Higher harmonic imaging of stress corrosion cracking using a direct contact technique

Ultrasonic testing as an in-service inspection on nuclear power plant components is usually performed by a pulse echo technique. If crack faces to be detected are closed and in contact, the ultrasonic waves pass through the crack faces, therefore no waves is scattered on the crack tip. Then, the crack tip location is not identified, thus accurate measurement of the crack size is impossible. So far, we have visualized artificial closed cracks of sub-micron gap by using higher harmonics with water immersion method. The harmonic wave amplitude reflected from the semi-closed crack increases with the incident wave amplitude. In order to apply the method using higher harmonic technique to ultrasonic testing for the piping of actual plants, the probe must be directly placed on the surface of the pipe. We tried to measure artificial stress corrosion cracking within Ni-based alloy weldments, which show the strong scattering and higher attenuation of ultrasonic waves, In this paper, we show higher harmonic image such SCC cracks by using the direct contact angle transducer. (author)