基于环形阵列聚焦式电磁超声钢管腐蚀成像检测方法研究

Although the EMAT technique has made some progress in theories and applications, some key problems in inspection for steel pipe with corrosion defects have still not been solved. On the one hand, corrosion on the wall will cause distortion of the physical fields, such as electric and magnetic field; on the other hand, the attenuation and scattering of reflection wave near the corrosion defects is quite seriously. Therefore, combined with the various research means such as theoretical analysis, numerical calculation and physical experiment, the wave motion equation introduced the boundary conditions of simulated corrosion will be established and solved, the different defects on the physical fields distortion are analyzed, the transmitting and scattering mechanism will be studied deeply, the analytical model of such characteristic parameters as echo wave reflection, transmission and scattering under different boundary conditions can be obtained, and the accurate quantitative and evaluation EMAT testing system for simulated corrosion defects will be built. On the basis, a focusing circular EMATs array is selectively studied combining with the structure characteristics of the pipe, so as to realize a magnetization increasing and weak echo signal detecting at a long distance on the corrosion defects of the pipe, and also to conduct a quantitatively extracting and accurately imaging. Then the high precision tomography spectrum can be formed. The successful implementation of this project will improve the performance of EMAT system and extend its application in the engineering practice of the steel pipe corrosion testing, and has important theoretical significance and practical value.

尽管电磁超声检测技术研究已取得较成熟的理论和成果，但在钢管腐蚀缺陷检测方面尚存在诸多技术瓶颈。一方面，管壁腐蚀引起电、磁物理场发生畸变；另一方面，超声回波在腐蚀缺陷处的衰减和散射较为严重。因此，本项目拟采用理论分析、仿真计算、物理实验等方法，引入模拟腐蚀边界条件建立并求解波动方程，分析腐蚀边界形式与电磁物理量的作用关系，研究电磁超声在腐蚀缺陷处的传播和散射机理，获得不同边界条件下超声回波反射、透射、散射特性参数的解析模型，构建准确的模拟腐蚀缺陷电磁超声检测量化评价体系；在此基础上，结合钢管的结构特点，重点研究一种基于环形阵列聚焦式电磁超声探头，以突破钢管腐蚀缺陷面大提离检测下的磁化与微弱回波信号检测，以及腐蚀缺陷的定量提取与精确成像等关键技术，从而形成钢管的高精断层扫描图谱。本项目的成功实施将会提升电磁超声检测系统性能并扩展其在钢管腐蚀检测工程实践中的应用，具有重要的理论意义和实用价值。

由于电磁超声在钢管腐蚀实际检测应用方面仍存在灵敏度低、定量难等问题，首先通过调研发现构件腐蚀主要分为大面积管壁减薄类均匀腐蚀和孔洞状应力集中型局部腐蚀两大类。为此，初步提出使用脉冲反射和底波衰减两种不同的腐蚀缺陷检测方法，通过在COMSOL仿真软件中建立电-磁-力-声多物理场耦合模型，观察了超声波在钢管中的传播特性以及遇到腐蚀缺陷的反射情况，进一步开展大量试验以对比研究螺旋型线圈、方型线圈、蝶型线圈和吕型线圈四种常用腐蚀检测电磁超声线圈的特性；然后，研究了适用于曲面结构的线聚焦电磁超声SV波线圈设计方法，相比于平面线圈和弯曲的平面线圈，所设计的曲面线圈产生的超声波在较大曲率曲面工件中表现出更高的指向性精度，在曲率为20的铝管中的缺陷检测峰值比平面线圈和平面线圈弯曲分别提高了338.84%和44.73%。推导了引起结构腐蚀的内应力检测经验公式并研究了基于超声纵/横波相结合的检测方法，应力测量误差不超过5%。提出了一种将电磁超声SV波探头和表面波探头集成于一体的检测方法，仅利用单个电磁超声通道即可实现工件的多维覆盖检测；最后，研究了可大幅度提高检测精度并削弱磁吸力的增强型磁聚焦磁化器，可将钢中检测信号幅值提高62.01%或者将磁通减弱38.02%。在此基础上，设计了新型的外穿过式环形阵列钢管电磁超声探头，可以实现单个壁厚点的测量或者多个壁厚点的同时测量，同时还能够检测在外径为Φ50mm钢管的内表面预制的Φ6mm盲孔缺陷。研究了腐蚀缺陷回波信号的定量提取与精确成像算法，在钢板中实现了C扫描成像，可以检测最小3mm宽，1mm深的腐蚀缺陷，成像后得图像通过滤波和阈值分割方法处理，可以定量评价腐蚀缺陷。