Subsurface detection and characterization of Hertzian cracks in advanced ceramic materials using optical coherence tomography.

Optical Coherence Tomography (OCT) is an active optical imaging technique that is capable of three-dimensional resolution better than 10 microns in all dimensions. OCT was originally developed as a non-invasive technique in biomedical field. It also found uses in the NDE of various materials including ceramics, plastics and composites. In various ceramics OCT can be used to detect microscopic, subsurface defects at depths approaching hundreds of microns. The depth of penetration depends on the material and on the wavelength of light. Here we demonstrate an application of OCT to the subsurface imaging in various materials and, in particular, to the detection of a surface-penetrating Hertzian crack in a Si3N4 ceramic ball. We present measured subsurface trajectory of the crack and compare it to theoretical predictions. These cracks represent one of the most important failure mechanisms in advanced ceramic materials. The ability to map subsurface trajectories of cracks is a valuable tool in the evaluation of different existing theories. Better theoretical understanding of various properties of crack initiation and propagation can lead to engineering of improved ceramic materials.