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

本研究利用A. C.电位法这两个课题都是关于带防腐蚀和热障涂层的镍基高温合金燃气涡轮机叶片的损伤评估。一个是测量由于热应力在涡轮机叶片上形成的裂纹数量和裂纹深度。另一种方法是测量涂层与基体合金界面处缺陷的大小和深度。C.电位-频率关系随裂纹数目和裂纹深度的不同而不同。这意味着裂纹的数量和裂纹深度可以通过利用A的频率依赖性来确定。C.潜力实际上，这两者都可以通过将目标试样的关系与存储有许多裂纹试样的关系的数据库中的关系进行比较来确定。C.缺陷的大小与电位的大小相对应，而A. C.电位-频率关系反映了缺陷的深度。助理C.通过测量最大值和梯度，证明了电势法能够测量缺陷的大小和深度，并对实验结果进行了谐波电磁有限元分析。分析中得到的绝对值与实验结果并不严格一致，但由远离缺陷的电位归一化的电位比与实验结果一致。这清楚地表明，在这项研究中获得的实验结果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.