Naoya Nishimura

Evaluation of minute cracks generated by spallation with nonlinear ultrasonic measurement

The ensemble of minute cracks nucleated by plate impact tests was detected by nonlinear acoustic measurement using PZT contact transducers. The incident wave was the burst wave of 5 MHz and 12 cycles. The wave propagated through the minute cracks were received by a 10 MHz transducer. The ratio of the 2nd harmonic component to the fundamental one is proportional to the incident wave amplitude for the spall-damaged sample, however, this ratio is independent of the incident amplitude for the sample without the spall damage. Thus, the group of minute cracks which can’t be detected by linear ultrasonics is detected by nonlinear ultrasonic measurement.

Sizing of small voids less than focused beam diameter with C-scan acoustic microscope

The small objects which are far less than the focused beam diameter were imaged with a C-scan acoustic microscope, and the acoustic image sizes were compared with the actual sizes. The sizes of flat strips of width over 0.01 mm and steel wires of diameter over 0.02 mm were estimated within 30% error by using the reflection ratio, which is the ratio of the intensity of reflection signal from an object to that from a reference plane. The validity of this method is also shown for small spherical voids within ceramics.

ENHANCEMENT OF B- AND C-SCAN IMAGES OF C-SAM WITH AN ACOUSTIC MATCHING LAYER

Ultrasonic immersion methods have been used for nondestructive evaluation of material properties as well as for B- or C-scan imaging of internal microstructure and/or defects. However, the direct application of this method is difficult for dissolving or hygroscopic materials as well as rusting metals. Thin waterproof coating on such materials enables us to apply immersion methods for those materials and it is also effective for reducing reflection at the water/material interface, if the coating has an appropriate acoustic impedance.