Quantitative analysis of laser-generated ultrasonic wave characteristics and their correlation with grain size in polycrystalline materials

Quantitative relationship between nanosecond pulsed laser parameters and the characteristics of laser-generated ultrasonic waves in polycrystalline materials was evaluated.The high energy of the pulsed laser with a large irradiation spot simultaneously generated ultrasonic longitudinal and shear waves at the epicenter under the slight ablation regime.An optimized denoising technique based on wavelet thresholding and variational mode decomposition was applied to reduce noise in shear waves with a low signal-to-noise ratio.An approach for characterizing grain size was proposed using spectral central frequency ratio(SCFR)based on time-frequency analysis.The results demonstrate that the generation regime of ultrasonic waves is not solely determined by the laser power density;even at high power densities,a high energy with a large spot can generate an ultrasonic waveform dominated by the thermoelastic effect.This is ascribed to the intensification of the thermoelastic effect with the proportional increase in laser irradiation spot area for a given laser power density.Furthermore,both longitudinal and shear wave SCFRs are linearly related to grain size in polycrystalline materials;however,the shear wave SCFR is more sensitive to finer-grained materials.This study holds great significance for evaluating metal material properties using laser ultrasound.

The technique of detecting the laser-ultrasonic vector displacement with a confocal Fabry-Perot interferometer waves in a plate

Generally, a confocal Fabry-Perot interferometer is only able to detect the out-of-plane component of a displacement field; while the in-plane component often has the information about the material which cannot be found in this out-of-plane component. In this paper, based on a confocal Fabry-Perot interferometer set-up for detecting the out-of-plane component of a laser generated acoustic field, a technique is developed to detect both the out-of-plane and in-plane displacement components simultaneously with a novel two-channel confocal Fabry-Perot interferometer.

Research on Online Texture Measurements in Metal Rolling Field

In rolled strip material,the orientation of the crystallites,known as texture,is influenced by various kinds of thermo-mechanical processes,such as casting,plastic deformation,annealing and phase transformation. The modern industry production requires stable product performance,real-time monitoring and full controlling of the quality.The online texture measurement in metal rolling can be used to real-time monitor the whole process, and then feedback control to the production process can be implied to adjust the process parameters to ensure the stability of the products.The principles,advantages and disadvantages of related detection methods(2D X-ray diffraction,neutron diffraction,laser-ultrasonics and electromagnetic acoustic transducers(EMAT)) and the possibility of online measurement are discussed.Finally,2D X-ray diffraction and laser-ultrasonics are employed on online texture measurement,and the schemes of online texture measurement are proposed.