Étude et caractérisation par microscopie acoustique de l’endommagement par fissures

Assessment of crack damage by acoustic microscopy

¹ LAIN/CNRS
² SARL Acoustique Métrologie, Montpellier
³ LDV, Université Montpellier 2

Abstract

Different non destructive methods of high frequency scanning acoustic microscopy are applied to image both sub-surface and surface microcracks. Spherical acoustic sensors with an aperture of 50° were used working at 50 MHz and at 570 MHz. Two types of waves generated by the acoustic sensors can be used: Rayleigh surface wave and longitudinal wave. Surface wave (Rayleigh wave) properties allow to detect and quantify surface cracks and subsurface cracks not farther than a wavelength from surface. A fringe pattern can be observed along the cracks wich periodicity is half of the rayleigh wavelength, and a model of the phenomenon can be computed.

Firstly an experiment of scratching test on thick hard chromium coatings compares optical an acoustic images of microcracks. A second experiment on a single cracked glass bar put in evidence, like the first one, the capability of the surface wave to detect sub-surface cracks (not deeper than a wavelength). This sample presents a linear and regular single crack profile with an interesting sub-surface diving zone. By decreasing the frequency, acoustic sensors offer to “see” deeper. The reflection of the longitudinal wave both on the surface of a sample and on a sub-surface crack plane, allows to image inclined deep cracks. The observed fringe pattern presents a periodicity directly linked to the angle of inclination. We show in this work that acoustic microscopy is an adaptative method to characterize various types of cracks, using different sensors geometries and frequencies.