利用电磁超声横波检测二冷区尾端连铸坯壳厚度

使用电磁超声横波对二冷区尾端的连铸坯壳厚度进行检测,并建立了有限元仿真模型。选取Q235连铸小钢坯作为被测对象。为减小永磁铁的提离距离、在被测体内部生成更大的感应涡流,文章利用多物理场有限元仿真软件建立了一种不同于一般结构的电磁超声换能器仿真模型：圆柱形永磁体两侧并行排列螺旋线圈。分析了永磁铁尺寸对磁场涡流的影响以及电磁超声横波在被测钢坯内不同时间、不同位置的传播情况。计算了被测体中的磁通密度模和电流密度模,同时计算了产生的涡流大小及质点位移情况。研究结果表明：该结构的电磁超声换能器可以在连铸坯壳内成功激发并接收电磁超声横波,且横波穿透力强、效率高,能够成功探测连铸钢坯的回波信号并计算出坯壳的厚度。     

连铸坯尾端电磁超声横波换能机制分析

为了研究电磁超声横波检测表面温度为500℃连铸尾端的坯壳厚度的问题，分析此温度下电磁超声换能器的换能机制。本文以坯壳厚度为30 mm的Q235小钢坯为被测对象，利用有限元软件COMSOL建立圆柱形永磁体和螺旋线圈的电磁超声换能器模型。分析在钢坯表面温度为500℃时的换能机制的主导因素，在相同偏置磁场强度和激发电流下，通过对比仅考虑洛伦兹力作用下的位移振幅和洛伦兹力与磁致伸缩应力的叠加作用下的位移振幅大小来分析主导因素，并利用现有的实验条件进行实验验证，同时分析了激励频率对换能机制的影响。研究结果表明，当坯壳表面温度为500℃时，随着频率增大，横波激发换能机制由两种机制转变成洛伦兹力占主导因素，且铁磁材料降低了电磁超声换能器的激发难度，为设计电磁超声换能器系统提供理论基础。     

电磁超声横波测量连铸钢坯壳厚度

为解决连铸坯壳厚度在线检测问题,提出采用电磁超声横波反射法检测连铸坯壳厚度。对电磁超声横波换能器的激发和接收过程进行模拟仿真,分析横波在连铸坯中的传播情况,获得横波在连铸钢坯内固液两相区分界面处的反射回波。以坯壳厚度为10～50 mm的Q460连铸小方坯为研究对象,通过有限元仿真软件COMSOL,仿真得出温度在800～1300℃范围内时横波速度随温度的变化曲线。根据被测对象内部的温度场分布,得出坯壳内平均声速。利用回波时间与平均声速计算坯壳厚度。实验证明,上述方法可用于测量连铸坯壳厚度,当声时测量误差在1～2μs时,厚度测量误差为2～5 mm。     

电磁超声钢板测厚装置中脉冲电磁铁的设计

针对传统电磁超声换能器(electromagnetic acoustic transducer,EMAT)在钢板表面移动困难、探头易磨损等问题,提出利用脉冲电磁铁替代永磁铁为换能器提供偏置磁场的方法。该方法通过控制激励电流来控制脉冲电磁铁仅在超声体波发射接收瞬间激发脉冲磁场,提高换能器在试件表面多点测量时的移动性。通过对脉冲电磁铁励磁线圈、铁芯以及驱动电路的设计,使得脉冲磁场满足EMAT对于偏置磁场强度及维持时间的需求。为测试该脉冲电磁铁的性能,搭建电磁超声钢板测厚实验平台。结果表明:激励电流仿真值与实测值相似度达90%,脉冲电磁铁可产生峰值约0.8 T、维持时间120μs的脉冲磁场,以该方法设计的换能器可对50 mm厚钢板进行测厚同时具有良好的移动性。     

用于钢板厚度测量的常用电磁超声线圈特性研究

为解决电磁超声检测信噪比低,为电磁超声线圈设计提供参考依据,该文针对常用的测厚电磁超声线圈开展一系列实验研究。首先对比螺旋线圈、矩形线圈、吕形线圈和蝶形线圈4种线圈结构的测厚反射回波信噪比,实验对象采用阶梯型钢板,结果发现螺旋线圈对于厚板测厚效果最佳,信噪比平均值达到14 dB以上。然后研究上述线圈的幅频特性,实验结果表明激励频率为1.8 MHz时,所有线圈的信噪比达到10 dB以上。最后通过线圈作用范围实验,得出不同类型线圈的主要作用区域也不同的结论。     

