Crack damage detection by ultra-high frequency A. C. potential method for coated single crystal superalloys

本研究以交流电位法研究两个主要课题。这两个课题都涉及到镍基高温合金燃气轮机叶片防腐和热障涂层的损伤评估。一是测量由于热应力在涡轮叶片上形成的裂纹数量和裂纹深度。二是测量涂层与基体高温合金界面处缺陷的尺寸和深度。交流电位-频率关系的梯度随裂纹数量和深度的不同而不同。这意味着裂纹的数量和裂纹深度可以通过利用交流电位的频率依赖性来确定。两者实际上都可以通过比较目标标本与数据库中存储的许多破碎标本的关系来确定。交流电位的最大值与缺陷的大小相对应，而交流电位-频率关系的梯度反映了缺陷的深度。交流电位法可以通过测量缺陷的最大值和梯度来测量缺陷的大小和深度。对实验结果进行了谐波电磁有限元分析。分析所得的绝对值与实验结果不完全一致，但经远离缺陷的电位归一化后的电位比与实验结果吻合较好。这清楚地表明，本研究得到的实验结果具有电磁理论的物理背景。

This research project investigated two main topics by using A. C. potential method. The both topics are related with the damage assessment of nickel base superalloy gas turbine blades with anti-corrosive and thermal barrier coatings. One was to measure the number of cracks and the crack depth formed on the turbine blade due to thermal stressing. The other was to measure the size and the depth of defects occurred at the interface of the coating and base superalloy.The gradient of the A. C. potential - frequency relationship was deferent depending on the number and the depth of cracks. This means that the number of cracks and the crack depth are determinable by utilizing the frequency dependence of A. C. potentials. The both are actually determinable by comparing the relationships of the target specimen to those in the database where the relationships of many cracked specimens are stored.The maximum value of A. C. potential was corresponded with the size of defects while the gradient of the A. C. potential-frequency relationships reflected the depth of defects. The A. C. potential method was concluded to be capable to measure the size and the depth of defects by measuring the maximum value and the gradient.Harmonic electro-magnetic finite element analyses were carried out to confirm the experimental results obtained. Absolute values obtained in the analyses were not rigorously agreed with the experimental results but the potential ratio normalized by the potential away from the defects agreed well with the experimental results. This clearly indicates that the experimental results obtained in this study had a physical background of electro-magnetic theory.