瞬态涡流检测层析成像理论及缺陷三维重构技术研究

Hidden defect into the aircraft fuselage can cause disaster. It is critical challenge to inspect hidden defect effectively and nondestructively for aerospace field. Transient eddy current testing is characteristic of multi-frequency components. Frequency is analogous to focus of camera. So transient eddy current testing has the potential to evaluate each depth of the conductor. Based on this merit, the principal and method of tomographic detection of hidden defect in the conductor are researched. Firstly, to solve the limitations caused by analyzing single eddy current field for forward problem of the transient eddy current testing, mathematics model of coupled three-dimensional transient eddy current field-circuit problem is built and solved using numerical calculation method. Furthermore, research on tomography mechanism of transient eddy current testing is accomplished. Secondly, the uniform field merits of alternating current field measurement is applied to transient eddy current testing. The shortage caused by picking up one-dimension signal for defect detection is made up. And novel array probe is designed and used to enhanced tomography capability. Thirdly, based on nonstationary characteristics of signal, time-frequency methods are investigated to accomplish signal decomposing and hierarchical feature extraction. Suppression method of liftoff noise for transient eddy current testing is attained. So image reconstruction is optimized. Finally, quantitative evaluation of hidden defect is accomplished based on tomography C-Scan image and B-Scan image. The research results of this project are advantageous to perfect tomography theory of transient eddy current testing and present new method for three-dimension reconstruction of defect.

飞机机身隐含缺陷会导致灾难性事故，如何对其实现有效地无损检测是目前航空航天领域面临的严峻挑战。瞬态涡流检测具有多频特性，所含"频率"恰好类似相机的"焦距"，具有对导体不同纵深位置进行损伤评估的潜力，本项目基于这一技术优势研究导体层析成像检测机理与方法。首先为解决瞬态涡流正问题建模时仅采用单一场分析存在的局限，拟将瞬态涡流场-电路直接耦合建模并求解，研究瞬态涡流检测层析成像机理；其次将ACFM技术"均匀场"优势引入瞬态涡流检测中，突破现有的单维信号测量局限，研制能实现多维测量的新型阵列探头结构，提高层析成像"硬"实力；然后立足瞬态响应信号的非平稳特性，寻找合适的时频分析方法实现信号多层次分解和分层特征提取，有效抑制提离等强干扰，提高重建图像质量；最后基于断层C-Scan成像和剖面B-Scan成像，完成对导体深层缺陷的定量评估。本项目研究可完善瞬态涡流层析成像理论，拓展缺陷三维重构技术途径。

本项目围绕飞机机身结构的瞬态涡流探伤问题，完成的主要研究内容及解决的技术问题如下：①本项目在建立理论模型时就将探头外接电路考虑进来，引入线圈磁链作为耦合因子，联立涡流场方程和探头电路约束方程，建立了两者直接耦合求解模型，打破了仅对采用单一涡流场进行建模分析存在局限性，提高了理论模型与实际系统的一致性，并增加了模型的理论价值和应用范围。②本项目将交变磁场测量技术ACFM的“均匀场”优势引入瞬态涡流检测TEC中，优化设计了探头结构，实现了将响应信号随时间t的变化特征和空间（X，Y，Z三个维度）的分布特征相结合对缺陷各个参数的定量评估。与“涡旋场”的常规探头模式相比，可将一维测量拓展到三维测量，显著提升了探头对缺陷轮廓变化和位置深度的敏感性，有利于成像检测。③瞬态涡流响应信号形式虽简单但正确解释难度大，特别是在工程实践中极易混入了提离干扰，更会显著降低缺陷识别率。针对这一问题，本项目分析了瞬态电磁场在被测导体内部的渗透规律，从理论上阐述了利用信号的非平稳特性实现缺陷检测的可行性。并在此基础上，将信号分析方法从时域拓展到时–频域，提出了一种利用相位谱特征实现检测信号混杂提离干扰时深层缺陷识别的新方法。④本项目实现了ACFM和TEC两项技术原理层面的融合，并以此为基础完成了多层导体结构瞬态涡流层析成像。包括针对实际检测中C-Scan图像仅对裂纹参数进行定性反映、存在受污染区域、图像裂纹区域与实际裂纹区域存在位置偏差等不足，提出了一种定量反映缺陷参数的改进型C-Scan成像方法；同时采用基于径向基神经网络实现了缺陷纵剖面的重构。