脉冲涡流检测提离交叉点的形成机理与调控方法研究

Pulsed eddy current (PEC) method has big potential for nondestructive evaluation of structural integrity due to rich of information and large penetration depth. For manual inspection, liftoff effect (local variation of the distance between the probe and target) has much influences on inspection accuracy. Therefore, reduction of liftoff effect plays a critical role in effective PEC testing. Liftoff intersection point (LOI) is a point where PEC signals intercept when only liftoff distance changes. LOI coordinates are inherently immune to liftoff variations, which makes them desirable signal features for PEC evaluation. This proposal is concentrated on the physics and regulating method of LOIs, which can be divided into four parts. (1) Analytical models of PEC signals will be established to present a tool for simulation and analysis. (2) The characteristics of phase from spectral PEC signals will be investigated in terms of liftoff immunity, and Fourier transform will be adopted to relate phase signatures to a LOI. Next, comparison will be made between PEC signals with a LOI and those without a LOI to analyze the difference. As a result, the existence of a LOI can be interpreted, and the physics of a LOI can be revealed as well. (3) The relationship between relative permeability and transient magnetic flux density will be established. With this relationship, normalization-based method will be proposed to process PEC signals so as to observe a LOI from PEC signals which do not have a LOI before. Subsequently, the behavior of LOIs will be displayed and represented as a function of standard skin depth. With this function, excitation frequency will be tuned to obtain an optimum LOI for improvement of PEC inspection. (4) An experimental setup using a LOI as signal features will be developed to verify the research. The findings of this research allow the elimination of liftoff effect, the extension of LOI application and the improvement of PEC evaluation.

脉冲涡流法在机械结构完整性检测与评估中应用广泛。手动检测时提离效应（探头与试件间局部距离变化）显著影响检测精度，因此，提离效应抑制是保障检测精度的关键。提离交叉点（LOI）不受提离效应影响，是理想信号特征。本项目围绕LOI的物理机理与调控方法开展研究：1 建立脉冲涡流信号解析模型，确立各参数与信号的映射关系，为后续研究提供分析工具。2 研究脉冲涡流信号相位谱的提离不变特性，应用傅立叶变换联系信号相位与LOI，观察LOI存在和不存在时信号相位谱的差异，确定LOI存在条件并揭示其机理。3 研究相对磁导率与瞬态磁感应强度的关系，应用归一化为核心的方法使不存在LOI的脉冲涡流信号出现LOI；研究LOI特性，构建LOI与渗透深度的映射关系，探索调整激励频率控制LOI处于最优区间以提高检测性能。4 开发基于LOI的实验系统进行验证。研究结果对抑制提离效应、拓宽LOI应用范围、提高检测性能具有重要意义。

提离效应对脉冲涡流检测信号的影响非常明显，有时掩盖了有用信号，如不采取抑制措施，则会降低检测精度，甚至导致无效的检测结果。提离交叉点信号不受提离效应影响，且具有明确的物理意义，是脉冲涡流检测与评估的理想特征。立项前，提离交叉点的形成机理不明，且无法从铁磁性试件脉冲涡流信号中获取提离交叉点，限制了提离交叉点的应用范围，未见资料报道提离交叉点信号的调控方法。针对上述问题，本项目深入研究了铁磁性和非铁磁性材料参数的提离交叉点形成机理与变化特性。首先，建立了基于区域截断法的常规涡流解析模型，应用傅里叶变换构建了脉冲涡流解析模型，采用分段插值方法提高了脉冲涡流模型效率；接着，通过仿真与实验研究，发现了铁磁性材料提离交叉点，并将其用于铁磁性材料平板的厚度测量；随后，提出了在脉冲涡流响应信号频域中获取提离交叉点的方法，并通过实验和仿真对该方法进行了验证。紧接着，研究发现通过改变激励参数可以对提离交叉点进行调控，改善了提离交叉点测量范围和测量精度。最后，提出了利用提离交叉点坐标值测量非铁磁性镀层厚度的方法，研究了检测精度的调控策略。本项目共发表学术论文15篇（全部标注本项目批准号51677187，第一标注12篇，第二标注3篇），其中SCI检索论文14篇，CSCD检索论文1篇；参加国内会议和展览31人次；申请发明专利4项，授权4项；培养博士研究生1人，硕士研究生7人（已毕业7人）。项目成果为提离交叉点在铁磁性材料中应用提供了理论依据，有望发展成为一种方便快捷且不受提离变化的电磁无损检测方法，提出了检测性能调控策略，对推动脉冲涡流提离交叉点工程应用具有重要理论意义。