Internal defect detection using laser-generated longitudinal waves in ablation regime

Laser ultrasonics could be an attractive solution for the nondestructive testing of structures in harsh environments. Longitudinal waves generated in the ablation regime are especially well suited to internal defect detection because they provide a higher signal-to-noise ratio in comparison to ultrasonic waves generated under a thermo-elastic regime, while their propagation direction normal to the surface enables the simplified interpretation processing of received signals when the defect echoes are analyzed. The internal defect detection using laser-generated longitudinal waves in the ablation regime was investigated numerically and experimentally, and a numerical model to simulate the generation and propagation of ultrasonic waves in the ablation regime was developed. This model was based on the simulation of ultrasonic generation and propagation caused by the net reaction force that was directly converted from the laser intensity absorbed onto the surface. This model was also extended as a model for internal defect detection. A steel specimen containing artificial internal defects was fabricated and inspected by using the through-transmission (T-scan) and pulse-echo (B-scan) modes. Clear amplitude reduction was observed in the transmitted waves at the defect positions in T-scan images, while B-scan images clearly showed the defect echoes arriving at different times depending on the depth location of internal defects. These results demonstrate that longitudinal waves excited in the ablation regime can be effectively used for internal defect detection.     

Lamb wave propagation modeling for structure health monitoring

This study aims to model the propagation of Lamb waves used in structure health monitoring. A number of different numerical computational techniques have been developed for wave propagation studies. The local interaction simulation approach, used for modeling sharp interfaces and discontinuities in complex media (LISA/SIM theory), has been effectively applied to numerical simulations of elastic wave interaction. This modeling is based on the local interaction simulation approach theory and is finally accomplished through the finite elements software Ansys11. In this paper, the Lamb waves propagating characteristics and the LISA/SIM theory are introduced. The finite difference equations describing wave propagation used in the LISA/SIM theory are obtained. Then, an anisotropic metallic plate model is modeled and a simulating Lamb waves signal is loaded on. Finally, the Lamb waves propagation modeling is implemented.     

基于PWAS的层合板导波传播特性和损伤响应FEM分析及试验验证

导波无损检测（Non-Destructive Testing, NDT）技术由于适用范围广、检测速度快等优势,已成为结构安全性检测的重要技术手段之一。不过,导波在各向异性介质中的传播表现为频散特性与波传播方向的关联性、多模态特性与各向异性的相互作用以及不同模态与损伤的相互作用。导波在复合材料无损检测应用中的复杂性对检测技术和检测方法提出了更高的要求,所以对碳纤维复合材料层合板中的导波传播特性和损伤响应进行分析具有十分重要的意义。文中采用理论分析、仿真和试验相结合的方法,对不同碳纤维增强聚合物基（Carbon Fiber Reinforced Polymer, CFRP）层合板中的导波传播特性和各模态波与损伤的相互作用进行了分析。首先根据群速度、波长等参数建立有限元模型,并在边界设置阻尼递增吸收区域来弱化边界反射波的干扰;然后分析基于压电晶片有源传感器（Piezoelectric Wafer active Sensors, PWAS）的不同层合板中导波传播差异,之后在结构中引入损伤来分析不同模态波与不同损伤的相互作用;最后通过试验分析和验证有限元仿真的准确性。     

基于谱元法的板结构中导波传播机理与损伤识别

建立板结构的三维谱元法模型和结构-压电晶片耦合模型,对铝合金板中Lamb波的传播机理和与损伤作用规律进行研究。以单点激励力模型研究了板中Lamb波的频散关系,从理论上对所建立的模型进行了验证;基于压电晶片模型研究了在施加电压激励条件下导波的传播规律;研究单一压电晶片、板中上下表面施加同相位或反相位激励电压的两个压电晶片三种情况下结构的响应;对板中传播Lamb波的幅值与传播距离的关系进行分析;采用减小结构单元刚度的方法模拟结构中的裂纹损伤,研究Lamb波与损伤的作用;结合试验分析,验证所仿真结果结论的正确性。相对二维谱元法模型,所建立的板结构三维谱元法模型能更真实地模拟结构中导波的激励与接收以及波的传播。     

