单向表面波电磁超声换能器优化设计

电磁超声表面波因在板材中的水平方向上传播时具有能量衰减较慢的特性,故广泛应用于板材的表面或近表面缺陷检测。相较于双向表面波,单向表面波能量集中在换能器的一个方向,从而使增强侧的表面波更能够识别微小缺陷。为提高单向表面波换能器对微小缺陷的检测灵敏度和精度,该文提出一种基于Halbach阵列的永磁铁结构,对比优化前后的磁场、涡流、洛伦兹力及位移场分布;研究分析在增强侧表面波遇到不同裂纹缺陷的响应特征,对微小裂纹进行定位分析,并通过仿真分析得出反射系数、透射系数与微小裂纹深度、宽度、倾角之间的变化曲线,实现对微小裂纹缺陷的量化分析。     

电磁声换能器性能测试研究

本文简单地介绍了纵波电磁换能器产生和检测超声的机理，设计与制作考虑。对所研制开发的纵波ＥＭＡＴ的方向性，非接触脱离特性、分辨率和务灵敏度等性能的测试方法进行了系统研究和测试，并给出了其测试结果。     

声表面波换能器激励的有限元仿真

采用有限元法分析了声表面波换能器电极上的激励问题。从声场波动方程、麦克斯韦方程以及压电本构方程出发,利用哈密顿原理,推导了在压电介质中声表面波有限元方程,然后采用Newmark法对有限元方程进行时域变换。分析了换能器电极上的静态电荷分布和动态电荷分布。对压电介质中声表面波振动振幅进行计算并分析了质点振动振幅随深度的变化情况。     

电磁超声单向表面波对铝板微小缺陷的检测

电磁超声表面波被广泛用来检测表面或近表面缺陷。双向表面波电磁超声换能器(ElectromagneticAcoustic Transducer, EMAT)会在两侧同时产生能量较低且均衡的超声波,而微小缺陷(缺陷深度远小于表面波波长)的反射信号非常微弱,易被噪声淹没,根据回波信号,难以识别和定位缺陷。为此基于惠更斯叠加原理设计了单向表面波EMAT,对其声场进行了有限元分析;研究了增强侧表面波遇到不同缺陷的响应特性,得出缺陷深度、角度与反射波幅值的关系;并对含不同微小缺陷的铝板进行了实验研究。仿真和实验结果表明,所提方法提高了表面波检测微小缺陷的灵敏度,并实现了缺陷位置及深度的量化。     

基于有限元法的声表面波换能器电荷分布的静电解

从麦克斯韦方程和声场波动方程出发，在准静态近似的条件下，建立分析声表面波换能器的有限元方程，计算声表面波换能器表面的静电荷分布，并把有限元计算结果和格林函数法结果进行了比较，为声表面波器件的结构优化和设计提供精确的参数。     

设计用于圆环链裂纹检测的表面波EMAT

电磁声传感器是一种非接触式的电磁能与声能的换能装置,象运动圆环链这类带有锈蚀的粗糙表面上,常规的压电晶体传感器无法使用的场合,电磁声传感器充分显示出了它的优越性。本文介绍了所设计研制的表面波电磁声传感器的结构、工作原理以及部分试验结果分析。     

电磁泵声学特性研究

对单通道CLIN-3/28和流向旋转180°的CLIN-4/32的流体金属电磁液压系统,在低传递和轻微受阻 传递条件下,对振动和噪声及其产生低噪声的可能性进行了研究。声学特征的初步研究结果表明:这种冷却系统 的电磁泵的振动级和声级都相当低。     

声表面波温度传感器抗干扰技术研究

目的设计一种声表面波(SAW)温度传感器抗干扰技术,以提高温度测量的稳定性。方法分析SAW谐振器(SAWR)回波特性,建立SAWR回波信号熵能量模型,发现SAWR回波信号的衰减过程与熵能量的上升过程对应。当回波信号达到噪声水平时,熵能量的单调上升过程消失。为了抑制正弦干扰设计一种改进型自相关算法,利用该算法对信号进行去噪的同时使谐振器回波信号的衰减特性和正弦干扰的等幅特性得到保持。结果根据模拟仿真结果设置了SAWR回波信号的检测阈值(V_(thre)=1),并对该阈值进行了蒙特卡罗仿真实验。仿真结果表明,当信号信噪比大于4dB时,SAWR回波信号的检测率达到86%,而正弦干扰误检率小于0.5%。最后应用该算法对实际的正弦信号和SAWR回波信号进行了检测,得到的误检率接近于0。结论实验结果显示,所设计的算法可以用作声表面波温度传感器的抗干扰技术。     

