3D + 1 dimensional simulations and experimental results for the laser induced breakdown of air are presented. The simulations include the laser propagation, multi-photon and impact ionization and heating of the electrons using accurate atomic and molecular data. For laser pulses of duration from 100 fs to 1 ns mechanisms for the breakdown of air based on the pulse duration and intensity ranging from optical field ionization to electron impact ionization are found. The laser energies at which the breakdown occurs are found to be in good agreement with experimental results.
This paper discusses a research on 2-dimensionl visualization of a spot weld piece using laser-ultrasonic technique.This laser-ultrasonic apparatus can scan a surface on a weld piece, and get ultrasonic longitudinal signals on several hundred measurement points per 10mm order. We analyzed these signals by Frequency Response Characteristic from 10MHz to 100MHz, and showed that the ultrasonic attenuation was related to the gain, and the ultrasonic propagation velocity was related to the phase, for frequency domain spectrum. At result, it was appeared that the gain of nugget parts was less than the gain of parents, and the phase of HAZ was latter than the phase of parents and nuggets. From these results, we made maps on frequency domain spectrum, and could realize to visualize the parent and the nugget, the HAZ on a weld piece. In addition, using Electron Back Scatter Diffraction, it was shown that the gain was effected by the crystal diameters, the phase was effected by the crystal directions.
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