A time reversal damage imaging method for structure health monitoring using Lamb waves

This paper investigates the Lamb wave imaging method combining time reversal for health monitoring of a metal-lic plate structure.The temporal focusing effect of the time reversal Lamb waves is investigated theoretically.It demonstrates that the focusing effect is related to the frequency dependency of the time reversal operation.Numerical simulations are conducted to study the time reversal behaviour of Lamb wave modes under broadband and narrowband excitations.The results show that the reconstructed time reversed wave exhibits close similarity to the reversed nar-rowband tone burst signal validating the theoretical model.To enhance the similarity,the cycle number of the excited signal should be increased.Experiments combining finite element model are then conducted to study the imaging method in the presence of damage like hole in the plate structure.In this work,the time reversal technique is used for the recompression of Lamb wave signals.Damage imaging results with time reversal using broadband and narrowband excitations are compared to those without time reversal.It suggests that the narrowband excitation combined time reversal can locate and determine the size of structural damage more precisely,but the cycle number of the excited signal should be chosen reasonably.     

GUW-based structural damage detection using WPT statistical features and multiclass SVM

Recently, guided ultrasonic waves (GUW) are widely used for damage detection in structural health monitoring (SHM) of different engineering structures. In this study, an intelligent damage detection method is proposed to be used in SHM applications. At first, GUW signal is de-noised by discrete wavelet transform (DWT). After that, wavelet packet transform (WPT) is employed to decompose the de-noised signal and the statistical features of decomposed packets are extracted as damage-sensitive features. Finally, a multiclass support vector machine (SVM) classifier is used to detect the damage and estimate its severity. The proposed method is employed for GUW-based structural damage detection of a thick steel beam. The effects of different parameters on the sensitivity of the method are surveyed. Furthermore, by comparing with some other similar algorithms, the performance of the proposed method is verified. The experimental results demonstrate that the proposed method can appropriately detect a structural damage and estimate its severity.     

卫星太阳能电池阵破损自动检测系统的研究

介绍了国内首次研制的卫星太阳能电池阵破损自动检测系统的检测原理、系统结构与设计及现场试用结果。系统采用专用照明系统,电池阵列经光学系统清晰成像至CCD传感器上,经信号放大、A/D转换后输入计算机管理和控制系统。图像传感探头自动扫描整块电池板,并由距离传感器控制,始终保持采集图像清晰。利用特有的系统控制和检测软件,处理每幅电池图像信息,检测结果通过列表形式给出损伤电池的位置、破损类型、破损度等参数。现场试验结果表明:该系统的检出率达98.5%以上,检测速度为每分钟20片电池,达到了快速、高精度、自动化检测太阳能电池阵的使用要求。     

Lamb波多特征参数的复合材料损伤程度 评估方法*

航空航天飞行器、风电等工程机械结构的安全性保障至关重要,复合材料等新材料的大量应用,进一步提升了对结构健康监测的难度和要求。裂纹、脱层等复合材料结构典型损伤形成机理复杂、易扩展,损伤程度累积严重时会造成结构的突然失效。面向复合材料结构的损伤监测与诊断,现有的方法大多很难实现对结构损伤程度的跟踪评估,进而无法对结构的安全性评价提供依据。采用Lamb波监测理论,基于其传播距离远、对小损伤敏感等特点,通过研究分析典型损伤对Lamb波结构响应信号传播的多个特征作用过程和机理,采用多尺度分析,提取结构损伤时频域多特征参数;借助于神经网络的非线性映射能力,构建多特征复合材料结构损伤程度评估模型,实现对不同程度下典型结构损伤的评估。在环氧玻璃纤维复合材料板结构上的实验验证结果表明：提取的Lamb波时频域多特征参数,对不同程度典型损伤有一定的敏感性,所构建的损伤程度评估系统可以较为有效的实现损伤的程度估计。     

卫星太阳能电池阵破损自动检测系统的研究

介绍了国内首次研制的卫星太阳能电池阵破损自动检测系统的检测原理、系统结构与设计及现场试用结果。系统采用专用照明系统,电池阵列经光学系统清晰成像至CCD传感器上,经信号放大、A/D转换后输入计算机管理和控制系统。图像传感探头自动扫描整块电池板,并由距离传感器控制,始终保持采集图像清晰。利用特有的系统控制和检测软件,处理每幅电池图像信息,检测结果通过列表形式给出损伤电池的位置、破损类型、破损度等参数。现场试验结果表明:该系统的检出率达98.5%以上,检测速度为每分钟20片电池,达到了快速、高精度、自动化检测太阳能电池阵的使用要求。     

Study of bubble damage detectors for neutron detection

Bubble damage detectors have been prepared by using polyacrylamide as detector solid and freon as detector liquid. Tests show that the prepared detectors are sensitive to fast neutrons and have proportionality between bubble number and neutron fluence within a certain range of neutron fluence. Therefore, it can be used as a fast neutron detector and a dosimeter. Our experiments also show that the sensitivity of the detector increases with temperature.     