基于电磁超声换能器阵列的管道螺旋导波传播规律研究

螺旋导波因在管道超声导波层析成像中的巨大应用价值,近年来受到研究者们的重视。阐述管道螺旋导波的激发/接收条件、传播路径和波前形状等规律。建立FE模型,研究由圆环波前S₀模态兰姆波在管道上形成螺旋导波的过程。组建了双环24阵元的电磁超声换能器阵列及试验系统,170 kHz下激发圆环波前S₀模态兰姆波在管道中产生螺旋导波,试验研究了激励源所在圆周及管段上的波动场信号特征。仿真和试验结果表明,管道螺旋导波实质上是兰姆波在曲面上的传播形态,可由管道某处点源激发兰姆波产生,主要存在于波动场的近场。由于管道结构的封闭性,兰姆波的波前在管道上反复交叉前行,形成了螺旋传播路径。从波源到管道上任意一点的螺旋导波传播路径有无数条,各阶螺旋角不连续。利用螺旋导波进行管段检测提供了缺陷的多角度入射信息,对缺陷高分辨率检测具有重要意义。     

Theoretical aspects of applying love and SH-waves to nondestructive testing of stratified media

Research into applications of acoustic SH-waves and Love waves to nondestructive testing of stratified media is reviewed. A mathematical model has been developed for describing the propagation of waves with horizontal polarization (SH-waves and Love waves) in anisotropic (monoclinic) stratified media. A modified transfer-matrix method (TM-method) has been developed in order to obtain the solution. Noncanonical-type waves with horizontal transverse polarization have been studied. The dispersion curves have been obtained for a multilayer composite that is in contact with an anisotropic semi-infinite space. It has been shown that the physical characteristics and geometry of any internal layer can be determined based on changes in the dispersion curves.     

超声疲劳裂纹扩展与摩擦生热研究

在高频循环载荷作用下,材料疲劳裂纹的萌生与扩展过程伴随着明显的温度变化,该温度变化反映材料内部结构的损伤特征。通过20 kHz的超声疲劳试验,研究一种碳锰钢在超高周疲劳加载条件下的内部疲劳裂纹萌生与扩展过程中温度的演化过程。通过对该材料在疲劳损伤过程中,内部裂纹间的摩擦生热机理分析,从微观角度出发,结合分形理论,建立内部裂纹微观结构的摩擦模型,数值模拟超声疲劳过程中材料内部疲劳裂纹面间的摩擦生热情况,并定量地计算该过程中由裂纹间摩擦所导致的温度上升,将模拟结果与试验结果进行比较。探究高频疲劳载荷下微裂纹扩展与摩擦生热的关系,并结合超高周疲劳裂纹扩展公式,建立超声疲劳过程中的裂纹扩展与裂纹面温度演化关系的模型。     

脉冲爆震火箭发动机数值模拟研究

脉冲爆震火箭发动机是一种利用脉冲式爆震波产生高温、高压燃气发出的冲量来产生推力的新型推进系统。与常规液体火箭发动机相比,脉冲爆震火箭发动机具有更高的性能,并且结构更简单。本文应用特征线法给出一维爆震波在爆震管内的传播过程的解析解。对爆震波到达爆震管口后发动机的非定常排气流动过程进行了二维数值模拟,并对比了无喷管和带3种不同形式喷管(收敛喷管、收敛扩张喷管和扩张喷管)对发动机推力等性能参数的影响。     

磁致伸缩纵向导波管道检测数值建模与分析

检测模型是磁致伸缩纵向导波检测技术发展的基石。现有磁致伸缩纵向导波检测模型存在不包含接收过程和不包含静态磁场计算等不足,难以满足设计优化接收传感器、增强导波检测信号等方面的研究需求。因此,以磁致伸缩纵向导波管道检测为背景,分析激励和接收过程中的控制方程,在此基础上建立了包括激励过程、声场传播和接收过程的有限元模型。分别采用仿真和实验分析了接收线圈的提离效应,仿真和实验结果吻合较好,验证了该数值模型的正确性。     

