基于时间反转理论的聚焦Lamb波结构损伤成像

从理论和实验上研究了时间反转法在频散和多模式的Lamb波结构健康检测方面的应用.当Lamb波在包含有损伤的板类结构中传播时,损伤的存在表现为一个被动波源.采用分布式传感器网络,基于传递函数的观点,通过推导由损伤这个被动波源产生的时间反转波场幅值的表达式,证实了当观察点位于损伤位置时,时间反转波场的幅值最大.为验证时间反转方法的聚焦效应,提出了一种适合于分布的激励/接收传感器网络的成像方法,该方法可以对损伤定位并近似确定损伤尺寸.结合有限元的实验结果显示了Lamb波检测信号的能量可在损伤处聚焦,表明时间反转     

压电晶片传感器监测金属薄板的腐蚀

激励压电晶片传感器产生Lamb波并用于监测金属板材的腐蚀,由信号的相关系数表征腐蚀对在板中Lamb波传播特性的影响。实验结果表明在Lamb波模态中A₀模式受腐蚀影响大,适合用于监测金属的腐蚀。Lamb波通过腐蚀区域后由于频散以及信号幅度和相位的变化对采集的损伤信号与健康状态信号的相关系数有影响。在腐蚀区域直径一定的情况下,对应的相关系数并不随腐蚀深度的增加而单调递减;在腐蚀深度一定时,相关系数随腐蚀区域直径增加而单调递减。     

有粘弹薄层负载时薄板中的Lamb波传播

从弹性波理论出发,结合边界条件,导出了一面有粘弹薄层负载时薄板中Lamb波的色散方程,通过数值计算,研究了薄板中Lamb波传播色散、衰减等特性及负载层参数对Lamb波传播的影响。最后提出了一种结合Lamb波和SH波微传感实现粘弹薄膜材料复Lame参数λ*和μ*的定征方法。     

多通道超声兰姆波检测板状结构中的裂纹

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。     

木塑复合材料缺陷及损伤的声发射信号特征分析及神经网络模式识别

针对木塑复合材料五种典型的缺陷及损伤机制，选择合适的木塑试样，应用三点弯曲的加载方法采集声发射信号。对主损伤区附近的声发射事件，应用小波变换提取特征参数，确定五类主要损伤机制所对应的声发射信号特征。采用B—P型反向传播神经网络构成的智能化模式分类器，对此五类声发射信号进行识别，获得了满意的效果。     

太赫兹无损检测的多特征参数神经网络分析技术

提出一种基于太赫兹无损检测的多特征参数神经网络分析技术,用于分析耐高温复合材料的粘贴质量无损检测.采用抽片式方法设计了一种耐高温复合材料的脱粘缺陷样品,抽片厚度为0.1mm.采用太赫兹时域光谱无损检测技术对耐高温复合材料的多层脱粘缺陷进行了检测试验研究,对比了上下脱粘缺陷所对应的太赫兹时域波形及频谱信息的异同,针对性地建立了耐高温复合材料粘贴质量的上层脱粘参数、下层脱粘参数、频域吸收质心参数等多特征参数,将特征参数进行优化作为反向传播神经网络的输入并对其进行上下脱粘分类识别.通过对反向传播神经网络的训练测试,实现了耐高温复合材料上层脱粘0.1mm、下层脱粘0.1mm的脱粘缺陷的识别.     

窄频带Lamb波频散特性研究

利用连续小波变换对从铝合金板结构中俘获的Lamb波信号进行分析,获得波信号在时间-尺度域的等高线和等高线脊线.根据Lamb波的频散特征、时间-尺度域等高线脊线的斜率和波在不同尺度下的到达时间,识别了Lamb波信号中各信息包的模式,并匹配出基础阶模式窄频带Lamb波在铝合金板结构中传播的实际频散曲线.对试验的Lamb波信号分析的结果表明该方法对于研究和应用窄频带Lamb波的频散特性是有效的. 关键词： 连续小波变换 Lamb波 频散     

