增强SH波电磁超声传感器信号的方法与实验

电磁超声传感器技术作为一种非接触无损测量方法,可用于恶劣工况的无损检测,尤其是可以实现水平剪切(SH)波的激励和接收。基于磁致伸缩效应的常规SH波电磁超声传感器信号较弱,为得到较强SH信号,改善信噪比,提出了一种增强电磁超声传感器信号的方法。该方法是在主永久磁铁闭合磁路上施加辅助永久磁铁,提高电磁超声线圈下部构件磁化强度,从而增强信号。实验证明:此方法可以使信号得到明显增强,并且有助于提高缺陷检测能力。     

融合电磁超声载波的声发射传播特性分析

电磁声发射是通过对缺陷导电结构件施加非接触式电磁载荷,进而激发声发射应力波,并据此效应来进行构件无损检测.本文针对电磁声发射信号不适合较远距离传播问题,融合电磁超声技术,实现对声发射应力波的载波远距离传输.文章首先从自由电子运动理论出发,从微观层面上研究电流对金属位错激活能的影响,推导出定向漂移的自由电子与位错碰撞时所产生的能量交换;其次搭建了融合电磁超声载波的电磁声发射有限元模型,分析了多个特征源激励下的应力波传播位移;最后利用希尔伯特黄变换方法对载波实验信号进行了分析比较.本文通过对传统的电磁声发射激励端研究、改进,提升电磁超声对电磁声发射应力波的传输特性,拓宽了电磁声发射检测的应用范围.     

电磁超声水平剪切导波的缺陷检测研究进展

超声导波检测技术是新型的无损检测方法之一。水平剪切(SH)导波是超声导波的一种。在板和管结构的检测中，SH导波检测技术占据着越来越重要的位置。电磁声传感器(EMAT)常被用于SH导波的激励或接收。SH-EMAT具有适应性强、效率高的特点，基于SH-EMAT检测技术的应用场景越来越多。以近十几年的相关文献为基础，介绍了SH-EMAT设计以及SH导波缺陷检测技术的研究现状和进展，提供了SH导波检测在不同应用背景下的传感器选择，总结了不同频率SH导波的检测能力，分析了不同SH导波检测技术的优缺点。同时还对电磁超声水平剪切导波检测的未来研究趋势进行了展望，为相关研究人员提供了研究方向和思路。     

基于超声衍射时差法的缺陷检测技术仿真分析

采用有限元方法对超声衍射时差(TOFD)检测技术进行分析.首先使用COMSOL有限元分析软件对超声TOFD检测技术建模;而后基于该模型分析超声波的产生、传播和接收过程;最后通过改变模型参数,研究缺陷几何尺寸对超声TOFD检测信号的影响.研究结果表明:超声TOFD信号中直通波、上端衍射波、下端衍射波、底面反射波信号的相位依次变化180°;当缺陷位于工件上表面或下表面时,探头只能接收到四种基本超声信号中的一部分.     

桩柱周期分布的平板中SH波的传播特性分析及其在聚焦透镜设计中的应用研究

本文首先建立求解水平剪切波(即SH波)在桩柱周期分布的板中传播的理论模型,计算其带隙结构,并与有限元仿真的结果进行对比,验证该理论模型的准确性.在此基础上,应用此模型研究桩柱高度对最低阶水平剪切波(即SH0)的影响,发现随着桩柱高度的增加,SH0的相速度降低.基于桩柱高度与相速度的敏感特性,设计了一款可实现SH0聚焦的透镜,并进行相关的仿真分析,研究该透镜的适用范围和聚焦效果.研究结果表明,该透镜聚焦位置准确,聚焦能量高,且在不改变结构的情况下在一定频率范围内均可实现聚焦功能,从而为能量收集、超声医学、无损检测、吸声降噪等应用提供新的思路.     

