Li Yong, Jing Haoqing, Zainal Abidin Ilham Mukriz, Yan Bei
State Key Laboratory for Strength and Vibration of Mechanical Structures, Shaanxi Engineering Research Center of NDT and Structural Integrity Evaluation, Xi'an Jiaotong University, Xi'an 710049, China.
Leading Edge NDT Technology (LENDT) Group, Malaysian Nuclear Agency, 43000 Bangi, Kajang, Selangor, Malaysia.
Sensors (Basel). 2017 Apr 25;17(5):943. doi: 10.3390/s17050943.
Coated conductive structures are widely adopted in such engineering fields as aerospace, nuclear energy, etc. The hostile and corrosive environment leaves in-service coated conductive structures vulnerable to Hidden Material Degradation (HMD) occurring under the protection coating. It is highly demanded that HMD can be non-intrusively assessed using non-destructive evaluation techniques. In light of the advantages of Gradient-field Pulsed Eddy Current technique (GPEC) over other non-destructive evaluation methods in corrosion evaluation, in this paper the GPEC probe for quantitative evaluation of HMD is intensively investigated. Closed-form expressions of GPEC responses to HMD are formulated via analytical modeling. The Lift-off Invariance (LOI) in GPEC signals, which makes the HMD evaluation immune to the variation in thickness of the protection coating, is introduced and analyzed through simulations involving HMD with variable depths and conductivities. A fast inverse method employing magnitude and time of the LOI point in GPEC signals for simultaneously evaluating the conductivity and thickness of HMD region is proposed, and subsequently verified by finite element modeling and experiments. It has been found from the results that along with the proposed inverse method the GPEC probe is applicable to evaluation of HMD in coated conductive structures without much loss in accuracy.
涂层导电结构在航空航天、核能等工程领域中被广泛应用。恶劣且具有腐蚀性的环境使得服役中的涂层导电结构容易受到保护涂层下隐藏材料退化（HMD）的影响。人们迫切需要使用无损评估技术对HMD进行非侵入式评估。鉴于梯度场脉冲涡流技术（GPEC）在腐蚀评估方面相对于其他无损评估方法的优势，本文对用于HMD定量评估的GPEC探头进行了深入研究。通过解析建模得出了GPEC对HMD响应的闭式表达式。通过涉及不同深度和电导率的HMD模拟，引入并分析了GPEC信号中的提离不变性（LOI），该特性使得HMD评估不受保护涂层厚度变化的影响。提出了一种利用GPEC信号中LOI点的幅值和时间同时评估HMD区域电导率和厚度的快速反演方法，并随后通过有限元建模和实验进行了验证。结果表明，结合所提出的反演方法，GPEC探头适用于评估涂层导电结构中的HMD，且精度损失不大。