三层介质超声谐振模式随材料和界面粘接性能变化的演变规律

在用超声波谐振对粘接材料的粘接强度进行无损评估时, 不同模式对粘接强度的敏感程度受到众多因素和参数的影响, 对检测结果的可靠性至关重要. 基于多层介质中声传播和界面弱粘接边界条件的理论模型, 将一个上下非对称的金属-粘接剂-金属三层结构的平面波反射系数函数中的谐振模式看作是上下铝金属层各自的Lamb波频散模式通过夹心粘接剂层相互耦合后叠加组成. 改变影响结构粘接强度的因素, 即粘接剂的性能参数(声阻抗、密度、厚度)和界面切向劲度系数k_t来分析三层结构谐振模式耦合方式的变化,得出结论: 粘接结构粘接性能的变化基本上不改变与被粘铝层相关的固有部分的Lamb波模式, 而它们的耦合模式则在谐振频率上产生平移并会与固有模式进行交换和替代; 不同参数的变化引起的模式演变有各自的规律, 大多可彼此区分.     

利用声带动力学模型参数反演方法进行病变嗓音分类

提出一种声带动力学模型参数反演方法,从发声机理角度对声带病变嗓音进行有效区分。依据声带生理组织和伯努利定律构建声带动力学模型,确定模型优化参数向量,耦合声门气流获取模型声门波;利用迭代自适应逆滤波算法获得实际嗓音声门波作为目标声门波;采用遗传优化算法提出通过匹配目标和模型声门波特征参数实现模型参数反演。实验结果表明,表征声门波的各时频域参数匹配相对误差不超过2%;依据反演所获模型参数提出去除声门下压影响的平均归一化缩放系数,克服声带非对称性特征在区分病变嗓音方面的不足,实现病理嗓音的全面有效区分。      

铝板中Lamb波检测的实验研究

Lamb波在传播过程中具有频散及多模态特性,若相关参数选择不当,会导致在实际应用中信号相互叠加而无法识别。本文基于Lamb波的频散曲线是其频散方程实数解分布的特点,采用二分法绘制了铝板中Lamb波的频散曲线、波结构曲线和入射角曲线。根据曲线选择S0模态的Lamb波对1mm厚铝板中不同类型的缺陷进行检测。实验结果表明,S0模态的Lamb波对裂纹型缺陷和贯穿型缺陷十分敏感,但对于裂纹型缺陷,其幅值变化并不与缺陷大小成线性关系,并且S0模态Lamb波的声场指向性十分集中,在偏离声束轴线时无法检测到缺陷。     

兰姆波检测信号的匹配追踪时频分析

为有效去除兰姆波检测信号中的冗余信息和识别多个模态,应用匹配追踪方法对兰姆波信号进行处理。在Chirplet原子基础上添加弯曲算子进行改进,以更好地匹配频散和多模式兰姆波信号的特征。由改进的Chirplet原子组成过完备字典,使用基于遗传算法的匹配追踪(GAMP)信号稀疏分解方法,从过完备字典中选出与待分析信号相匹配的最佳原子,利用最佳匹配原子和对应的分解系数进行信号重构和时频分析。研究结果表明,改进后的Chirplet原子更能反映出兰姆波信号的非线性时频变化特征,得到的时频分布与频散曲线的弯曲特性能很好的吻合。采用改进后的Chirplet原子匹配追踪方法可以获取更加精确的走时信息,为后续兰姆波损伤定位成像奠定基础。      

Acousto-ultrasonic sensing for delaminated GFRP composites using an embedded FBG sensor

In this paper, an active damage detection system for composite laminates is introduced. A fiber Bragg grating (FBG) was employed as an embedded sensor for detecting ultrasonic Lamb wave generated by a piezoelectric (PZT) actuator, inside the laminates. There were two different configurations for the system, making use of a broadband light source and a laser source, respectively. It was found that the use of a laser source could provide a higher sensitivity, so it was preferred in an ultrasonic signal acquisition unit for damage detection. When a delamination is developed in composite structures, the propagation characteristics of Lamb wave is then changed. According to altered signals, the types and natures of delamination inside the structures can be evaluated. In this experiment, glass fiber-reinforced epoxy (GFRP) composite beams were used to study the feasibility of our proposed detection technique. FBG sensors were embedded in different layers of the composite beams. The responses of FBG sensors from both intact and delaminated composite beams were then compared. Finally, the acquired Lamb wave signals corresponding to different delamination sizes and locations were examined.     

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.     

