Techniques for NDE of closed cracks

Usually ultrasonic techniques tend to evaluate the closed cracks as smaller than the actual size. In this paper, we describe two strategic techniques for accurately evaluating the closed cracks by means of ultrasound. First, we show the advanced technique utilizing the thermal stress induced by cooling a cracked part. It is shown that a small amount of temperature decrease is effective to open a closed crack and hence to increase the ultrasonic response of the closed crack. Another advanced technique gives the success in quantitative evaluation of both the crack depth and the crack closure stress by nondestructive way. The technique is based on the analysis of inverse problem, and capable of evaluating tightly closed cracks as well as open cracks under a no load condition. (author)