声表面波传感器结构模型与敏感特性研究

本文主要从声表面波传感器基础理论出发,首先分析了声表面波技术的发展,对声表面波传感器结构模型和工作模式特性做了必要的分析和研究,进而展开了对声表面波传感器敏感特性研究。并对仿真效果进行了扼要的总结,对声表面波技术在相关感领域的应用,本文也展开了一定的探讨。     

超声换能器的宽带阻抗匹配器研究

在功率超声领域,换能器的阻抗匹配占有相当重要的地位,但有关在较宽频带内都能适用的匹配方案尚少有报道。为满足实验室研究工作需要,我们研制了两套简易匹配装置,可分别对２０～５０ｋＨｚ和０．２～２．０ＭＨｚ频率范围内的超声换能器实现阻抗匹配。     

Fatigue load monitoring in steel bridges with Rayleigh Waves

Fatigue load monitoring is a useful tool for safety assessment of highway bridges. Monitoring has been conventionally done using strain gages. Installation of these gages is labor-intensive and requires safety precautions. Noncontact electromagnetic-acoustic transducers (EMATs) offer an attractive alternative. EMATs were used to transmit and receive Rayleigh Waves (RW). Changes in time of flight of RW due to the acoustoelastic effect can in principle be used to monitor stresses resulting from vehicular traffic. We have performed proof-of-concept experiments to demonstrate the feasibility of this approach. Specimens were subjected to bending to simulate the load environment in bridges. RW EMATs were used to measure the relatively low stresses (less than 14 MPa) typically experienced by bridge girders. The signal-to-noise ratio achievable with our system should allow adequate stress resolution for fatigue load monitoring. Factors which could impede technology transfer were considered. The primary obstacle appears to be variability in time of flight (TOF) due to magnetostriction. If the magnetic state is changed (e.g., by scanning of the EMATs) the TOF can change, even at constant stress. We have characterized this effect. If a proper installation procedure is followed, fatigue load monitoring with RW EMATs is feasible.Contribution of the National Institute of Standards and Technology.     

Evaluation of pipeline girth welds using low-frequency horizontally polarized waves

The practical implementation of alternative acceptance criteria for pipeline girth welds requires the use of inspection tools capable of determining the principal dimensions and positions of planar flaws. A new ultrasonic inspection method is described that permits complete volumetric inspection of the girth welds. The new system uses noncontacting electromagnetic-acoustic transducers (EMATs) that operate at low ultrasonic frequencies (454 kHz). Theoretical models of the measurements are developed and verified experimentally. In addition, practical performance limits of the new system are established in terms of minimum flaw sizes that can be detected. The results are related to accept-reject curves based on a model of the failure processes. An inspection protocol for field applications is also described.     

基于粉碎区控制的钻孔爆破岩石-炸药匹配方法

合理的炸药-岩石匹配关系对于提高炸药能量的有效利用率和改善破碎效果有着重要意义。传统上强调的波阻抗匹配理论并不合理,文章从爆破破碎机理出发,提出了一种岩石-炸药匹配的新方法,在保证相邻炮孔间岩石充分破碎的前提下,通过对粉碎区的合理控制来确定钻孔爆破最优的炸药性能参数。该方法可以直观地反映爆破破碎效果及能量有效利用率,可操作性强。考虑相邻炮孔爆炸荷载的联合作用,修正了钻孔爆破破坏分区计算模型。在此基础上,针对具体工程目标给出了混装炸药耦合装药条件下不同等级岩石的炸药性能匹配参数。     