针对不连续带洞结构的声发射源定位方法研究

由于兰姆波在舷窗等带孔结构表面的传播路径不再是线性路径,传统方法难以解决撞击的声源定位问题。 本文针对该 问题,提出了一种基于声发射的网格映射概率成像定位方法。 该方法通过建立网格映射模型解决了信号传播的非线性路径难 题,并通过对网格节点的信号最快传播路径计算得到相应的理论时间差值。 之后利用自适应能量阈值方法与 AIC 方法计算信 号的实际到达时间差,通过比较理论值与实际值的偏差构建概率图像以实现定位解算。 通过有限元模拟分析了带洞结构对兰 姆波传播路径的影响,给出信号传播路径的确定依据。 通过对 32 个撞击点的定位实验验证了定位方法的可行性,平均绝对误 差为 1. 15 cm。     

利用Monte Carlo模拟技术研究OCT图像对比度

建立了光在生物组织中传播的模型,利用MonteCarlo模拟技术快速计算和可移植性的优点,研究了OCT图像对比度与显微物镜数值孔径、焦距深度、时间门参数的关系,并给出它们的关系曲线.通过在所建立的模型基础上加入透镜透过率函数和光学传递函数,弥补了以往程序只能模拟光在生物组织中传播行为的缺点.该模型不仅可以分析生物组织图像与生物光学特性的关系,而且还可以指导OCT结构的完善和创新.模拟结果表明:在构建OCT时,参考臂与样品臂的这两个显微物镜的数值孔径越大,生物组织的采样深度越浅,处理信号的时间门宽度越小(但时间门宽度不能小于激光脉冲时间),混浊生物组织图像对比度越好.     

Improved Three-Dimensional Image Reconstruction Technique for Multi-Component Ground Penetrating Radar Data

For electromagnetic imaging the vectorial character of the emitted field and the radiation characteristics of both source and receiver play an important role. Recently, a new imaging algorithm was presented dedicated to the electromagnetic case. The radiation characteristics of GPR source and receiver antennas and the vectorial nature of the electromagnetic waves are taken into account for a monostatic fixed offset GPR survey. This resulted in a representative image of a point scatterer. Comparison with scalar imaging algorithms shows that for a homogeneous space the SAR image has an opposite sign compared to the multi-component image, whereas the Gazdag image has a phase shift of about 90° with respect to the multi-component image. In this paper, modified scalar imaging algorithms are introduced that minimize these differences. However, between the images obtained in a homogeneous half-space phase differences of 10–20° are still present. These differences indicate the possible error in nature of the physical property contrast when it is determined with the modified scalar imaging algorithms. The multi-component imaging algorithm returns a representative image because it uses the appropriate Greens functions to eliminate the propagation effects. For practical imaging strategies, only far-field radiation characteristics can be used to compensate for the propagation effects due to the large computing time needed to evaluate the total-field expressions. Synthetic analysis of the imaging of a point scatterer calculated using total-field expressions shows that using the far-field expressions in the multi-component imaging algorithm the images better approximate the actual contrast than using the modified scalar imaging algorithms. Experimental results are presented from imaging several buried objects with different medium properties and different orientations. The phase differences in the experimental data are similar to those obtained synthetically. This likeness indicates that using the multi-component imaging algorithm, a more reliable image is obtained than with the modified scalar imaging algorithms.     

Ultrasonic based structural damage detection using combined finite element and model Lamb wave propagation parameters in composite materials

This paper presents a combined finite element and model Lamb waves propagation parameters method as a tool for structural health monitoring in composite materials. Modal analysis allows identifying the mode conversions induced by the defects. A simulation combining a lossless finite element approach and Lamb wave propagation parameter for finding natural frequencies and mode shapes of the structures in undamaged and damaged condition is proposed. This analysis is performed on two carbon-fiber-reinforced plastic bars in both undamaged and damaged state, where the two damaged states are (1) having a cut partway through the bar, perpendicular to the long axis of the bar and (2) having a circular hole. The lamb wave propagation parameters are calibrated using the ultrasonic pulse generator test setup. The natural frequencies for the theoretical, finite element and experimental results are compared and close agreement is found between the frequencies obtained experimentally and computationally.     

