Research on Highly Sensitive Quantitative Nondestructive Evaluation Techniques of Closed Cracks and Branched Cracks

1.通过优化平均条件来降低超声散射噪声，以提高信噪比。为了降低来自晶界的超声散射噪声，得到了最佳采样条件和最佳扫描条件。对含有未知疲劳裂纹的焊接件进行了闭合后壁裂纹的超声检测，通过求解超声反问题，成功地确定了裂纹闭合应力和裂纹深度。此外，研究还表明，用干冰和乙醇冷却厚壁焊接试件后壁应力腐蚀开裂的超声响应显著增强。采用非线性超声技术对闭合裂纹进行检测，并对闭合裂纹的行为与…的关系进行分析得到了较大的非线性超声波数和裂纹闭合应力。此外，利用二次谐波可以观察到弱结合、应力腐蚀开裂等现象。DCPD技术灵敏度的提高提出了一种灵敏的直流电势降技术，该技术通过对管道外表面进行局部加热来控制被测管道中的电流，并提供了该技术的能力。此外，还研制了一种新型的具有可重复接触单元的DCPD传感器，实现了对电势降的精确和可重复测量。结合各种可靠的质量检测技术，比较了超声检测的能力，包括非线性超声和敏感的DCPD技术，用于检测各种类型的有缺陷的试件。实践证明，这些技术的结合是实现闭合裂纹和分支裂纹可靠定量无损检测的有效方法。较少

1. Enhancement of signal-to-noise ratio by optimizing the averaging conditions for reducing the ultrasonic scattering noiseAustenitic stainless steel, AISI316L, was treated in this study. For reducing the ultrasonic scattering noise from the grain boundary, the optimum sampling conditions and the optimum scanning conditions were obtained.2. Ultrasonic inspection of closed back-wall cracks in welded partSpecimens containing the unknown fatigue cracks were prepared, and the crack closure stress and the depth of cracks have been successfully determined by solving the ultrasonic inverse problem. Furthermore, it was shown that the ultrasonic responses for stress corrosion cracking at the back-wall of the thick-walled welded specimen were enhanced remarkably by cooling the surface of the specimen with Dry Ice and ethanol.3. Evaluation of closed cracks by nonlinear ultrasonicsThe nonlinear ultrasonic technique was adopted to inspect the closed cracks, and the relationships between the behavio … More r of nonlinear ultrasonic waves and the crack closure stress were obtained. Furthermore, by using the second harmonic waves, weak bonding, stress corrosion cracking, and so on were visualized.4. Enhancement of sensitivity of DCPD techniqueThe sensitive direct current potential drop technique, which controls the current flow in the tested pipe by locally heating the outer surface of the pipe, has been proposed, and the ability of the present technique has been provided. Furthermore, a new DCPD sensor with repeatable contact unit for the probes has been developed, and the precise and repeatable measurements of potential drop have been achieved.5. Combination of various techniques for reliable QNDEAbilities of ultrasonic inspection including that of nonlinear ultrasound and sensitive DCPD techniques were compared for inspecting the various types of specimens having flaws. It was verified that the combination of these techniques were effective for realizing the reliable QNDE of closed cracks and branched cracks. Less