多层平板构件深层缺陷的脉冲远场涡流定量评估关键技术研究

Quantitative evaluation of deeply hidden flaws in aircraft multilayer aircraft structure has been a big challenge to aviation nondestructive testing (NDT) society for a long time. Conventional NDT techniques are incapable of detecting for inner layer fatigue cracks and corrosion from aircraft exterior, therefore,to inspect inaccessible areas of an aircraft component usually requires the removal of obstructions or disassembly prior to inspecting the desired component. The removal of the exterior is expensive and can not be adapt to the detecting demand in the airport. Remote field eddy current (RFEC) technique is not restrict by the skin effect and becomes the top effective method to solve this problem. On the basis of previous research work, pulsed excitation can provides wideband exciting frequency components and this excitation mode is introduced in electromagnetic NDT technique. The key problems of quantitative evaluation of deeply hidden flaws in flat geometry with pulsed signal excitation RFEC are studied in this project. The disadvantages of single frequency excitation technique are overcomed by making remote field phenomenon happen on an object of flat geometry, incorporating of pulsed eddy current and RFEC techniques, blocking energy diffusion along the direct energy coupling path and focusing the energy going downwards through the conducting wall to enlarge the detecting signal, improving the ability of deeply embedded defect quantitative evaluation and classification. This work will extend and improve the research domain and level of pulsed RFEC technique, and also has an important theoretics meaning and practicality value for improve the integrated NDT technique and accelerating the progress of aviation nondestructive testing.

飞机多层结构中深层缺陷的定量评估是目前航空无损检测领域面临的一个难点问题，传统无损检测方法需要对飞机结构拆卸后进行检测，这样既增加了费用又无法满足飞机外场检测的需求，远场涡流技术由于不受集肤效应的限制，其可实现原位检测的优点成为解决此难题的有效途径。本项目在前期研究工作的基础上，利用脉冲激励频谱宽的优势，将脉冲激励方式引入电磁无损检测领域，深入研究脉冲激励下远场涡流技术对平板构件中深层缺陷定量评估的关键技术难题。通过突破平板构件远场涡流的检测机理、脉冲涡流与远场涡流的原理集成、脉冲远场涡流检测的磁场抑制与信号增强以及深层缺陷的定量评估与分类识别等关键科学问题，从而在解决正弦激励的远场涡流检测技术不足的基础上，进一步拓展和提高脉冲远场涡流检测技术的研究领域和理论水平。本项目是前期研究工作的深化和拓展，对于提高集成化无损检测的研究水平和推进航空无损检测的进程都具有十分重要的理论意义和实用价值。

本项目针对飞机多层结构中深层缺陷的检测难题，综合利用脉冲激励频谱宽、缺陷信息丰富，以及远场涡流检测不受集肤效应限制的优势，对多层平板构件中深层缺陷的脉冲远场涡流定量评估问题展开研究。项目中首先对铁磁性平板的远场涡流检测机理进行了研究。接着针对非磁性板材的远场涡流检测效应展开了研究。项目研究中设计了一种特殊的传感器结构，将U字型构件（U-type component）罩于非磁性平板上，用于在平板构件上模拟管道结构，以达到对磁场直接耦合分量的衰减作用,从而实现对非磁性金属平板的远场检测效果。其次，将脉冲激励方式与远场涡流探头进行了集成，明确了瞬态感应信号特征量与近场区、过渡区和远场区的关系，提取了时域特征量实现了对裂纹缺陷的定量检测。为了提高传感器对大厚度材料的检测能力，需要实现对远场涡流的信号增强与磁场抑制。研究表明，信号增强单元对间接耦合磁场的增强效能以及磁场抑制单元对直接耦合分量的屏蔽效能直接决定了传感器对铆接结构中缺陷的检测能力。在信号增强方面，通过给激励线圈与检测线圈分别加装聚磁结构，实现了对磁场间接耦合分量的增强，提高了检测线圈对二次穿透平板磁场的收集能力。在磁场抑制方面，通过在激励线圈与检测线圈之间加装屏蔽盘，实现了对磁场直接耦合分量的抑制，提高了传感器对缺陷的检测能力。为了克服噪声较强时缺陷特征难以提取的问题，项目研究了远场涡流检测的缺陷分类识别和定量评估方法，采用时频分析技术克服了检测噪声对信号波形造成的干扰，实现了噪声干扰条件下缺陷的分类识别和定量评估。