电磁超声体波方法对内部孔洞缺陷检测研究

电磁超声是一种新型的超声检测方法,其具有非接触、不受工作环境影响、易于激发多种类型的超声波等优点。本文研究了电磁超声体波对结构件内部孔洞缺陷探测问题,首先利用离散点源实现对电磁超声声场的分析;而后利用有限元方法对电磁－结构多场耦合建模,研究超声体波在体内缺陷处的波型转换以及模式转换后爬波在缺陷表面传播情况;最后利用电磁超声体波对孔洞缺陷的尺寸、位置进行了定量检测。有限元计算结果与简化解析求解得到较好的验证,本文为电磁超声对体内孔洞缺陷的应用评估提供良好基础。     

水下垂直发射出筒载荷响应分析

在运载火箭水下发射载荷预报问题的研究中,运载火箭水下垂直发射过程中,存在明显的横向流、弹性适配器与箭体的动态耦合,造成箭体出现复杂载荷波系,严重影响运载火箭垂直发射安全.为了解决上述问题,在分析了发射火箭的流体动力特性、弹性适配器的变形应力特征及其与箭体强耦合主动激励特性的基础上,针对出筒过程中箭体的法向载荷振荡响应特性进行了动力学仿真,并对波形特征进行了相应的对比分析.仿真结果表明:箭体法向载荷可归结为由主动激励低频波与适配器弹性阻尼高频波的综合叠加,并提出以载荷主体波系预报箭体表面法向载荷分布危险截面域的方法.     

用于板结构损伤检测的磁致伸缩传感器

基于铁磁性材料的磁致伸缩特性,提出一种用于非铁磁性板结构损伤检测的磁致伸缩传感器。该传感器分为激励和接收两部分,由8字型线圈、镍带以及偏置永磁铁组成。根据磁致伸缩效应,激励部分在板中激励导波,波在板中传播,遇缺陷及边缘反射,通过磁致伸缩逆效应,由接收部分接收其反射信号。根据反射信号的到达时间和波在板中传播的速度,可判断出板中缺陷所在位置。改变恒定偏置磁场与时变磁场的方向可以在板中激励不同波型的导波。实验结果证明:该传感器设计是可行的,且具有价格便宜、灵敏度高、可与被测结构分离等优点。     

变波长水平剪切波电磁超声换能器设计

针对大型常压储罐底板厚度在线测量需要,设计并制作了一种可变波长的低阶水平剪切波电磁超声换能器,该换能器主要由永磁铁阵列、可变间距机械夹具、PCB板、阻抗匹配模块、接口、保护层及外壳等组成.对研制的换能器进行了性能测试,结果表明:研制的电磁超声换能器可在一定波长变化范围内激励出单一的低阶水平剪切波,且信噪比较好.设计的可变波长电磁超声换能器为压力容器在线检测提供了可能的传感器手段.     

基于电磁超声的天然气管道机器人测控系统设计

分析了国内天然气管道现状及检测维护情况,针对目前天然气管道裂纹缺陷检测问题,采用电磁超声检测方法,并结合虚拟仪器技术,设计了天然气管道机器人测控系统;系统以PXI模块化仪器平台为核心,结合数据采集模块和运动控制模块构建了硬件平台,以LabVIEW编程语言为基础开发了人机交互软件平台;该系统能够控制机器人在管道内的运动,并可在管道内壁激励和接收超声导波;整个系统可靠性高、扩展性强、人机交互界面友好,满足了机器人管内电磁超声检测作业时的运动控制、数据采集及处理要求.     

Exact periodic,cusp, solitary and loop wave solutions of the EX-ROE

The extended reduced Ostrovsky equation (EX-ROE) is investigated by using the dynamical system theory. The bifurcation phase portraits are drawn in different regions of parameter plane. The bounded travelling wave solutions such as periodic waves, periodic cusp waves, solitary waves, peakon, solitary loop waves and periodic loop waves are obtained. The dynamic characters of these solutions are investigated.     

香港维多利亚港内船行波浪摄影测量应用研究

在香港维多利亚港对船行波浪实体进行同步陆地立体摄影测量。用解析摄影测量绘制波浪等值线图,对船行波高、船速、入/反射波的相关信息进行统计分析,进而研究了维多利亚港内常见船舶类型的兴波能力及入/反射波的运动过程。     

串脉冲超声导波断轨检测技术研究

针对传统的钢轨断轨检测效率低、无法进行实时检测的不足,对串脉冲超声导波断轨检测方法进行了研究.首先为了增强发射能量,同时达到脉冲叠加的目的,研发了脉冲个数、脉宽、间隔均可调的高压串脉冲超声导波发射系统;为了保证接收信号的信噪比和空间精度,用匹配滤波的方法对接收信号进行脉冲压缩.采用研制的串脉冲超声导波断轨检测系统,选用SH导波换能器在钢轨上进行了检测实验.结果表明,检测系统性能稳定,经过串脉冲叠加,使接收信号的幅值约为单脉冲接收信号的3倍,经过匹配滤波后,信噪比得到明显提升.该方法的换能器置于轨腰,能够对钢轨实现在线监测.     

