多重反射を利用した薄板の超音波試験に及ぼす超音波ビーム集束特性の影響

抄録

In a newly developed ultrasonic testing of thin plates with a focused probe, the flaw echo or back wall echo appears with an observable time delay behind surface echo under the condition of shorter water path than the focal length of probe. The sound field of the probe is an important factor for this method, which is determined by the details of probe such as test frequency, diameter of active element and radius of acoustic lens. In this study, the relation between the sound field and the details of focused probe was investigated experimentally and theoretically. These results were used for the discussion of the effect of sound field on the flaw echo or back wall echo of thin plate in the ultrasonic testing based upon the multiple reflections.
The experiments and numerical calculations showed that a focused probe with steep increase of sound pressure at the focal point was useful for the ultrasonic testing of thin plates based on the multiple reflections. The steep increase of sound pressure was evaluated by the calculation of sound pressure of probe with Hirose's model modified for pulse wave. Application of the developed ultrasonic testing with a focused probe to the thin plates was limited by the critical thickness, which was minimum detectable thickness, and depended upon the relation between water path and sound pressure of probe. These results were confirmed by the ultrasonic testing of thin plates and spot welds.