Abstract
ABSTRACT A new capability of TV holography, also known as electronic speckle pattern interferometry (ESPI), is presentedfor locating and imaging slightly thinned or thickened areas in metallic plates. It is based on the measurementof the wavenumber variation of narrowband Lamb waves as they propagate through these plate-thickness in-homogeneities. The relation between frequency and phase velocity of all Lamb modes depends on the elasticconstants of the material the plate is made of (two parameters in isotropic materials) and on the plate thick-ness. Therefore, the associated dispersion curve of each mode present wavenumber changes that are sensitiveto a thickness reduction. We have formerly developed a double-pulsed TV holography system which allows thefull-eld measurement of the instantaneous out-of-plane displacement eld induced by surface acoustic wavesand, by further processing, to calculate maps of the acoustic amplitude and phase. A method based on furtheranalysis of the acoustic complex-displacement map is therefore proposed to locate and characterize such smooththickness reductions. In particular, we calculate a map of the local wavenumber of the acoustic wave as themodulus of the two-dimensional gradient of the mechanical phase. Hence, as the variations in the wavenumbercorrespond to variations in the plate thickness, the local thickness reductions and increments can be detected inthis map. Within the resolution limits imposed by the wavelength of the Lamb wave, this method allows also tocontour the shape of the inhomogeneities. The technique is demonstrated herein by imaging a X-shaped recessmachined on an aluminium plate.Keywords: ESPI, TV holography, Lamb waves, non-destructive testing
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