Faculty and Staff
- Yoshikazu Ohara
In our laboratory, we conduct cutting-edge research on ultrasonic nondestructive measurement methods, which are indispensable for realizing a safe, secure, and sustainable society. Specifically, we leverage our unique technologies in nonlinear ultrasonics, phased arrays, and laser measurements, to create "innovative measurement technologies that make the invisible visible" and to realize the industrial metaverse "digital twin."
While ultrasonic methods are widely used in industry, conventional methods are unable to measure closed cracks due to the ultrasonic waves passing through the crack faces in contact. Such defects are not limited to closed cracks, but also include kissing bonds, delaminations, and micro-defects. Nonlinear ultrasonics, which utilize the opening and closing vibrations of crack faces, is attracting attention as a measurement method for these defects. In our laboratory, we are developing “nonlinear ultrasonic phased array” for closed-crack imaging by combining nonlinear ultrasonics with ultrasonic phased arrays. The SPACE (subharmonic phased array for crack evaluation) imaging method, which uses subharmonic waves, was the first method in the world to achieve the accurate measurement of closed-crack depth. Subsequently, we proposed simpler techniques, such as the GPLC (global preheating and local cooling) that uses thermal stress loading and the FAD (fundamental wave amplitude difference) that can indirectly infer all nonlinear components with a single array probe. Recently, we have also been developing ultra-high-speed imaging techniques in combination with large displacement pump excitations of more than 1000 nm.
Defects that occur in actual structures have complex 3D shapes, but current ultrasonic phased arrays use 1D array transducers consisting of multiple rectangular-shaped piezoelectric elements, yielding only 2D images. In principle, 3D imaging is possible with 2D array transducers, but the maximum number of elements currently available is 256 (i.e., 16x16), which does not provide sufficient resolution. To overcome this limitation, our laboratory has developed the PLUS (piezoelectric and laser ultrasonic system), an ultra-multiple-element phased-array imaging method that combines a single-element piezoelectric transducer for transmission and an ultra-multiple-point 2D scan with a laser Doppler vibrometer for reception. The laser Doppler vibrometer enables ultra-wideband reception from 0 to 25 MHz, allowing phased array imaging at any desired frequency by simply changing the frequency of the transmitter. Furthermore, we are also working on the development of a real-time 3D phased array imaging method by fabricating a piezoelectric 1024-element 2D matrix array transducer.