Cyclotron Resonance of Composite Fermions

The introduction of suitable fictitious entities occasionally permits to cast otherwise difficult strongly interacting many-body systems in a single particle form. We can then take the customary physical approach, using concepts and representations which formerly could only be applied to systems with weak interactions, and yet still capture the essential physics. A most notable recent example occurs in the conduction properties of a two-dimensional electron system (2DES), when exposed to a strong perpendicular magnetic field B. They are governed by electron–electron interactions, that bring about the fractional quantum hall effect (FQHE). S. Das Sarma and A. Pinczuk (eds.), Perspectives on Quantum Hall Effects (Wiley, New York, 1996). Composite fermions, that do not experience the external magnetic field but a drastically reduced effective magnetic field B*, were identified as apposite quasi-particles that simplify our understanding of the FQHE. J. K. Jain, Phys. Today, 39–45 (2000). J. K. Jain, Phys. Rev. Lett. 63, 199–202 (1989). They precess, like electrons, along circular cyclotron orbits, with a diameter determined by B* rather than B. B. I. Halperin, P. A. Lee, and N. Read, Phys. Rev. B 47, 7312–7343 (1993). O. Heinonen, (ed.), Composite Fermions (World Scientific, Singapore, 1998). R. R. Du, H. L. Stormer, D. C. Tsui, L. N. Pfeiffer, and K. W. West, Phys. Rev. Lett. 70, 2944–2947 (1993). R. R. Du et al., Phys. Rev. Lett. 75, 3926–3929 (1995). R. L. Willett, R. R. Ruel, K. W. West, and L. N. Pfeiffer, Phys. Rev. Lett. 71, 3846–3849 (1993). V. J. Goldman, B. Su, and J. K. Jain, Phys. Rev. Lett. 72, 2065–2068 (1994). J. H. Smet, D. Weiss, R. H. Blick, G. Lütjering, and K. von Klitzing, Phys. Rev. Lett. 77, 2272–2275 (1996). The frequency of their cyclotron motion remained hitherto enigmatic, since the effective mass is no longer related to the band mass of the original electrons and is entirely generated from electron–electron interactions. Here, we demonstrate the enhanced absorption of a microwave field that resonates with the frequency of their circular motion. From this cyclotron resonance, we derive a composite fermion effective mass that varies from 0.7 to 1.2 times the electron mass in vacuum as their density is tuned from 0.6× 10¹¹/cm² to 1.2× 10¹¹/cm².     

声波测井压电换能器多频点阻抗匹配技术研究

针对声波测井压电换能器的多频点阻抗匹配技术展开研究,首先采用多模态等效电路精确描述了换能器的导纳特性;然后通过分析多模态阻抗匹配理论,设计电感-电容复合阻抗匹配网络,并结合换能器等效电路进行参数优化和电路仿真。实验表明,相比于换能器没有阻抗匹配的测试结果,该阻抗匹配技术可大幅提高换能器在谐振频率附近多个频率处的有功功率,频带内的有功功率平均提高了30倍,从而改善换能器的激励带宽和激励效率,提高测井仪器的适应性、探测深度和分辨率。     

“水鸣天梯”声景观成因分析

基于周期台阶表面声阻抗理论,解释了\"水鸣天梯\"声景观的物理机制。首先在低频准平面波近似的情况下,利用阻抗转移公式得到了周期台阶单元表面声阻抗。数值解表明,在低频段台阶表面的声阻抗呈现纯容抗性,证实了表面波的存在。有限元软件仿真的结果与理论分析保持一致。应用表面波的性质,设计并进行了现场试验。试验数据分析表明:在台阶底部脚踏石阶或者发射脉冲声波,声波经过台阶表面作用,到达台阶顶部时只有低频部分的声能量得到了较大保留,与理论分析和模拟结果相一致,最终确定了正是周期台阶的表面波滤波效应使得脚踏声转变为悦耳的水滴声。     

多元助剂改性羰基铁粉雷达波低频吸波性能研究

采用三种处理剂对羰基铁粉样品进行表面复合改性,研究了多元助剂对羰基铁粉样品表面改性后的微观结构及电磁参量的影响。结果表明,多元助剂的使用使羰基铁粉表面形成了一层致密的有机绝缘薄膜,能有效降低羰基铁粉的复介电常数,增加复磁导率虚部,提高吸波材料的电磁匹配性能,改善吸收剂的低频吸收效果。根据传输线理论计算吸波材料的反射损耗(Reflection loss,RL),在厚度为2mm时,三元助剂改性羰基铁粉的反射损耗峰值在2GHz附近达到-15dB,在RL-10dB的有效吸收频宽为1GHz(1.6~2.6GHz),具有较好的雷达波低频吸波性能。     

天然纤维增强复合材料吸声性能研究

采用热压成型法制备天然纤维增强复合材料层合板和蜂窝夹芯结构,利用双传声器阻抗管进行吸声性能测试,并与合成纤维增强复合材料层合板和蜂窝夹芯结构进行对比。结果表明:与合成纤维增强复合材料层合板相比,天然纤维增强复合材料层合板虽然具有更优异的吸声性能,但是仍不能满足吸声材料的要求,需通过材料设计进一步提高这种材料的吸声性能。而天然纤维增强蜂窝夹芯结构具有优异的吸声性能,吸声系数峰值高达0.4,可以被用作吸声材料。