基于递归量化观测的复合材料损伤贝叶斯估计方法

针对复合材料损伤定位定量识别中损伤指数构建困难与损伤预测的不确定性问题,提出一种基于递归量化特征观测的 损伤状态贝叶斯概率估计方法。 该方法无需基准无损信号参考与损伤波包识别,采用 Lamb 波信号的递归量化分析(RQA)特 征,从结构非线性动力学角度对复材板内部损伤进行观测,根据特征与损伤位置大小的相关性与单调性进行了敏感特征筛选。 根据各传感路径信号的 RQA 特征与损伤形态参量间的线性/ 非线性关联特性,采用多变量二次交互模型建立了损伤状态的特 征描述空间。 综合考虑各传感路径中 RQA 特征对损伤观测的不确定因素,建立了复材板损伤状态的观测模型,采用贝叶斯更 新算法对复材板损伤位置尺寸进行估计。 复材板的脱层损伤仿真分析结果表明,本文提出方法无需分析结构损伤与 Lamb 波 的复杂交互机制,就能同步实现复材板内部损伤的量化评估与定位分析,估计出的损伤位置大小在仿真模型的损伤参数设定值 的 75% 置信区间内,损伤估计成像区域与仿真损伤区域相符合。 复合材料板的疲劳损伤试验分析结果表明,采用本文方法经过 24 次迭代更新后,得到的损伤面积与大小完全覆盖了 X 射线扫描检测的真实损伤。 该方法充分考虑了复材板损伤评估中的不 确定因素,更加贴合复合材料结构损伤检测与评估的工程实际应用,具有较好的应用前景。     

厚梁结构中的超声导波传播与损伤识别

基于导波的结构健康监测技术研究中,结构厚度与不同损伤形式对导波传播特性的影响是该技术在工程应用中对实际结构进行损伤识别的关键。通过导波在厚梁结构中传播时所表现出的特性,分析并试验研究损伤对导波传播的影响。通过在结构中引入切槽损伤,理论分析与试验研究导波在有损结构中的传播特性。以此为基础,重点研究疲劳裂纹损伤。试验得出导波在厚梁结构中的实际传播速度,并由此分析损伤反射波中的波包成分,研究并总结损伤大小对导波幅值、相位和到达时间的影响规律。分别对结构中的对称和非对称损伤进行研究,验证具有非对称损伤结构中模式转换波包的存在,分析模式转换波包的形成与传播机理,研究不同的非对称损伤对模式转换波包形式的影响。此外,还介绍了试验试件和疲劳裂纹的加工过程。总结了切槽损伤与疲劳裂纹损伤对导波传播的不同影响。     

面向SAR匹配辅助导航的景象区域适配性分析

对SAR景象匹配辅助组合导航的关键技术之一——SAR景象区域适配性进行分析。首先,结合SAR成像过程、组合导航过程及SAR图像的特点对SAR景象区域适配性问题进行综合描述;然后,围绕减小几何畸变和提高匹配概率两个目标剖析SAR景象区域适配性的影响要素,其中包括基本约束条件、适配性特征指标和预测函数构建等。最后在此基础上,以限制几何畸变,优化图像特征为原则构建了基于多目标遗传优化的SAR景象适配区选择模型。基于真实SAR图像进行的实验中根据模型选出最佳适配区匹配概率达到94%,从而验证了分析和建模的合理性。该结论可为选择高性能的SAR景象适配区提供参考。     

Three-dimensional laser interferometric CT (LICT) measurement of shock wave interaction around a circular cylinder