Solitary wave simulations of Complex Modified Korteweg-de Vries Equation using differential quadrature method

Complex Modified Korteweg-deVries Equation is solved numerically using differential quadrature method based on cosine expansion. Three test problems, motion of single solitary wave, interaction of solitary waves and wave generation, are simulated. The accuracy of the method is measured via the discrete root mean square error norm L₂, maximum error norm L_∞ for the motion of single solitary wave since it has an analytical solution. A rate of convergency analysis for motion of single solitary wave containing both real and imaginary parts is also given. Lowest three conserved quantities are computed for all test problems. A comparison with some earlier works is given.     

Improved Beam Steering Method Using OAM Waves

Orbital Angular Momentum (OAM) is an intrinsic feature of electromagnetic waves which has recently found many applications in several areas in radio and optics. In this paper, we use OAM wave characteristics to present a simple method for beam steering over both elevation and azimuth planes. The design overcomes some limitations of traditional steering methods, such as limited dynamic range of steering, the design complexity, bulky size of the steering structure, the limited bandwidth of operation, and low gain. Based on OAM wave characteristics, the proposed steering method avoids design complexities by adopting a simple method for generating the OAM-carrying waves. The waves are generated by an array of Planar Circular Dipole (PCD) elements. These elements are designed to cover a wide bandwidth range between 3 and 30 GHz. The transmitting array shows an enhanced gain value from 8.5 dBi to almost 11.5 dBi at the broadside angle. Besides the enhanced PCD-based OAM generation, the novelty of the design lies in a new method of beam steering. Beam steering is then performed by controlling the electrical feeding of the PCD elements; the beam azimuthal location is managed by turning off some of the PCD elements, while the elevation is determined by changing the gradient phase of excitation for the turned-on PCD elements. Detailed analysis of the steering method is carried out by finding the mathematical model of the system and the generated waves. The performance has been verified through numerical simulators.     

Generalized vortex methods for free surface flow problems. II: Radiating waves

Generalized vortex methods are applied to the study of free surface motion in incompressible, irrotational, inviscid, layered flows in which waves are generated by external means, such as the motion of submerged bodies. Radiation conditions are developed that allow outward-traveling waves to escape the computational domain without significant reflection. Numerical sponge layers are used to absorb outward-traveling waves. Applications are given to wave generation by surface pressure distributions and by translating submerged bodies.     

Vortex pattern and wave motion produced by a bottom blunt body moving at a critical speed

An underwater object moving at a near-critical speed in a shallow-water domain had been observed to generate a sequence of upstream propagating solitary waves with an elongated depression of water surface and a train of dispersive waves followed in the downstream. This study presents the development of a two-dimensional stream function–vorticity based viscous fluid model with satisfied nonlinear free-surface conditions to study the generation of solitary waves and the induced vortex motion under the forcing of a moving object. A combined finite analytic and finite difference method is adopted to solve the flow field equations and free-surface boundary conditions in a transient curvilinear coordinate system. The model is shown to produce free-surface elevations in fairly good agreements with published results for a test case of a moving smooth bump. Other tests for the generation of recirculation zone behind a body of square shape in a confined fluid domain are also conducted to further verify the model performance. The results showing the generation of upstream advancing solitary waves and downstream vortex pattern by a blunt rectangular body moving at a critical speed along the bottom in a domain with free surface are presented. Comparisons of results from potential flow and viscous flow conditions are made to demonstrate the importance of viscosity to the wave generation. Different from the relatively regular vortex pattern occurred under the case of Re = 3500, the transition of the vortex motion for a larger Reynolds number (e.g. Re = 35,000) evolves without a regular pattern throughout the generation process of the advancing solitons. The effects of the size and bluntness of a moving object on the generated flow field and free-surface elevations are also analyzed and discussed.