Numerical reconstruction of acoustic bulk waves in aluminium from TV holography surface displacement measurements

Abstract

The paper describes a hybrid technique, aimed at nondestructive inspection of materials, that combines whole-field optic measurements, acoustic excitation and a numerical reconstruction method. The interior of a thick specimen is probed by short bursts of narrowband ultrasonic bulk waves. The acoustic wavefronts that constitute the burst emerge at the opposite face of the sample and induce periodic displacements of its surface. These displacements are measured by TV holography, a whole-field optical technique, also known as electronic speckle pattern interferometry (ESPI). The measurement process yields the complex amplitude (i.e., amplitude and phase) of the acoustic wavefronts at the plane of the surface as a series of 2-D, complex-valued maps. Lastly, a numerical reconstruction algorithm that uses the Rayleigh-Sommerfeld diffraction formula is employed to calculate the amplitude and phase of the acoustic wavefronts at any other plane in the interior of the specimen. This procedure is analogous to the numerical reconstruction of optical object wavefronts in digital holography (with light and free space taking the place of acoustic waves and the material medium, respectively), so the present method could also be designated as digital opto-acoustic holography. If the wavefronts are affected by the presence of inhomogeneities in the medium, information about the shape and position of such defects could be retrieved from the reconstructed wavefront at the appropriate depth. The technique herein proposed was successfully tested in an alluminium specimen with an artificial defect.