Defect identification in rods subject to forced vibrations using the spatial wavelet transform

Use of the wavelet transform (WT) to study the forced vibrations in a rod in order to detect the presence of a defect is proposed. The axial vibrations in an inhomogeneous rod, produced by the application of different forces, are simulated by the Network Simulation Method and the obtained response is analyzed with the WT. An analysis of the detection and location of defects for several applied forces and defects generated by locally changing density or stiffness of the rod is presented. A noise test was carried out to check the robustness of the technique in real situations.     

Passive and active interior noise control of box structures using the structural intensity method

This paper presents passive and active vibro-acoustic noise control methods for attenuating the interior noise level in box structures which can be an analogy of cabins of vehicle and aircraft. The structural intensity (SI) approach is adopted to identify the predominant vibration panels and interior noise sources for box structures. In the study, the finite element method is used to determine the structural vibration and structural intensity in the box surfaces. According to structural intensity vectors plot and structural intensity stream lines presentation, the possible effective control positions where the dampers may be attached and the active control forces may act to reduce vibration and interior noise, are identified. From the study, it can be demonstrated that the structural intensity approach and stream line presentation are possible methods for identifying the vibro-acoustic interior noise source and predominant panels which may be modified to reduce the interior noise level. The structural intensity methodology, passive and active noise control results can be extended to the further study of the vibration and interior noise control of actual cabins of vehicles and aircraft.     

基于频率特征的薄膜损伤声学判别方法

利用声学法研究单层氧化硅薄膜在不同激光能量下的损伤情况,建立了激光致薄膜产生的声波采集系统,对其采集到的时域信号进行傅里叶变换,比较和分析薄膜损伤前后声波24~40kHz高频段的曲线,提取频率特征,并提出用曲线相似函数进行薄膜损伤的识别方法。实验数据结果表明,该方法简单易行,可降低环境噪声的影响,既能实现在线检测,又能准确判别薄膜损伤。     

Structural health monitoring using polymer-based capacitive micromachined ultrasonic transducers (CMUTs)

Transducers based on a capacitive micromachined ultrasonic transducer (CMUT) design have been fabricated using a rapid prototyping technique. This results in a device that is constructed principally from polymers, in a process which is simple and inexpensive. The resultant devices can be attached to the surfaces of solids. Their peak sensitivity is in the 80–100 kHz range, making them ideal for applications such as acoustic emission and structural health monitoring. Good low frequency sensitivity leads to applications in vibration monitoring.     

基于WLI原理K9基片的亚表层损伤检测

在现代激光技术的应用发展中，光学元件在其制造过程中形成的亚表层损伤是导致激光损伤的主要根源之一。光学元件亚表层的检测方法已成为光学加工领域的一个新的热点问题。利用非接触式白光干涉原理对K9基片进行亚表层损伤检测，获得基片表面至亚表面200μm深的三维立体图和纵向截切图，并精确地确定其亚表层损伤深度，以及在不同深度范围内的损伤密度。该方法与现阶段国内常用的亚表层损伤检测方法相比，具有非破坏性、纵向检测结果精确、检测过程简单快捷等特点。     

嵌入式超声传感器的混凝土损伤非线性检测研究*

对建筑结构在其生命周期中的损伤状态进行实时监测十分重要,借助埋在混凝土构件内部的压电传感器对结构进行无损检测是一种新型的检测手段。本文将压电传感器嵌入混凝土试件内部,通过超声二次谐波法对信号进行分析处理得到表征材料损伤的非线性参数,以此来监测混凝土的早期强度并对混凝土试件受压和钢筋混凝土梁受弯的不同损伤程度进行检测。结果证明,非线性参数可以反映混凝土养护过程的强度增长情况,也可以表征不同损伤形式下试件的损伤程度。     

Modeling nonlinearities of ultrasonic waves for fatigue damage characterization: Theory,simulation, and experimental validation

A dedicated modeling technique for comprehending nonlinear characteristics of ultrasonic waves traversing in a fatigued medium was developed, based on a retrofitted constitutive relation of the medium by considering the nonlinearities originated from material, fatigue damage, as well as the “breathing” motion of fatigue cracks. Piezoelectric wafers, for exciting and acquiring ultrasonic waves, were integrated in the model. The extracted nonlinearities were calibrated by virtue of an acoustic nonlinearity parameter. The modeling technique was validated experimentally, and the results showed satisfactory consistency in between, both revealing: the developed modeling approach is able to faithfully simulate fatigue crack-incurred nonlinearities manifested in ultrasonic waves; a cumulative growth of the acoustic nonlinearity parameter with increasing wave propagation distance exists; such a parameter acquired via a sensing path is nonlinearly related to the offset distance from the fatigue crack to that sensing path; and neither the incidence angle of the probing wave nor the length of the sensing path impacts on the parameter significantly. This study has yielded a quantitative characterization strategy for fatigue cracks using embeddable piezoelectric sensor networks, facilitating deployment of structural health monitoring which is capable of identifying small-scale damage at an embryo stage and surveilling its growth continuously.     

