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| 电磁超声兰姆波的传播机理及缺陷检测技术研究
摘 要
薄铝板构件广泛应用在交通、海洋、空天等领域，尤其在汽车、飞机、航天、舰船等领域的一些轻量化、耐腐蚀关键部件上，薄铝板构件具有不可替代性。对于薄铝板构件的缺陷检测，超声无损检测技术(UT)是主要的检测技术之一。目前，超声检测技术主要有压电超声、激光超声和电磁超声，电磁超声与其他超声检测方法相比因其工作时无需对试件表面进行预处理和添加耦合剂而具有广阔的应用前景。然而，由于电磁超声换能器存在换能效率低、激发的兰姆波存在多模态和频散现象、兰姆波在板内传播时其传播速度和能量衰减易受到温度影响等缺点，因此具有广阔的研究价值。本文从理论推导、有限元仿真和实验验证等方面对提高换能器的换能效率、激发单一模态兰姆波、探究温度对兰姆波的传播机理影响等方面做了研究，并研发了一款电磁超声无损检测激发系统。
首先，根据麦克斯韦方程组，推导了兰姆波S0模态和A0模态的激发与洛伦兹力方向、偏置磁场方向的关系。根据偏置磁场方向对激发兰姆波模态的影响对电磁超声换能器进行了优化，设计了可激发单一兰姆波S0模态的电磁超声换能器。根据理论知识推导了优化后的电磁场超声换能器的永磁铁结构可以产生抑制兰姆波A0模态的磁场。并通过仿真实验和实测实验验证了优化后的电磁超声换能器激发单一S0模态的效果。针对电磁超声换能器换能效率低的问题进行了研究，在优化后的可以激发单一S0模态的电磁超声换能器的基础上对电磁超声换能器的曲折线圈结构进行了优化，通过理论分析和仿真实验与普通电磁超声换能器的换能效率进行了对比，验证了优化后的曲折线圈结构可以增强换能效率。
然后，针对温度对电磁超声兰姆波检测精度和能量衰减的影响进行了研究。研究了温度对电磁超声换能器和待测件各主要参数的影响。通过有限元仿真，仿真了在温度变化时，接收到的兰姆波随温度的变化情况。为后续的温度补偿算法提供了理论依据。
最后，针对传统电磁超声无损检测激励系统存在的激发功率低、便携型差的问题设计了一款电磁超声无损检测激励系统。系统以FPGA提供脉冲激励，脉冲信号经过功率放大后驱动全桥逆变电路，输出具有高带负载能力的脉冲波，脉冲波可直接连接到电磁超声换能器上，激励出兰姆波。
关键词：EMAT；单一模态；超声兰姆波；温度；激发功率；33
Abstract
Thin aluminum plate components are widely used in transportation, ocean,aerospace and other fields,especially in some lightweight and corrosion-resistant key components in automobiles, aircraft, aerospace, ships and other fields. Thin aluminum plate components are irreplaceable.For the defect detection of thin aluminum plate components,ultrasonic non-destructive testing (UT) is one of the main detection technologies. At present, ultrasonic testing technologies mainly include piezoelectric ultrasound, laser ultrasound and electromagnetic ultrasound. Compared with other ultrasonic testing methods,electromagnetic ultrasound has broad application prospects because it does not require pretreatment of the surface of the specimen and the addition of couplant. However, the electromagnetic ultrasonic transducer has the disadvantages of low energy conversion efficiency, multi-mode and dispersion phenomenon of the excited Lamb wave,and the propagation speed and energy attenuation of the Lamb wave in the plate are easily affected by temperature. Therefore, Has broad research value.In this paper, from theoretical derivation,finite element simulation and experimentalverification, we have done research on improving the energy conversion efficiency of the transducer, exciting single-mode Lamb waves, and exploring the influence of temperature on the propagation mechanism of Lamb waves. An electromagnetic ultrasonic nondestructive testing excitation system.
First, according to Maxwell's equations,the relationship between the excitationof Lamb wave S0 mode and A0 mode and the direction of the Lorentz force and the direction of the bias magnetic field is derived. According to the influence of the direction of the bias magnetic field on the excitedLamb wave mode, the electromagnetic ultrasonic transducer is optimized,and an electromagnetic ultrasonic transducer that can excite a single Lamb wave S0 mode is designed. According to theoretical knowledge,it is deduced that the permanent magnet structure of the optimized electromagnetic field ultrasonic transducer can generate a magnetic field that suppresses the Lamb wave A0 mode. The simulation experiment and actual measurement experiment verify the effect of the optimized electromagnetic ultrasonic transducer to excite a single S0 mode. The problem of low energy conversion efficiency of the electromagnetic ultrasonic transducer is studied, and the tortuous coil structure of the electromagnetic ultrasonic transducer is optimized onthe basis of the optimized electromagnetic ultrasonic transducer that can excite a single S0 mode. , Through theoretical analysis and simulation experiment and comparison of the energy conversion efficiency of ordinary electromagnetic ultrasonic transducer,it is verified that the optimized zigzag coil structure can enhance the energy conversion efficiency.
Then, the influence of temperature on the detection accuracy and energy attenuation of electromagnetic ultrasonic Lamb waves is studied. The influence of temperature on the main parameters of theelectromagnetic ultrasonic transducer and the DUT is studied. Through the finite element simulation,the variation of the received Lamb wave with temperature is simulated when the temperature changes. It provides a theoretical basis for the subsequent temperature compensation algorithm.
Finally, an electromagnetic ultrasonic non-destructive testing excitation system was designed to solve the problems of low excitation power and poor portability of the traditional electromagnetic ultrasonic non-destructive testing excitation system. The system uses FPGA to provide pulse excitation,the pulse signal drives the full-bridge inverter circuit after power amplification, and outputs a pulse wave with high load capacity. The pulse wave can be directly connected to the electromagnetic ultrasonic transducer to excite the Lamb wave.
Key Words：EMAT; single mode; ultrasonic Lamb wave; temperature; excitation power;
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