Laser-Ultrasound Characterization of Spherical Objects (Ceramic Bearing Balls, Surface Wave Energy)
Ceramic bearing balls are desirable for use in high temperature and nonlubricative environments because of their ability to retain high mechanical strength and reduced wear. However, because ceramics are brittle, it is very important to inspect ceramic parts for the existence of small (1-10 um) surface defects. The resonance spectrum of a spherical object can provide information about its material properties such as shear and longitudinal wave velocities to a high degree of accuracy. Also, surface wave resonant modes that are observed at high frequencies (the half circumference of the sphere is a multiple of at least 100 times the half surface wavelength) provide information about the surface cracks density. As surface waves encounter a defect, the resonant energy will be attenuated. By comparing the surface wave mode Quality factors (Q) between a perfect and an imperfect sphere, we are able to quickly detect the existence of surface imperfection. We find that a single defect will reduce the surface wave resonant Q by about 30%. A non-contacting detection scheme is desired because contacting points will also attenuate surface wave energy and make the detection of surface cracks less sensitive. We present here a non-contacting detection method to measure resonances of a sphere. In this case, we constructed a computer controlled system to excite resonances on a sphere with a transducer by a single Hertzian contact, and we use an optical Heterodyne interferometer to detect both amplitude and phase of the surface variations on the opposite pole of the sphere. This system is capable of inspecting bearing balls with diameters ranging from 12 mm to 1 mm.
