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

1.阐明闭合裂纹的角束剪切波响应及其机理制备了含有不同裂纹深度和闭合应力的二维疲劳裂纹的试件，并对试件后壁垂直分布的裂纹进行了检查。对于大范围的裂纹，为获得剪切波响应的相当大的灵敏度，波束角估计为50°。2.闭合裂纹斜梁剪切波响应理论模型的建立考虑到裂纹深度和闭合应力同时反映的响应曲线形状，提出了闭合裂纹斜梁剪切波响应模型.（1）二维闭合小裂纹的验证确定了开口裂纹的裂纹回波幅度与裂纹深度之间的关系。进而得到了由闭合裂纹回波高度与张开裂纹回波高度之比定义的反射系数与裂纹闭合应力的另一种关系。最后，利用包含这些关系的闭合裂纹响应模型，通过求解反问题，确定了未知二维裂纹的裂纹闭合应力和裂纹深度。(2)将二维闭合裂纹的计算方法推广到三维闭合裂纹。由于三维浅裂纹的一小段长度可以近似为一段二维裂纹，因此分别对每段裂纹中心处测得的响应进行了反分析。(3)浅三维闭合裂纹的验证裂纹闭合应力和三维闭合裂纹的裂纹尖端轮廓是用上述第（2）节中开发的技术确定的。

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).