Nondestructive detection of damage produced by a sharp indenter in ceramics

The feasibility of an ultrasonic technique using normal-incident compressional waves and a thermal wave measurement technique was evaluated for their sensitivity to surface and subsurface damage in ceramics. Well-defined damage in the form of surface and subsurface cracks was introduced by Vickers indentation in soda-lime glass and silicon nitride. The indentation impressions were first examined by optical microscopy to identify the types of cracks and to measure the size of the indents and cracks. As expected, indentation produced median/radial cracks emanating from the ident corners and subsurface lateral cracks. The ultrasonic technique was successful in detecting the subsurface lateral cracks in both materials. The signals obtained by focusing the trandsucer into the material (i.e., defocusing) was used to estimate the depth of subsurface cracks. The lateral cracks and the median/radial cracks were detected by the thermal wave measurement technique using the optical beam deflection method. The lateral cracks and the median/radial cracks were identified separately by using two deflection components of the probe beam. The transverse deflection component of the probe beam was used for the detection of the median/radial cracks, whereas the normal deflection component was used for the detection of the lateral cracks. The results are discussed in terms of the applicability of these two techniques as nondestructive methods for the detection of machining-induced damage in ceramics.