Finite-Element Analysis of Laser-Generated Ultrasounds for Wave Propagation and Interaction with Surface-Breaking Cracks

Abstract

A finite-element method was used to simulate the wave propagation of laser-generated ultrasound and its interaction with surface-breaking cracks in an elastic material. The thermoelastic laser line source on the material surface was approximated as a shear dipole and loaded as nodal forces in the plane-strain finite element (FE) model. The shear dipole FE model was tested for the generation of ultrasound on the surface with no defect. The model was found to generate the Rayleigh surface wave and to give the correct directivity patterns for longitudinal and shear bulk waves. The model was then extended to examine the interaction of laser-generated ultrasound with surface-breaking cracks of various depths. Both fixed and scanning laser sources (SLS) were considered. For the case of the fixed source, the crack-reflected Rayleigh waves were monitored to detect a crack. In the SLS technique, the laser-generated signal arriving at the Rayleigh wave speed was monitored as the laser source was scanned. The proposed model clearly reproduced the experimentally observed features that can be used to characterize the presence of surface-breaking cracks.

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.…”

“…Enhancement for the SLLS method therefore considers interference of the incident and reflected Rayleigh waves, but also the changes in the laser generation shape and boundary conditions as the laser passes over the crack [2,[5][6][7]12,13].…”

“…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].…”