Three-dimensional (3-D) laser interferometric computed tomography (LICT) measurement has been applied to high-speed and unsteady flow including shock waves. The purpose of our investigation is to clarify 3-D flow phenomena quantitatively by using LICT. We used a diaphragm-less shock tube to clarify the unsteady shock wave behavior with the shot by shot method. The diaphragm-less shock tube enables us to obtain shock wave propagation reproduction in quick succession. By LICT the 3-D internal density structure of the shock–vortex flow field can be clarified. In this LICT method the observation system consists of a CCD camera, Mach–Zehnder interferometer, and nitrogen pulse laser as a light source with a suitable delay signal controller. The 3-D density CT data can be reconstructed typically from the 19 2-D density images which have a 5° interval among every shot of the diaphragm-less shock tube. As for the method to get many projection images, the optical system is fixed, while the model with a cylindrical duct and a cylinder is rotated by a fixed degree of intervals in the designated observation angle for every shot. In this paper the shock wave interaction with a finite length circular cylinder is observed by LICT measurement. For reconstruction of density distribution the Algebraic Reconstruction Technique (ART) is applied to reduce the artifact, together with precise observation.     

氢气-空气预混火焰传播特性的数值模拟研究

应用Fluent流体计算软件,基于标准湍流κ-ε模型和EDC燃烧模型,采用SIMPLE格式算法对常温常压下对常温常压下氢气／空气预混火焰在光滑管道中的传播特性进行二维数值模拟,获得火焰传播速度、火焰结构、表面积、火焰到达位置和时刻沿管道变化情况。结果表明：火焰传播速度先增加后逐渐减小。在离点火端105mm处到达最大值,之后逐渐降低。在t=9．1ms时,火焰开始逐渐呈现典型的郁金香结构。火焰在初始加速阶段的主要物理机理是前方未燃气体受到前驱压缩波作用而被加热和压缩的正反馈微分加速机制,此后在管右端的反射压缩波影响下火焰传播速度略有降低。     

Analysis of the Internal Characteristics of a De ector Jet Servo Valve

In current research on deflector jet servo valves, the receiver pressure estimated using traditional two-dimensional simulation and theoretical calculation is always lower than the experimental data; therefore, credible information about the flow field in the prestage part of the valve can hardly be obtained. To investigate this issue and understand the internal characteristics of the deflector jet valve, a realistic numerical model is constructed and a three-dimensional simulation carried out that displays a complex flow pattern in the deflector jet structure. Then six phases of the flow pattern are presented, and the defects of the two-dimensional simulation are revealed. Based on the simulation results, it is found that the jet in the deflector has a longer core area and the fluid near the shunt wedge cannot resist the impact of the high-speed fluid. Therefore, two assumptions about the flow distribution are presented by which to construct a more complete theoretical model. The receiver pressure and prestage pressure gain are significantly enhanced in the calculations. Finally, special experiments on the prestage of the servo valve are performed, and the pressure performance of the numerical simulation and the theoretical calculation agree well with the experimental data. Finally, the internal mechanism described by the theoretical and numerical models is verified. From this research,more accurate numerical and theoretical models are proposed by which to figure out the internal characteristics of the deflector jet valve.     

非浸入式超声导波液位测量方法研究

从Navier运动方程出发,求解了自由平板和覆水平板中导波的特征方程,并绘制了其频散曲线。自由平板和覆水平板频散特性对比分析后,发现了覆水平板中导波模态较自由平板多了一种Quasi-Scholte模态导波。通过数值计算、有限元模拟和实验研究,分别对自由平板和覆水平板中导波的传播特性和模态进行验证。在低频区,A_0模态导波存在于自由平板中,而Quasi-Scholte模态导波只存在于覆水平板中。利用Quasi-Scholte模态和A_0模态导波的传播特性设计液位测量实验,采用一激一收压电传感器布置方式,频率130 k Hz的2.5周期汉宁窗调制正弦波作为检测信号,结果表明,液位与导波传播时间呈线性关系,能够用于容器内液位测量。