments and the simulations analyze the process of interaction of the shear wave and the defects, discussing the relationship between the ultrasonic signal and the defects from the influence of the shear wave amplitude by the defects. Analysis of the variation in the shear wave amplitude from the scanning detection. Making a systematic analysis on the basis of the same depth the different size defects and the same size the different depth defects, summed up the influence of the defect on the ultrasonic signal.This research result will provide a theoretical basis for laser ultrasonic detection of internal defects in metallic material.
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