电磁超声换能器新型线圈阻抗及匹配电容的计算

为提高电磁超声换能器(EMAT)的换能效率及信噪比,研究了EMAT线圈阻抗匹配的方法。考虑到检测材料属性、线圈提离以及线圈在高频激励下的集肤和邻近效应等因素会影响线圈的阻抗大小,线圈的分布电容也会对线圈的匹配产生影响,针对EMAT线圈的等效电路,采用有限元方法计算了不同提离情况下的线圈阻抗和分布电容,以此为基础,对线圈的匹配电容进行了计算,并通过实验验证了计算方法和结果的正确性。计算和实验结果表明,线圈电阻随提离增大而减小,线圈电感随提离增大而增大,线圈分布电容随提离增大而减小,变化规律均符合指数规律。     

连铸方坯温度场数值分析中的几个问题

本文针对连铸方坯的凝固特性,对铸坯在结晶器及二冷区的边界传热模型进行了修正,取得了与实验相符的结果。本文还就计算坯壳厚度的临界条件进行了讨论,并首次提出有效坯壳厚度的概念,讨论了结晶器冷却能力、拉速等对坯壳厚度的影响。     

SH电磁超声换能器优化设计及粘接强度测量

电磁超声换能器(Electromagnetic Acoustic Transducer,EMAT)使用时无需耦合剂，可以便捷地应用于严苛工况下结构的超声检测。但由于EMAT复杂的多物理场换能机理，通常存在换能效率低，接收信号信噪比小等缺点。针对上述问题，文章开展了线圈宽度、线圈间距及磁致伸缩贴片等参数对EMAT换能效率的影响研究，优化设计了SH电磁超声换能器。实验结果表明，一定厚度的磁致伸缩贴片对EMAT的换能效率有较明显的提高。基于SH0导波对界面变化的敏感性，采用优化设计的EMAT激发SH0导波，对固化温度等因素引起的多层铝板弱粘接结构件进行粘接性能的检测，实验测得的粘接强度与结构拉伸强度的变化趋势一致，表明优化后的EMAT可以适用于粘接结构状态的检测。     

热顶电磁成型系统磁场的实验与数值模拟

根据电磁场理论,建立了热顶-电磁连铸成型系统的物理和数学模型,用小线圈法实测了不同电源功率时系统内的磁场强度,用有限元软件数值模拟了感应线圈高度、结晶器高度、热顶结构等对成型系统内磁感应强度和分布的影响,结果表明:1)降低结晶器高度使系统内磁感应强度增强,但不十分明显;2)感应线圈的高度对系统内磁场影响显著,采用20mm线圈较40mm线圈的磁感应强度提高约85%;3)有载时的模拟结果显示,系统内的磁场呈现更明显的趋肤效应;热顶结构对磁场的强度和分布规律无明显的影响.研究结果表明热顶电磁连铸法将有利于提高电源效率和稳定液柱高度,改善铸坯的内外部质量.     

EMAT激发超声兰姆波的模式展开分析

0前言因超声兰姆波的多模和频散特性,使其在激励、接收和模式识别等方面存在较大的复杂性[1]。本文提出一种研究电磁超声换能器(EMAT)激发超声兰姆波的模式展开分析方法。从EMAT曲折线圈的电流分布出发,推导出铝板表面洛伦兹切向应力的形式解;对其进行空间傅立叶变换,得到激发超声兰姆波的表面应力之波数谱密度。在此基础上采用导波的模式展开分析方法,推导出与曲折线圈的     

Numerical simulations of an electromagnetic acoustictransducer-receiver system for NDT applications

The development of a multistage numerical model of an electromagnetic acoustic transducer (EMAT), with particular emphasis on an EMAT receiver, is presented. The model includes five separate modeling states: static magnetic field simulation of an electromagnet; pulsed eddy current distribution of a generic meander-line coil suspended over a conducting specimen; Lorentz force distribution due to the interaction of the static magnetic field with the eddy current distributions; acoustic wave generation and propagation based on the dynamic Lorentz forces; and acoustic wave detection by an EMAT receiver. In particular, it is shown how the transient particle displacement fields are converted into an induced voltage response as part of the EMAT receiver system. Numerical simulations show that the voltage response is dependent on the wire spacing of the receiver coil     

电磁驱动力对金属铸坯凝固的影响

利用低熔点Sn 3.5wt%Pb合金模拟钢的凝固末端电磁搅拌的实验条件,研究了电磁驱动力对铸坯凝固进程的影响。研究表明：在电磁驱动力作用下,半固态区存在一个边界层,边界层表征电磁驱动力作用的极限区域,边界层以内熔体的流速随着搅拌器线圈电流的增大而增大,电磁场对边界层以外的熔体影响不大;电磁搅拌驱动力使固－液界面或边界层向外推移,铸坯凝固坯壳变薄;电磁搅拌力对半固态的金属熔体具有熔蚀、冲刷作用,阻碍柱状晶生长,使凝固壳生长均匀,且内表面光滑无突起。     

Simulation study on horizontal continuous casting process of copper hollow billet under rotating electromagnetic stirring Part 2—effects of electromagnetic and casting parameters on solidification process

The comprehensive three-dimensional mathematical model proposed in Part 1 is used to investigate the effect of rotating electromagnetic stirring on the solidification process of copper hollow billet during horizontal continuous casting. In this part, the model is used to investigate the effects of electromagnetic parameters and casting speed on the electromagnetic field, temperature field, fluid flow and solidification during the horizontal continuous casting with rotating electromagnetic stirring. The results show that electromagnetic frequency, current intensity and casting speed have significant influence on the tangential velocity, temperature gradient, liquid fraction and sump depth.     

