Hideo Cho

Precise velocity measurement of surface acoustic waves on a bearing ball

Using the photoacoustic effect of interference fringes scanned at the phase velocity of surface acoustic waves (SAW), we excited tone bursts of SAW with a center frequency of around 30 MHz on a 8 mmu200aφ steel bearing ball. A surprisingly large number (around 20 turns) of round-trip propagations was observed. The time interval between the SAW at the first and the twelfth turn was as large as 93 μs, however it could be determined with a 2 ns resolution since an exact overlapping of the two wave forms was possible. Thus, we achieved a very high resolution of 0.002% in the velocity measurement, and a velocity change of 2 m/s due to the deposition of a 50-nm-thick Ag film was easily detected. Because of its noncontact nature, this method would be useful for nondestructive evaluation of bearing balls.

Observation of diffraction-free propagation of surface acoustic waves around a homogeneous isotropic solid sphere

This letter shows an unexpected phenomenon where a surface acoustic wave (Rayleigh wave) excited by a line source with a finite length on a solid sphere propagates along the great circle in a direction perpendicular to the line source without beam spreading due to diffraction. In experiments, a piezoelectric transducer, 1.5 mm in width and 20 mm in length, was glued on a surface of a glass ball, 80 mm in diameter, as a line source with a finite length. A beam of Rayleigh waves with frequencies centered at 1.1 MHz was excited in either direction perpendicular to the transducer length. A receiving transducer with a circular aperture, 2 mm in diameter, was used in direct contact with the surface to detect the distribution of vibration over the surface of the ball. It was observed that the excited Rayleigh wave propagated along a great circle of the ball for at least four roundtrips. The beam was confined within a narrow path around the ball, the width of which was no more than 20 mm.

Analysis of spurious bulk waves in ball surface wave device

We analyzed the acoustic waves propagating in a sphere to establish a useful guideline for the design of NDE apparatus and ball surface acoustic wave (SAW) device exploiting the diffraction-free propagation of SAW on a sphere. First, we calculated the laser-generated acoustic displacements both under ablation condition and under thermoelastic condition and verified experimentally the validity of the calculation. Next, the acoustic waves excited by out-of-plane stress and those excited by in-plane stress were compared. The results showed that when the out-of-plane stress was applied, the relative amplitudes of the bulk waves to that of the SAW were larger and the number of bulk waves was larger than that when the in-plane stress was applied, while the SAW had similar waveforms in each case. The ratio of the relative amplitude of the bulk waves for the out-of-plane stress and the in-plane stress was 3.1:1 at phi(1)=90 degrees and 1.67:1 at phi(1)=0 degrees. The large amplitude for the out-of-plane stress can be explained by wide directivities of bulk waves. Consequently, we found that it is necessary for ball SAW device to select a piezoelectric material and form of interdigital transducer so that the in-plane stress becomes dominant.

Time of Flight Diffraction Method Using Laser Ultrasound for Noncontact Flaw Height Measurement

We developed a laser TOFD (Time of flight diffraction) algorithm which utilizes not only longitudinal wave but also shear wave. This algorithm made it possible to obtain accurate flaw depth without knowing the specimen velocity and probe distance previously. We constructed the laser TOFD system and applied it to estimate the slit depth of aluminum alloy plate. Time of flight of lateral wave, flaw tip diffraction waves and mode converted shear wave at flaw tip were used to estimate the slit depth using new algorithm.

Laser ultrasonic TOFD method

We applied laser ultrasound for the time of flight diffraction (TOFD) measurement. To find an effective testing condition of TOFD method, laser ultrasonic transmission around the slit in glass specimens was observed using the photoelastic ultrasonic visualization system. After the observation we constructed the laser TOFD system and applied to the slit depth measurement of aluminum plate, and then, obtained highly accurate results.

Visualization of laser ultrasound by photoelasticity

For investigating the directivity of laser ultrasounds experimental ultrasonic visualization system using photoelasticity that we had used for piezoelectric transducers was improved to be highly sensitive and applied to laser ultrasounds. As a result, propagation of the laser ultrasounds in glass specimen could be observed. Furthermore sound field of laser ultrasounds also could be estimated quantitatively and we compared it with the theoretical line source model.

Detection of Defects in Micro-Machine Elements by Using Acoustic Waves Generated by Phase Velocity Scanning of Laser Interference Fringes

High aspect ratio microstructuring is a key process for fabricating microelectromechanical systems (MEMSs). Many microfabrication methods have been developed. Among them, inductively coupled plasma (ICP) source etching is especially attractive because it can fabricate high aspect ratio microstructures, at high speeds with a dry etching process. However, in such microstructures, it is difficult to nondestructively evaluate the quality of high aspect ratio etched bottoms. Therefore, we developed a technique for detecting defects in high aspect ratio microstructures on Si wafers machined by ICP source etching. Using acoustic waves generated by phase velocity scanning of laser interference fringes, we nondestructively detected 13-µm-high defects located on the bottom of narrow and deep grooves from the other side of the Si wafer, which could not be detected by means of optical techniques or a scanning electron microscope.