A breakthrough in ultrasonic detection and sizing of partially closed cracks?

Ultrasonic testing is one of the most efficient methods available for finding and sizing of cracks in structural components such as for example nuclear reactor circuits. However, some fatigue or stress corrosion cracks (SCC) may be closed or partially closed due to the presence of residual stresses pressing the crack surfaces against each other. In these cases the ultrasound may pass through the closed crack almost without any reflection or scattering. Water or oxide in the crack may also influence the transparency of the crack. This means that the crack may be invisible to ultrasound and, hence, the crack may not be found at all or its size may be seriously underestimated. This may lead to catastrophic failure of the component. Tohoku University in Japan has developed a method in which cracks are monitored both at the insonifying frequency f and at the subharmonic frequency f/2 using a phased array for focused reception of ultrasound and a LiNbO3 single-crystal transmitter for generation of intense ultrasound. The subharmonic ultrasound is generated only at the closed parts of the crack. The method therefore combines normal phased array testing with phased array testing at subharmonic frequency. A commercial version of the equipment for research is under development by a Japanese company. This imaging method is called subharmonic phased array for crack evaluation (SPACE). Different nonlinear ultrasonic methods for evaluation of closed cracks have been proposed for decades (based on superharmonics), however, SPACE seems to include several new innovative ideas and it seems to have the best potential for becoming a standard method for inspection of nuclear reactor circuits for closed cracks. Subharmonics have a better signal to noise ratio than superharmonics because subharmonics are generated only at closed cracks while superharmonics are generated also in transducers, liquid couplers and electronics. We therefore mainly restrict ourselves to the description of the subharmonic SPACE method in this work. A more traditional sizing method using phased arrays is, however, briefly presented as reference.