Subharmonic ultrasonic measurement and imaging for quantitative evaluation of stress corrosion and fatigue cracks

为建立疲劳和应力腐蚀裂纹的亚谐波超声测量和成像的基础，首先推导了考虑粘附力的解析解，并实现了亚谐波檐产生阈值与裂纹闭合应力之间的关系。其次，我们发展了弹性体振子模型，因为需要详细的信息来开发实际的设备。通过对该模型的数值分析，研究了在A7075铝合金疲劳裂纹实验中，亚谐波与入射波幅值和闭合应力的关系，以及测量波形与计算波形的比较，并开发了非线性超声成像系统，用于测量闭合裂纹厚度方向的深度。作为发射机，我们采用了LiNbO3单晶振荡器，它对高压电信号具有优异的耐久性，并采用了阵列传感器，用于通过接收信号聚焦来产生图像。此外，在试样内的延迟规律制定和设计一个IIR数字滤波器作为成像前的预处理，构建一个系统，可以用来获得裂纹的线性和非线性图像。此外，我们应用上述开发的系统，以开放和封闭的裂纹在铝合金，并表明，分谐波图像是有用的成像封闭裂纹。此外，我们验证了该系统，以封闭裂纹的不锈钢（SUS316L）试样的厚度相当的再循环管道的原子能发电厂。证明了该方法在裂纹尺寸确定方面具有较高的精度。最后，我们将该系统应用于加工过程中产生的钢试样中的裂纹，我们表明该系统是有用的斜裂纹和分支裂纹。

To establish the basis of subharmonic ultrasonic measurement and imaging of the fatigue and stress corrosion cracks, we first derived an analytical solution considering the adhesion force, and realized the relation between the threshold of subharmonic eave generation and the crack closure stress. Nest, we developed the elastc body-oscillator model since detailed information is needed to develop practical apparatus. Through numerical analysis of this model, we investigated the dependence of subharmonic wave upon the input wave amplitude and upon the closure stress, and comparison between measured and calculated waveform in experiments of fatigue cracks in an aluminum alloy (A7075).As the next step we developed nonlinear ultrasonic imaging system for detailed measurement of the depth of closed cracks in thickness direction. As a transmitter, we employed a LiNbO3 single crystal oscillator that has excellent durability for high voltage electric signal and employed an array sensor for producing image by the received signal focusing. Moreover, the delay law within the specimen was formulated and designed a IIR digital filter as a preprocess before imaging to construct a system that can be used to obtain linear and nonlinear images of cracks.In addition, we applied the developed system above described to open and closed cracks in aluminum alloy and showed that the subharmonic image is useful for imaging closed cracks. Moreover, we verified the system to closed crack in stainless steel (SUS316L) specimen with comparable thickness to that used in recirculator piping of atomic power plants. Then we proved that it has high accuracy in crack sizing. Finally, we applied this system to cracks in steel specimen generated under processing, and we showed that the system is useful for oblique and branched cracks.