变厚度储罐壁板电磁超声导波检测技术基础研究

Although the EMAT guieded-wave technique has made some progress in theories and applications, some key problems in inspection for complex structures with variable thickness have still not been solved. Firstly, variation of wall thickness can excite much more guided-wave mode in structures at the same frequency; then the boundary of structures with variable thickness make guided-waves propagating complicated; finally, wall thickness variation continuously can cause guided-wave serious attenuation and distortion. Therefore, combined with the various research means such as theoretical analysis, numerical caculation and physical experiment, the guided-wave motion equation introduced the boundary conditions of variable thickness will be established and solved, the effects of wall thickness variation on guided-wave mode conversion are analyzed systematically, the excitation and propagation mechanism will be studied deeply, and the analytical model of such characteristic parameters as guided-wave scattering, reflection and transmission under different boundary conditions can be obtained. On the basis, the propagation characteristics of guided-wave in structures with stepped thickness will be selectively studied in view of the structural characteristics of tank wall, the acoustic field and its energy distribution of guided-wave in the area with geometrical discontinuity will be described, the law of guided-wave mode conversion and dispersion in tank wall can be revealed, some effective methods of signal identification for tank wall inspection will be investigated. And the theoretical and methodological basis of guided-wave technology for tank wall inspection will be established. Moreover, this research work will provide some theoretical basis for EMAT guided-wave while detecting some structures with variable wall thickness and large-size.

尽管电磁超声导波检测技术研究已取得较成熟的理论和成果，但其在变壁厚结构检测方面尚存在诸多技术瓶颈。首先，壁厚变化使得相同的频率下结构中会激发出更多的导波模态；其次，变壁厚结构的边界条件使得导波的传播非常复杂；再次，壁厚连续变化导致导波的衰减和畸变较为严重。因此，本项目拟采用理论分析、仿真计算、物理实验等方法，引入结构的变壁厚边界条件建立并求解其导波波动方程，分析结构边界形式与导波模态转换的作用关系，研究变壁厚结构中导波的激发和传播机理，获得不同边界条件下导波散射、反射、透射特性参数的解析模型；在此基础上，针对储罐壁板的结构工艺特点，重点研究壁厚呈阶梯状变化的结构中导波的传播特性，描述导波在几何不连续区域的声场和能量分布特征，揭示储罐壁板中导波的模态转换和频散规律，探索变壁厚结构中导波信号的特征识别方法，建立储罐壁板导波检测技术的理论与方法基础，并为变厚度、大尺寸结构的导波检测提供技术支撑。

针对结构厚度变化引起的导波模态增多和传播特性复杂等问题，项目首先理论研究了变壁厚板状结构中电磁超声导波的激励、接收机理，分析了电磁超声换能过程涉及的非线性问题及电场、磁场、力场和声场等多个物理场之间的能量耦合关系，理论推导了洛伦兹力、磁致伸缩力以及洛伦兹力电流密度和磁致伸缩电流密度的计算公式；其次，建立了变厚度板平面应变波动方程，求解得到了具有多模态和频散特性的导波频散方程和相速度、群速度与频厚积间的一般关系式，研究了在变壁厚板状结构中激励接收纯净SH1波的可行性；再次，推导了变厚度板中SH1波群延时理论模型，研究了SH1波在楔形板中信号幅值、走时随板厚的变化规律，并提出了一种双探头对称布置激励SH1波的方法，设计并优化了一种在变厚度板状结构上、下表面对称布置PPM EMATs，通过保证结构上、下表面探头的磁极同向、电流反向来激励和接收纯净SH1波的换能器结构，分析了上、下探头安装误差对激励信号幅值和对称模态导波抑制效果的影响。在此基础上，深入分析了SH1波在阶梯状不连续区域的模态转换规律和声场特性，重点研究了变厚度板状结构的倾斜角、缺陷位置及其特征几何参数（宽度、深度、斜角）对SH1反射波和透射波声时的影响关系，探索了基于二维傅立叶变换、希尔伯特黄变换提取缺陷回波信号特征包络信息的信号处理方法。研究和实验结果表明，基于双PPM EMATs上、下对称布置来激励和接收SH1波，能够较好地实现对变厚度板状结构的导波检测，其能为储罐壁板等变厚度板状结构的电磁超声导波检测提供有效的解决方案。