Point-Focusing Electromagnetic-Acoustic Transducer for Crack Inspection

Stress corrosion cracking in stainless-steel pipes is a critical failure in atomic power plants and chemical plants. In ultrasonic testing, piezoelectric transducers are generally used, in which reproducibility of amplitude measurements is not necessarily high because of effects of coupling materials and contacting conditions between the transducers and specimens. Comparing to the transducers, an electromagnetic acoustic transducer (EMAT) requires no coupling materials and is less sensitive to the contacting conditions, making the reproducibility higher. However, lower signal to noise (S/N) ratio has been a disadvantage. For increasing the S/N ratio, we developed a point-focusing EMAT (PF-EMAT) [1]. It generated shear-vertical (SV) waves from concentric line sources on a top surface of a specimen. Coil configuration of the EMAT was designed so that the SV waves were accumulated in phase at a focal point on the bottom surface, which increased the S/N ratio and improved the spatial resolution. We have designed PF-EMATs operated at different frequencies, and applied to artificially fabricated defects on stainless steel specimens. In this presentation, we show the results, and discuss availability of the PF-EMAT to crack inspection

Development of shear-vertical-wave point-focusing electromagnetic acoustic transducer

A shear-vertical-wave point-focusing electromagnetic acoustic transducer is developed for the purpose of detecting stress corrosion cracking in stainless steel. The transducer is composed of a permanent magnet and two identical concentric meander-line coils, one for the transmitter and the other for the receiver, and the shear-vertical waves are excited and detected by the Lorentz force mechanism. The meander-line coils are designed so that the phases of all the excited shear-vertical waves are focused in phase at a focal point. The focal area is evaluated experimentally, and it is confirmed that the developed transducer is capable of detecting slit defects deeper than 0.05mm at the bottom surface of a stainless-steel plate 20mm thick.Takashi Takishita, Kazuhiro Ashida, Nobutomo Nakamura, Hirotsugu Ogi and Masahiko Hirao. Development of shear-vertical-wave point-focusing electromagnetic acoustic transducer. Japanese Journal of Applied Physics, 2015, 54,(7S1), 07HC04. https://doi.org/10.7567/JJAP.54.07HC04

Point-Focusing Electromagnetic-Acoustic Transducer for Crack Inspection

Stress corrosion cracking in stainless-steel pipes is a critical failure in atomic power plants and chemical plants. In ultrasonic testing, piezoelectric transducers are generally used, in which reproducibility of amplitude measurements is not necessarily high because of effects of coupling materials and contacting conditions between the transducers and specimens. Comparing to the transducers, an electromagnetic acoustic transducer (EMAT) requires no coupling materials and is less sensitive to the contacting conditions, making the reproducibility higher. However, lower signal to noise (S/N) ratio has been a disadvantage. For increasing the S/N ratio, we developed a point-focusing EMAT (PF-EMAT) [1]. It generated shear-vertical (SV) waves from concentric line sources on a top surface of a specimen. Coil configuration of the EMAT was designed so that the SV waves were accumulated in phase at a focal point on the bottom surface, which increased the S/N ratio and improved the spatial resolution. We have designed PF-EMATs operated at different frequencies, and applied to artificially fabricated defects on stainless steel specimens. In this presentation, we show the results, and discuss availability of the PF-EMAT to crack inspection.</p