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研究了超声Lamb波混频的激励效率与模式选择。在非零能量流和相匹配条件的基础上,结合Lamb波在结构中的状态分布,提出了以激励效率参量作为超声Lamb波混频模式选择的依据。根据混频条件筛选混频模式对,计算相应的激励效率参量,并通过超声Lamb波混频的仿真模拟与实验测量进行验证。通过对比仿真与实验中得到的非线性参量与激励效率参量的关系,证明了激励效率参量作为模式选择依据的有效性。研究表明,激励效率参量可以有效地表征超声Lamb波混频的激发效率,从而筛选非线性效应明显的模式对,更好地实现损伤的检测与表征。 相似文献
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本文分析了小信号情况下飞秒脉冲的倍频特性,在非耗尽近似下,得出了飞秒脉冲二次谐波波形及效率的解析解,在时间域内,对一般条件的耦合波方程进行了数值解,分析了相位失配对飞秒脉冲倍频波形及效率的影响。 用0.5mm厚的一类匹配LBO晶体对碰撞锁模激光器产生的80fs超短脉冲进行了腔外倍频实验,实验结果与理论计算符合得很好。 相似文献
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本文采用固定入射角,改变入射光偏振方位角的方法从实验上了研究了锗薄膜的透射光学二次谐波产生及其变化规律,并从理论上推导了产生透射二次谐波的表达式,最后拟合求得描述面谐波电流的唯象参数a。 相似文献
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研究了二次谐波是负折射率而基频光是正折射率材料中,在考虑吸收系数、走离效应和相位失配的情况下,推导出平面波振幅缓变近似下,二次谐波的耦合波方程.根据该方程在基频光小信号近似下推导出二次谐波转换效率表达式.最后采用数值计算的方法分别研究了相位失配量Δk、走离角ρ和二次谐波的吸收系数α对转换效率的影响.结果表明,由于相位失配量的存在二次谐波的转换效率随负折射率材料长度的变化存在极大值,即负折射率材料长度存在最佳值,并且随着长度的增加,转换效率呈明显的周期性振荡并且极大
关键词:
负折射率材料
二次谐波
转换效率
相位匹配 相似文献
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以Nd:MgO:LiNbO_3同时兼作激活介质和非线性光学材料,研究自倍频激光器。用小型氙灯泵浦,在室温下获得二次谐波激光(0.547μm) 阈值小于4.8'J,最大输出400μJ/shot,工作温度范围大于20℃~45℃,无光损伤。 相似文献
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建立了由损耗段和铜段组成的波导结构的Ka波段二次谐波回旋行波放大器理论模型,并进行了非线性理论研究。基于稳定性分析,确定35 GHz TE02模二次谐波回旋行波放大器的基本工作参数:波导半径为1.02 cm,电子注工作电压为90 kV,工作电流为25 A,工作磁场为0.642 6 T,横纵速度比为1.2;然后通过模拟详细分析了工作电流、波导损耗和速度零散等因素对该放大器性能的影响。研究表明:在该结构中损耗段可以有效地抑制模式竞争从而提高输出功率和带宽;工作电流对输出功率的影响存在最大值;速度零散对输出功率有很大的影响。 相似文献
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Transmission analysis of ultrasonic Lamb mode conversion in a plate with partial-thickness notch 总被引:1,自引:0,他引:1
Mode conversions of Lamb waves can occur upon encountering damage or defect such as a notch, leading to newly-converted modes apart from wave reflection and transmission. In this paper, the transmission of the fundamental Lamb modes symmetrical S0 and anti-symmetrical A0 with anti-symmetrical notches were investigated in steel plates within the relatively short propagation distance. The group velocity and modal energy of the converted modes were analyzed using simulations and experiments. Two-dimensional finite difference time domain (2D-FDTD) method was employed to calculate the scattering field and extract numerical trends for simulation study and experimental confirmation. Both simulations and experiments revealed that the apparent group velocities of the converted modes in the transmitted signals subject to the notch positions. To describe the mode conversion degree and evaluate the notch severity, wave packets of the originally-transmitted modes and newly-converted modes were separated and corresponding mode energy percentages were analyzed at different notch severities. Frequency-sweeping measurements illustrated that the modal energy percentages varied monotonically over the notch-depth increase with a statistically consistency (R = 1.00, P < 0.0004). 相似文献
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When a bounded beam is incident on an immersed plate Lamb waves or Rayleigh waves can be generated. Because the amplitude of a bounded beam is not constant along its wave front, a specific beam profile is formed that influences the local efficiency of energy conversion of incident sound into Lamb waves or Rayleigh waves. Understanding this phenomenon is important for ultrasonic immersion experiments of objects because the quality of such experiments highly depends on the amount of energy transmitted into the object. This paper shows by means of experiments based on monochromatic Schlieren photography that the area within the bounded beam responsible for Lamb wave generation differs from that responsible for Rayleigh wave generation. Furthermore it provides experimental verification of an earlier numerical study concerning Rayleigh wave generation. 相似文献
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超声兰姆(Lamb)波在结构缺陷检测方面愈来愈受到重视,但目前Lamb波的应用局限于单信号源激励,单通道接收的方法,容易受到噪声干扰,其后续的信号分析处理也比较复杂。本文旨在采用多通道Lamb波对板状结构中的裂纹进行定量分析与诊断。在铝板的表面凿刻出不同深度的凹槽作为裂纹,通过线阵换能器采集在一定传播距离内的多通道Lamb波信号,并采用二维傅里叶变换分析在不同深度的裂纹下,Lamb波模式能量的变化规律。结果表明,相对于完好铝板中的Lamb波信号,裂纹的存在会使Lamb波发生模式转换现象,并且转换模式能量百分比随裂纹深度的增加而线性增加。其结果为Lamb波评价板状结构中的裂纹状况提供了一种可能的方法。 相似文献
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Piezoelectric transducer parameter selection for exciting a single mode from multiple modes of Lamb waves 下载免费PDF全文
