Imaging of transient surface acoustic waves by full-field photorefractive interferometry

Multi-mode laser-ultrasound imaging using Time-domain Synthetic Aperture Focusing Technique (T-SAFT)

The Synthetic Aperture Focusing Technique (SAFT) is an imaging algorithm used in laser ultrasonics (LU) to visualise the appearance of defects. However, ultrasound excited by a pulsed laser has the characteristics of wide bandwidth and multi-mode directivity patterns, leading to common problems in the SAFT process, such as low utilisation of ultrasound information and possible artefacts. To solve these problems, a Multi-mode Time-domain SAFT (MMT-SAFT) algorithm is proposed in this paper. The influence of ultrasonic directivity is discussed according to the imaging depth range, and imaging with multiple LU modes is performed to reduce artefacts. Simulations and experimental results prove the feasibility of the MMT-SAFT algorithm, which not only presents a clearer image of the upper part of defects but also improves image quality compared with time-domain SAFT using a single ultrasonic mode. The proposed technique can provide feasible directions for laser ultrasonic defect imaging.

Schlieren-style stroboscopic nonscan imaging of the field-amplitudes of acoustic whispering gallery modes

We report a schlieren-style stroboscopic phase-contrast field-amplitude imaging of two-dimensional acoustic whispering gallery modes in a circular shell cavity immersed in liquid. A schlieren signal is combined with a presplit reference beam to enable nonscan field-amplitude imaging. Excitation mechanisms of standing and traveling eigenmodes, respectively, are analyzed with acoustic ray simulations presented in a Poincaré surface of sections. The time evolutions for both standing and traveling eigenmodes are reconstructed using the stroboscopic capability.

Visualization of ultrasonic wave field by stroboscopic polarization selective imaging

A stroboscopic method based on polarization selective imaging is proposed for dynamic visualization of ultrasonic waves propagating in a transparent medium. Multiple independent polarization parametric images were obtained, which enabled quantitative evaluation of the distribution of the ultrasonic pressure in quartz. In addition to the detection of optical phase differences δ in conventional photo-elastic techniques, the azimuthal angle φ and the Stokes parameter S₂ of the polarized light are found to be highly sensitive to the wave-induced refraction index distribution, opening a new window on ultrasonic field visualization.

Weld quality inspection of small-diameter thin-walled pipes by a laser ultrasonic method

Defect inspection of small-diameter thin-walled pipes is a difficult problem in the field of nondestructive testing. In this paper, a new detection method based on laser ultrasonics and guided circumferential wave technology is proposed and used to inspect the defects in pipes. First, a theoretical model based on the theory of acoustic propagation in solids is proposed for the small-diameter thin-walled pipes. The dispersion characteristics of the guided circumferential waves of thin-walled pipes are obtained by numerical simulation. Second, a laser ultrasonic system is constructed and used to detect the flaws in the small-diameter thin-walled pipes. Finally, the laser ultrasonic system is used to analyze the welding quality of stainless steel pipes with a diameter of 4.20 mm and a wall thickness of 0.30 mm. The experimental results are in agreement with the theoretical analysis, which demonstrates the reliability and practicability of the laser ultrasonic method in the weld quality inspection of small-diameter thin-walled pipes.