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 30MHz 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.
