Full-field localization of plate-thickness inhomogeneities through the local changes in the wavenumber of Lamb waves measured with pulsed TV holography

Abstract

A new capability of TV holography, also known as electronic speckle pattern interferometry (ESPI), is presented for locating and imaging slightly thinned or thickened areas in metallic plates. It is based on the measurement of the wavenumber variation of narrowband Lamb waves as they propagate through these plate-thickness inhomogeneities. The relation between frequency and phase velocity of all Lamb modes depends on the elastic constants of the material the plate is made of (two parameters in isotropic materials) and on the plate thickness. Therefore, the associated dispersion curve of each mode present wavenumber changes that are sensitive to a thickness reduction. We have formerly developed a double-pulsed TV holography system which allows the full-field measurement of the instantaneous out-of-plane displacement field induced by surface acoustic waves and, by further processing, to calculate maps of the acoustic amplitude and phase. A method based on further analysis of the acoustic complex-displacement map is therefore proposed to locate and characterize such smooth thickness reductions. In particular, we calculate a map of the local wavenumber of the acoustic wave as the modulus of the two-dimensional gradient of the mechanical phase. Hence, as the variations in the wavenumber correspond to variations in the plate thickness, the local thickness reductions and increments can be detected in this map. Within the resolution limits imposed by the wavelength of the Lamb wave, this method allows also to contour the shape of the inhomogeneities. The technique is demonstrated herein by imaging a X-shaped recess machined on an aluminium plate.