Monitoring of the Structural Integrity of Reactor Circuit (RAKEMON): Linear and nonlinear ultrasonic techniques for evaluation of partially closed cracks

Cracks are usually initiated by fatigue, but when water enters the cracks, growth will mainly be controlled by corrosion and such cracks are called stress corrosion cracks (SCC). SCCs constitute a major threat to the structural integrity of nuclear power plant components. Unfortunately closure stresses and/or water containment may render the cracks more or less transparent to ultrasound used in traditional testing. Due to the transparency the crack size may be underestimated in an ultrasonic examination, or the crack may even not be observed at all. Different nonlinear ultrasonic techniques have recently been studied in order to enhance sizing and detectability of partially closed and possibly water confining cracks. Nonlinear NDE utilizes new frequency components that are generated at the crack. These new frequency components may be higher harmonics or subharmonics. In this article we concentrate on work done at Tohoku University in Japan and at Royal Institute of Technology in Sweden. Tohoku University has developed a technique called "Subharmonic Phased Array for Crack Evaluation" (SPACE) and some recent progresses in this field will be reviewed. The Royal Institute of Technology has done both experimental and theoretical work on the linear and nonlinear ultrasonic response of dry and water confining rough surfaces in partial contact. Selected experimental results will be presented. The Swedish work has the nature of fundamental research while the Japanese work is more practically oriented.