Excitation and propagation of Lamb waves by using rectangular and circular piezoelectric transducers surface-bonded to an isotropic plate are investigated in this work. Analytical stain wave solutions are derived for the two transducer shapes, giving the responses of these transducers in Lamb wave fields. The analytical study is supported by a numerical simulation using the finite element method. Symmetric and antisymmetric components in the wave propagation responses are inspected in detail with respect to test parameters such as the transducer geometry, the length and the excitation frequency. By placing only one piezoelectric transducer on the top or the bottom surface of the plate and weakening the strength of one mode while enhancing the strength of the other modes to find the centre frequency, with which the peak wave amplitude ratio between the S0 and A0 modes is maximum, a single mode excitation from the multiple modes of the Lamb waves can be achieved approximately. Experimental data are presented to show the validity of the analyses. The results are used to optimize the Lamb wave detection system. 相似文献
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In this paper, we describe a modal expansion approach for the analysis of the selective generation of ultrasonic Lamb waves by electromagnetic acoustic transducers(EMATs). With the modal expansion approach for waveguide excitation, an analytical expression of the Lamb wave's mode expansion coefficient is deduced, which is related to the driving frequency and the geometrical parameters of the EMAT's meander coil, and lays a theoretical foundation for exactly analyzing the selective generation of Lamb waves with EMATs. The influences of the driving frequency on the mode expansion coefficient of ultrasonic Lamb waves are analyzed when the EMAT's geometrical parameters are given. The numerical simulations and experimental examinations show that the ultrasonic Lamb wave modes can be effectively regulated(strengthened or restrained) by choosing an appropriate driving frequency of EMAT, with the geometrical parameters given. This result provides a theoretical and experimental basis for selectively generating a single and pure Lamb wave mode with EMATs. 相似文献
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考虑了压电传感器(PZT)与板之间的耦合作用,从理论上研究了单个压电传感器激发时产生单模式兰姆波的频率调节方法,实验给出了模式选择在兰姆波结构健康监测中的应用. 在板材、板厚以及PZT尺寸一定的情况下,从理论上能够预测到作为频率函数的各兰姆波模式的幅值变化. 根据某特定兰姆波模式的幅值最大而其他模式幅值相对最小时所对应的频率,即可识别出该兰姆波模式优化的激发频率. 数值仿真验证了理论的有效性以及单模式兰姆波选择的可能性. 在不同的激发频率下,分别激发了优化的A0 模式,优化的S0模式以及共存的A0和S0模
关键词:
兰姆波
压电传感器
激发频率
结构健康监测 相似文献
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Liquid level sensor using ultrasonic Lamb waves 总被引:5,自引:0,他引:5
This paper describes a novel, noninvasive method for measurement of liquid level in closed metal tanks that are under high pressure. It is based on the use of ultrasonic Lamb waves propagating along the tank wall. Contact with liquid substantially changes the characteristics of these waves and this can be used as an indicator of liquid presence. Theoretical analysis shows that the symmetric and antisymmetric Lamb wave modes, both fundamental and higher order, are sensitive to presence of the liquid. The optimal wave frequency depends on the thickness of the tank wall and wall material. A prototype level sensor based on this principle has been developed. It uses two pairs of wedge transducers to generate and detect Lamb waves propagating along the circumference of the gas tank. An operating frequency of 100 kHz is found to be optimal for use with tanks having a wall thickness of 30-50 mm. Prototype sensors developed under this program have been used successfully in oil fields in the far northern region of Russia. 相似文献
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该文运用解析的方式推导了考虑声波衰减时兰姆波二次谐波的累积和传播规律,并用半解析方式将该理论推广到缓慢变厚度板的情况。由于色散特性,兰姆波二次谐波和基频波相速度不匹配,传播通常会产生拍频效应,使得二次谐波的振幅沿着传播距离周期性的归零。当考虑声波衰减或板的厚度缓慢变化的情况时,拍频效应将不再严格地被满足。二次谐波的振幅依然会沿着传播距离而振荡,但不会归零。该研究可以用于分析如何高效地激发和接收兰姆波的二次谐波,表征和评估不同厚度变化的结构中的微观结构损伤。 相似文献
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利用兰姆波的扩散场信号,实现了距离传感器较近缺陷的全聚焦成像.通过两传感器接收的扩散场全矩阵信号进行互相关,恢复出两传感器之间的格林函数响应,重建新的全矩阵.该重建全矩阵削弱了直接耦合采集响应信号中存在的早期饱和非线性效应信号,恢复了被遮盖的近距离缺陷散射信号.在含缺陷的各向同性铝板中激发兰姆波,重建信号的早期信息与直接俘获信号的后期信息相结合形成混合全矩阵,结合全聚焦成像,优化成像效果.所提方法为薄板类结构中距离传感器较近缺陷的兰姆波无损检测提供了理论指导. 相似文献