博論修論卒論が一通り終わり、ほっと一息ついていた２月前半最後の日、Plasma and Fusion Research誌に投稿していたApplication of High-frequency Ultrasonic Test to the Non-destructive Inspection of W-Cu Bonded Interface（高周波超音波検査技術のタングステン－銅接合界面検査への適用性評価）とのタイトルの論文の受理通知が参りました。素晴らしい。
今回の内容は当研究室からのものとしてはちょっと珍しく、超音波検査に関するものです。核融合炉のダイバータというところにはタングステンブロックに銅の冷却管が接合された構造をしている1のですが、銅が薄肉ということもあり、その接合面の検査がなかなか難しいというのが大きな課題でした。本研究は、一般的な構造物検査に用いられるよりもより高周波、大体30MHzくらいの周波数の超音波を使うと、かなりいい感じで接合界面評価ができるということを示した、というものです。試験体たくさん作って高周波超音波測定をやり、画像処理して、サーモグラフィー試験の結果と比較し、さらに試験体の断面観察と盛りだくさんの内容になっています2。
論文の概要は以下。
Authors: Noritaka Yusa, Ryouji Suzuki, Takashi Furukawa, Masayuki Tokitani, Suguru Masuzaki
Title: Application of High-frequency Ultrasonic Test to the Non-destructive Inspection of W-Cu Bonded Interface
Abstract: This study evaluated the applicability of high-frequency ultrasonic tests to the non-destructive inspection of the bonded interface between a cooling pipe and a divertor monoblock. Samples prepared in this study were an ITER-grade tungsten block bonded with a 2.5 mm-thick oxygen-free copper using diffusion bonding. The high-frequency ultrasonic test was performed using an acoustic microscope. A probe, operated in pulse-echo mode, scanned the copper surface of the sample two-dimensionally. Five probes with operating frequencies ranging from 15 to 50 MHz were used. The measured ultrasonic signals were converted into ultrasonic images on the assumption that the samples had a uniform and isotropic speed of sound to evaluate reflections from the interface. Whereas an interface without any artificial flaw partly reflected ultrasonics, setting the decision threshold properly, based on the distribution of the surface echo amplitudes, enabled the smallest flaw to be detected clearly. Ultrasonic signals measured around 30 MHz showed the best signal-to-noise ratio in detecting an artificial flaw introduced at the bonding layer. The results of the ultrasonic tests were consistent with those of subsequent lock-in thermography and destructive test. However the thermography test could not detect small flaws that the high-frequency ultrasonic test confirmed.