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

1. 澄清角梁剪切波对闭合裂纹的响应及其机理制备了含不同裂纹深度和闭合应力的二维疲劳裂纹试件，并对垂直位于试件后壁的裂纹进行了检测。对于大范围的裂缝，在横波响应中达到相当灵敏度的梁角估计为50°2。考虑到同时包含裂纹深度和闭合应力信息的响应曲线的形状，提出了角梁剪切波响应模型。(1)二维小闭合裂纹的验证。确定了开放裂纹裂纹回波幅值与裂纹深度的关系。然后，得到闭合裂纹的反射系数与张开裂纹的反射高之比与裂纹闭合应力之间的另一种关系。最后，利用前面第2节构建的包含这些关系的闭合裂纹响应模型，通过求解逆问题确定未知二维裂纹的闭合应力和裂纹深度值。(2)向浅三维闭合裂纹扩展将二维闭合裂纹评价方法扩展到浅三维闭合裂纹。由于三维浅裂缝的一小段长度可以近似为二维裂缝的一段，因此对每段裂缝中心测量的响应分别进行逆分析。(3)浅层三维闭合裂纹验证采用上述(2)开发的技术确定裂纹闭合应力和三维闭合裂纹尖端轮廓。

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).