Development of nonlinear ultrasonic imaging method with low frequency vibration for closed cracks in large structures

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The aging damage has been accumulating in the large structures built during high economic growth period. To maintain safety and reliability of society, the means of evaluating them is necessary. The objective of this study is to develop the nonlinear ultrasonic imaging method with low-frequency vibration and phased array using pulse waves in order to visualize closed cracks in large structures with high resolution. In this study, we performed [1] development of the apparatus, [2] formation of closed-crack specimen, and [3] its verification test. As a result, we succeeded in imaging closed crack, formed in aluminum-alloy specimen, with high spatial resolution. Furthermore, we found that the selectivity for closed cracks in ultrasonic images was enhanced by 24 times. In addition, although the crack depth was underestimated by approximately 10 mm, we succeeded in measuring the crack depth with an accuracy of less than 1 mm using the developed method.