Ceramic fiber composites: Under the acoustic microscope

Acoustic microscopy is very powerful for high resolution, nondestructive inspection of composites in critical applications. An acoustic frequency of 50 MHz is typically used, giving a subsurface resolution of about 0.1 mm (0.004 in.). For more detailed studies of the elastic structure of composites, much higher frequencies are needed. Commercial acoustic microscopes are available that work at frequencies up to 2 GHz. At that frequency, the acoustic wavelength in water is 0.75 [mu]m, which is similar to the wavelength of red light. A contrast mechanism which has proved particularly fruitful in the study of materials uses the excitation of acoustic waves in the surface of a specimen. These waves are known as Rayleigh waves. They have both longitudinal and transverse components, which decay exponentially into the depth of the surface. At 2 GHz they are confined to a depth of about 2 [mu]m. By adjusting the defocus of the acoustic microscope the contrast can be made extremely sensitive to small changes in the propagation of Rayleigh waves. These may be due to variations in the elastic properties of the material, or to changes in orientation when the properties are anisotropic, or to discontinuities in the elastic properties where there aremore » cracks or interfaces. By examining acoustic micrographs of sections through ceramic fiber composites, it is possible to study fiber/matrix interfaces, and to see the relationship between microstructure and the degradation of an interface.« less