The excitation and detection of lamb waves with planar coil electromagnetic acoustic transducers

Planar coil electromagnetic acoustic transducers (EMATs) are investigated for the excitation and detection of Lamb waves in nonferromagnetic metallic wave-guides. Such EMATs are attractive for certain applications due to their omni-directional sensitivity to wave modes with predominantly in-plane surface displacement, such as the So Lamb wave mode. A model is developed that enables the modal content of the radiated Lamb wave field from a transmitting EMAT to be calculated, and the output voltage from a receiving EMAT to be predicted when a Lamb wave mode is incident on it. The predictions from this model are compared with experimental data obtained from 12 different EMATs tested on a 5-mm thick aluminum plate, and good agreement is obtained. The model then is used to analyze the different effects that contribute to the overall Lamb wave modal sensitivity of an EMAT. The relationship between coil geometry and wavelength is examined.     

Simulation and application of dynamic heat transfer model for improvement of continuous casting process

Abstract

Numerical simulation is being increasingly used to improve the existing cooling systems. In order to attain highest quality strand, a two-dimensional dynamic mathematical heat transfer model of billet continuous casting of low carbon steel has been presented. This model can be used to compute the billet temperature distribution and shell thickness, especially it can be used to simulate the solidification process which is caused by frequently variational casting conditions. The fluctuation of measured temperature has been reduced to <10°C with thermal imaging system. The online model can monitor surface temperature and shell thickness in the casting process. So it provides the possibility for the online process control. For the validation of the dynamic model, a lot of billet surface temperature and shell thickness measurement were carried out on an actual casting machine. Finally, the dynamic model has been used for adjusting the operating parameters to improve the casting speed.     

Finite element numerical simulation on thermo-mechanical behavior of steel billet in continuous casting mold

The thermo-mechanical behavior of a thin, growing shell during the early stages of solidification in a continuous casting mold is very important to the ultimate quality of the final billet. A two-dimensional, transient finite element model has been developed to treat the heat flow and deformation of the solidifying shell in the continuous casting billet mold as a coupled phenomena. The major application of the model is to predict the extent of the gap between the mold and the shell and focus on the influence of mold taper on the thermo-mechanical behavior of the steel billet to help to understand the formation of off-corner cracks and break-outs in the solidifying shell. The calculations indicate that the gap is initially formed at the corner of the billet, where heat transfer is greatly reduced. Insufficient mold taper contributes to a hot spot in the off-corner region, which corresponds to the lowest shell thickness. At the same time, the solidifying front on the diagonal of the billet is subjected to an excessive mechanical strain, which causes the off-corner cracks and even the break-outs.     

Evaluation of Fatigue Specimens Using EMATs for Nonlinear Ultrasonic Wave Detection

It is suggested that the waveform of the received ultrasonic wave is slightly different from that of the incident ultrasonic wave when the displacement of an ultrasonic wave and the opening width of a crack are almost same. Thus, as regarding the incident ultrasonic wave, its harmonic frequency components change as it penetrates the crack. Since, the nonlinearity of a solid material is very small compared to that of a liquid medium with a high nonlinear efficiency when used as a coupling medium, we applied an electromagnetic acoustic transducer (EMAT), which does not require a coupling medium. In addition, we tried to develop the EMAT that could alternately drive S₀-Lamb wave and fundamental Shear Horizontal (SH₀)-plate wave to detect any nonlinearity in an ultrasonic wave. We actually tested the performance using the fatigue specimens we fabricated. As a result, we observed that the harmonic components changed when we used the specimen with a specific loading condition and a specific ultrasonic mode. This is indicated that the harmonic components detection using the trial EMAT could also provide useful information on the degree of damage to any structures or any materials.     

Pulse distortion due to eddy currents induced in a multiply excitedcylindrical shell of finite length and thickness

The electromagnetic field due to pulse excitation of two crossed pairs of coils surrounded by a cylindrical conducting shell is determined by accounting for the eddy-current effect. Each coil is modeled as a pair of straight, parallel wires of infinite length; the surrounding conducting shell is a cylindrical tube of finite length and thickness. The problem is formulated in the frequency domain, and the resulting expressions are used to calculate the transient magnetic flux density which originates from the quadruple coil and the eddy currents induced within the metallic shell. The part due to the eddy currents represents an unwanted distortion, which is investigated through a numerical example. The analytical and numerical results are complemented by a brief discussion on compensation techniques and potential applications in magnetic resonance imaging