Interaction of a scanning laser-generated ultrasonic line source with a surface-breaking flaw

Abstract

The scanning laser source (SLS) technique has been proposed recently as an effective way to investigate small surface-breaking cracks. By monitoring the amplitude and frequency changes of the ultrasound generated as the SLS scans over a defect, the SLS technique has provided enhanced signal-to-noise performance compared to the traditional pitch-catch or pulse-echo ultrasonic methods. In previous work, either a point source or a short line source was used for generation of ultrasound. The resulting Rayleigh wave was typically bipolar in nature. In this paper, a scanning laser line source (SLLS) technique using a true thermoelastic line source (which leads to generation of monopolar surface waves) is demonstrated experimentally and through numerical simulation. Experiments are performed using a line-focused Nd:YAG laser and interferometric detection. For the numerical simulation, a hybrid model combining a mass-spring lattice method (MSLM) and a finite difference method (FDM) is used. As the SLLS is scanned over a surface-breaking flaw, it is shown both experimentally and numerically that the monopolar Rayleigh wave becomes bipolar, dramatically indicating the presence of the flaw.

Frequency Characteristics of Surface Wave Generated by Single-Line Pulsed Laser Beam with Two Kinds of Spatial Energy Profile Models: Gaussian and Square-Like

“…However, by using a line laser beam, the directivity of the surface wave can be improved [5]. A line laser beam can be produced by several methods such as the use of a cylindrical lens and a line-slit mask [2,4,7]. In these methods, the spatial laser energy profile can be largely classified into two kinds: Gaussian and non-Gaussian.…”

Near Field Enhancements from Angled Surface Defects; A Comparison of Scanning Laser Source and Scanning Laser Detection Techniques

“…One paper [11] reports the detection point passing over a crack, whereas the SLS technique with the laser source scanned over the crack has received much recent attention [2,[5][6][7]12,14]. The crack angle dependence of the enhancement has previously been ignored.…”

“…We now compare these enhancements with the SLLS method, which has received much recent attention [2,[5][6][7]12,14]. In this case, the laser source passes over the crack, generating an incident Rayleigh wave which can be reflected and mode-converted at the defect.…”

“…If the laser generation point is over a defect, part of the beam is truncated and the boundary conditions change significantly, and research has shown that this leads to a similar signal amplitude enhancement during scanning of a sample [2,[5][6][7]12,14]. This enhancement has been the subject of much recent research [2,[5][6][7]12,14] and is known as the scanning laser source (SLS) technique, or scanning laser line source (SLLS) if the generation laser is focussed into a line. Frequency enhancements have also been reported before, with researchers investigating changes in the frequency at which the fast Fourier transform (FFT) shows a maximum magnitude [2].…”

“…Ultrasound is in common use, as testing can be performed in a non-destructive manner, and recent advances in understanding the interaction of surface waves, such as Rayleigh waves, with surface cracking is leading to new techniques for characterisation [1][2][3][4][5][6][7]. Recent work has investigated the reflection and transmission of waves following interaction with surface defects which are oriented normal to the sample surface, a geometry which is typical of calibration samples [1,[5][6][7][8]. It has been seen that a surface defect will act like a filter to an incident wave, hence the transmitted signal for a broadband Rayleigh wave, typical of that generated using non-contact techniques such as electromagnetic acoustic transducers (EMATs) or laser ultrasound, drops off approximately exponentially with depth [9,10].…”