采用单个压电传感器的单模式兰姆波激发频率的选择

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     

Scattering characteristics of Lamb waves from debondings at structural features in composite laminates

This article investigates the scattering characteristics of Lamb waves from a debonding at a structural feature in a composite laminate. This study specifically focuses on the use of the low frequency fundamental antisymmetric (A(0)) Lamb wave as the incident wave for debonding detection. Three-dimensional finite element (FE) simulations and experimental measurements are used to investigate the scattering phenomena. Good agreement is obtained between the FE simulations and experimental results. Detailed parameter studies are carried out to further investigate the relationship between the scattering amplitudes and debonding sizes. The results show that the amplitude of the scattered A(0) Lamb wave is sensitive to the debonding size, which indicates the potential of using the low frequency A(0) Lamb wave as the interrogating wave for debonding detection and monitoring. The findings of the study provide improved physical insights into the scattering phenomena, which are important to further advance damage detection techniques and optimize transducer networks.     

Health monitoring of a composite wingbox structure

This work was devoted to the development of a health monitoring system assigned to aerospace applications. Those applications concerned the detection of damaging impacts and debonding between stiffeners and composite skins, since they are the major causes of in-service damage of aircraft structures. The chosen health monitoring system was first based on the excitation and reception of Lamb waves along the structure by using thin piezoelectric transducers (active mode) and secondly on a continuous monitoring taking the same transducers used as acoustic emission sensors (passive mode). The composite specimen used was consistent with aircraft wingbox in terms of structure and loading. Several impacts with increasing energy increments were applied on the composite specimen. In passive mode, the study showed the ability of using the acoustic signature of an impact to detect possible damage. Moreover, the damage emergence in the case of damaging impact was confirmed in active mode. Further measurements during fatigue testing were performed. The aim was to demonstrate the ability of the system to monitor disbond growth between the stiffener and the composite skin. The sensitivity of the health monitoring system to the disbond growth was further demonstrated.     

Design of optimal configuration for generating A0 Lamb mode in a composite plate using piezoceramic transducers

This work concerned a technique for a health monitoring system based on the generation and sensing of Lamb waves in composite structures by thin surface-bonded piezoceramic transducers. The objective was to develop transducers that are adapted for the damage detection in orthotropic composites. The key problem with the investigated Lamb waves was to select a mode to be sensitive to the damage. A hybrid modeling technique was therefore used to conceive transducers that were adapted to achieve such a feature. This modeling technique enabled studying the influence of the transducer characteristics on the Lamb waves propagating in orthotropic plates. It was demonstrated that a Lamb mode could be generated dominantly to other modes by using a multi-element transducer. The effectiveness of this technique was successfully verified experimentally on composite plates. It was shown that the dominant Lamb mode, obtained by use of dual-element transducers, was an appropriate mode for successfully detecting a damage in composites.     

兰姆波结构健康监测中的概率损伤成像

应用概率成像方法对兰姆波结构健康监测中板的损伤进行识别。根据兰姆波损伤散射信号的传播时间以及传感器网络中一对发射-接收传感器的空间位置来确定一个椭圆轨迹,该轨迹显示了损伤可能出现的位置。将监测区域均匀网格化,计算各网格节点到椭圆轨迹的最短距离,将此距离映射为损伤出现的概率。采用灰度等级对所有网格节点处的概率值进行量化,则由每一个发射接收传感器对就确定了一个灰度图像。为了突出损伤,应用图像融合技术对传感器网络中所有发射接收传感器对所构成的灰度图像进行融合。对铝板中横穿孔损伤的实验结果显示了该方法能够有效地确定损伤的位置和范围,有助于推动兰姆波结构健康监测的实用化。      

不锈钢折弯板裂纹Lamb波检测技术*

金属弯板广泛应用于车辆、船舶、飞机等大型装备结构件中，折弯处易形成应力集中产生裂纹，直接影响构件的使用安全和寿命。本文开展了不锈钢弯板裂纹缺陷Lamb波检测技术研究，计算了钢板的频散曲线，优选三种不同频率的S0模态：0.25MHz、0.5MHz、1MHz。利用COMSOL软件建立频域仿真模型，模拟了Lamb波在3mm厚“L”形弯板内部的传播情况，开展折弯板裂纹缺陷检测实验，并使用小波包变换的方法对实验信号进行分析。结果表明，经小波变换后，0.25MHz、0.5MHz、1 MHz的S0模态Lamb波均可用于裂纹缺陷的识别检测.