圆形阵列无线传感器的鸟鸣声检测方法*

针对海岛湿地自然保护区鸟类等发声动物监测困难的问题,开发了基于物联网的圆形阵列无线传感器实时在线监测系统。鉴于保护区现场环境条件的特殊性,为保障传感器系统在野外长期稳定运行,需减少无鸣声数据的传输量来减轻系统负担,该文提出了基于子带能量谱熵比的动态双门限鸟类鸣声端点检测算法,对监测数据的有效鸣声段检测后实时回传到云平台。经过仿真和野外实测数据对比了不同信噪比下的鸣声检测性能,结果表明在暴风雨、风浪等海岛自然环境噪声下,鸟鸣声端点检测算法的准确率达90%以上,基本可实现海岛环境中的有效鸣声端点检测,提升了圆形阵列无线传感器系统的可靠性。     

铁磁纳米阵列膜温度稳定性的损伤扩散研究

利用损伤扩散方法研究了三角点阵量子磁盘的温度稳定性问题.为建立实际体系的伊辛模型,我们作了三方面考虑：1)计入自旋间的长程相互作用;2)考虑纳米线阵列中纳米线长度的不一致性,3)推导出矫顽力与自旋交换常数间的关系并利用矫顽力定量估算出自旋交换常数的数值.模拟结果表明,自旋间的作用范围越长,损伤扩散越难,即稳定性越好;而纳米线的长度混乱度越大,温度稳定性越差. 关键词： 量子磁盘 温度稳定性 损伤扩散方法     

Damage detection in composite panels based on mode-converted Lamb waves sensed using 3D laser scanning vibrometer

This paper introduces damage identification approach based on guided ultrasonic waves and 3D laser Doppler vibrometry. The method is based on the fact that the symmetric and antisymmetric Lamb wave modes differ in amplitude of the in-plane and out-of-plane vibrations. Moreover, the modes differ also in group velocities and normally they are well separated in time. For a given time window both modes can occur simultaneously only close to the wave source or to a defect that leads to mode conversion. By making the comparison between the in-plane and out-of-plane wave vector components the detection of mode conversion is possible, allowing for superior and reliable damage detection. Experimental verification of the proposed damage identification procedure is performed on fuel tank elements of Reusable Launch Vehicles designed for space exploration. Lamb waves are excited using low-profile, surface-bonded piezoceramic transducers and 3D scanning laser Doppler vibrometer is used to characterize the Lamb wave propagation field. The paper presents theoretical background of the proposed damage identification technique as well as experimental arrangements and results.     

一种改进的数字散斑相关方法用于检测人的口型变化

提出了一种基于数字散斑相关测量技术的口型变化检测技术,该技术以白光数字散斑相关测量方法为基础,根据实际应用的需要,引入重心算法并进行了改进,可以实时地检测人说话时的口型变化。通过实验表明,提出的方法是一种实用的、快速、高精度的口型变化测试方法,可为今后的唇语识别技术奠定基础。     

Direct detection searches for WIMP dark matter

A very active hunt is underway to discover the composition of dark matter in the universe. A large effort is devoted to the direct detection of dark matter through interactions with detectors in the laboratory. In this paper, we give an overview of the dark matter problem, discuss some of the design considerations taken in direct detection experiments, and describe some of the current efforts to discover Weakly Interacting Massive Particles (WIMPs), a well-motivated class of candidates for dark matter.     

Fabry-Perot干涉技术及其在疲劳损坏检测中的应用

从激光超声的特点出发,分析目前主要的超声检测技术,重点研究Fabry-Perot干涉仪的原理和工程应用。共焦Fabry-Perot干涉仪具有只对固体表面的振动速度灵敏、有较大的入射孔径、集光能力强等优点,克服了其它干涉检测技术对工作环境的严格要求,广泛应用于工业生产中。为检测碳钢的疲劳破坏,设计了一个激光超声检测系统,分析不同超声频率和加载不同循环压力造成的疲劳损坏情况。     

单元素靶粒子位移损伤计算方法及有效模式分析

通过SRIM程序的快速损伤计算与全级联计算两种常用模式,对单元素靶材料进行粒子辐照模拟计算,分别利用基于损伤能量间接计算移位数的NRT位移模型方法和直接通过输出文件读取的方法获得移位数,并对数据进行相应的处理及分析对比,结果表明：对于单元素靶来说,在SRIM快速损伤和全级联两种计算模式下,利用NRT位移模型数值计算得到的移位数基本一致,都可以用于进一步计算得到可靠的位移损伤剂量（dpa）;而通过SRIM两种模式下的输出文件数据直接获得的移位数则有两倍左右的差异,要想得到相对可靠的dpa相关参数,需要根据不同辐照情况选取合适的计算模式。