• Bruno, Francois
• Mezil, Sylvain
• Diboune, Hafsa
• Kiefer, Daniel
• Prada, Claire

Abstract

Background, Motivation & Objectives: Lamb modes are dispersive guided waves commonly used for nondestructive evaluation. Among these modes exist peculiar ones which have a finite wavelength for a zero group velocity: ZGV Lamb modes. The energy of such a mode is thus trapped under the excitation, enabling local measurements with high sensitivity. Laser ultrasonic techniques, being contactless, were shown to provide an ideal tool to observe these modes in order to make local measurements of various parameters, such as the thickness or Poisson ratio [1-3]. These local measurements are usually performed with pump and probe beams at the same point. Thus, in order to map the thickness of a plate, the surface needs to be perpendicular to the probe beam and of sufficient reflectivity. To tackle this strong limitation, we investigated the feasibility to retrieve the ZGV frequency associated with pump locations for a single probe location.Methods: The samples are millimeter-thick Duralumin plates containing either a thickness step or a thickness gradient. The acoustic waves are generated by a pulsed pump laser (λ=1064 nm with a pulse of 10 ns and ~5mJ) and detected by an interferometric probe (λ=532 nm). 1D and 2D scans are performed by shifting either the sample (reference measurement), the pump beam (fixed probe) or the probe beam (fixed pump). The latter is done for comparison and to better understand the phenomenon.Results / Discussion: One of the measurements was performed on a plate with a triangular ~8% thickness step as depicted in Fig. a. The local thickness, deduced from the ZGV frequency at the excitation point but measured in a fixed point (red cross) is presented in Fig. b The results can be compared to reference Fig. c when both beams are co-focused. While these results are conclusive, some others show a wrong thickness reconstruction. A numerical study has been done to better analyze this phenomenon and assess the necessary conditions under which non-propagative ZGV Lamb modes convert to propagative ones and can be detected elsewhere. We expect that these results will pave the way to non contact thickness measurement method for materials of low reflectivity.

Topics

• impedance spectroscopy
• surface
• ultrasonic