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

为建立疲劳与应力腐蚀裂纹的亚谐波超声测量与成像基础，首先推导了考虑附着力的解析解，实现了亚谐波屋檐产生阈值与裂纹闭合应力之间的关系。其次，我们开发了弹性体振荡器模型，因为需要详细的信息来开发实用的仪器。通过对该模型的数值分析，研究了某铝合金（A7075）疲劳裂纹实验中，次谐波对输入波幅值和闭合应力的依赖关系，并对实测波形和计算波形进行了比较。下一步，我们开发了非线性超声成像系统，用于在厚度方向上详细测量闭合裂纹的深度。作为发射器，我们采用了对高压电信号具有优异耐久性的LiNbO3单晶振荡器，并采用阵列传感器通过接收到的信号聚焦产生图像。建立了试件内部的延迟规律，并设计了IIR数字滤波器作为成像前的预处理，构建了一个可用于获取裂纹线性和非线性图像的系统。此外，我们将所开发的系统应用于铝合金的开闭裂纹，并证明了亚谐波图像对闭闭裂纹的成像是有用的。此外，我们还验证了该系统可以关闭与核电站循环管道中使用的厚度相当的不锈钢（SUS316L）试件的裂纹。结果表明，该方法具有较高的裂纹评定精度。最后，将该系统应用于钢试件加工过程中产生的裂纹，结果表明该系统对斜裂纹和分支裂纹是有效的。

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.