Responses of Angle-Beam Ultrasonic Shear Waves to Closed Cracks and Highly Sensitive Quantitative Nondestructive Evaluation of Crack Sizes Together With Closure Stresses

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1. Clarification of angle-beam shear wave responses to closed cracks and its mechanismSpecimens containing two-dimensional (2D) fatigue cracks with different crack depths and closing stresses were prepared, and the cracks situated vertically on the specimen back-wall were inspected. The beam angle for achieving considerable sensitivity in shear wave response for a wide range of cracks was estimated as 50°.2. Construction of theoretical model of angle-beam shear wave response to closed crackBy taking care of the shape of the response curve that contains the simultaneous information of crack depth and closing stress, the model of angle-beam shear wave response has been proposed.3. Evaluation of crack size and closing stress by inverse analysis(1) Verification for small two-dimensional closed cracksA relation between the amplitude of the crack echo and the crack depth for open cracks was determined. Then, another relation between the reflection coefficient, which is defined by the ratio of echo-height of closed, crack to that of open crack, and crack closure stress was obtained. Finally, by using the model of closed crack response constructed in the previous section 2, which contains these relations, the values of crack closure stress and crack depth of an unknown 2D crack have been determined by solving an inverse problem.(2) Extension to shallow three-dimensional closed cracksThe methodology of evaluating 2D closed crack was extended to shallow 3D closed crack. Since a small segment of length of a 3D shallow crack can be approximated as a segment of 2D crack, the inverse analysis for the response measured at the center of each segment was individually performed.(3) Verification for shallow three-dimensional closed cracksCrack closure stress and the crack tip contour of a 3D closed crack were determined by the technique developed in the above section (2).