基于三维弹性理论的碳纤维板中Lamb波建模

基于主动Lamb波的结构健康监测是目前复合材料结构损伤监测技术研究的热点之一,了解Lamb波的传播特性对进行可靠的损伤监测非常重要.本文结合经典三维弹性理论与Lamb波的运动位移方程,对碳纤维复合材料板中传播的Lamb波传播特性进行了建模研究,在此基础上推导了碳纤维板的相速度频散曲线,并讨论了Lamb波传播方向与坐标轴之间的夹角及碳纤维铺层方向对频散曲线的影响,建模结果证明了这种建模方法的正确性.     

Lamb波传播特性研究

应用三维弹性理论对Lamb波频散曲线进行理论建模.在Lamb波主动监测系统中,应用Gabor小波变换理论分析Lamb波信号的时延及相位角,获得了板中Lamb波的相群速度频散曲线.比较理论曲线与实验数据,证明了三维弹性理论建模方法的有效性.     

复合材料无损检测中Lamb波的优化

如何解决Lamb波检测机理的复杂性并将之运用于复合材料损伤检测是个重要课题.以玻璃纤维增强的复合材料层合板为研究对象,搭建试验平台,利用集成在复合材料层合板上的PZT压电陶瓷片作为驱动元件和传感元件,对接收的Lamb波信号进行分析与研究,从信号频率范围、信号中心频率、波振幅值、波形个数和信号激励形式等五个方面完成Lam...     

多模式Lamb波在层合板检测中的应用研究

以层合板复合材料为实验对象,采用超声Lamb波为手段,通过检测信号的时频分析,有效确定缺陷类型对Lamb波在复合材料中传播的影响.     

基于Lamb波的风电叶片复合材料声发射源定位研究

声发射波在风电叶片薄板结构中传播时产生的多模态效应及频散现象使得波速的测定较为困难,导致声发射源定位误差过大。针对以上问题,从声发射波传播的物理机制出发,利用完备总体经验模态分解(CEEMD)结合Lamb波理论对风电叶片复合材料在不同应力状态下产生的声发射波进行断铅实验研究。研究结果表明:作用于叶片薄板结构的应力方向不同,将会产生不同频率、不同声速的应力波。垂直应力主要激发出弯曲波,该波波速较低(约1 357 m/s),幅值衰减较快,频率主要集中于50 kHz左右;平行应力主要激发出扩展波,该波波速较高(约3 634 m/s),幅值衰减较慢,频率主要集中于150 kHz左右。扩展波无频散效应且衰减较慢,更加适合风电叶片薄板结构声发射源定位。     

碳纤维复合材料损伤声发射信号模式识别方法

针对碳纤维复合材料层合板剪切过程中所产生的纤维断裂及基体开裂声发射信号的数据样本数量多、分布随机、变化形式较为离散等问题,提出一种可用于识别纤维断裂及基体开裂两种损伤类型的方法。首先,利用经验模态分解(EMD)对纤维断裂及基体开裂的声发射信号进行时频变换;然后,对分解后信号进行快速傅里叶变换(FFT)以获得特征频率集,再利用主成分分析法(PCA)对特征频率集进行降维处理;最后,利用支持向量机(SVM)实现纤维断裂及基体开裂信号进行损伤模式识别。结果表明,此方法可较为准确地识别纤维断裂及基体开裂两种信号。针对碳纤维复合材料层合板剪切过程所产生的声发射信号,模型的总识别率达85.8%。     

纤维增强复合板中声弹Lamb波的波结构分析

基于线性三维弹性理论和“增量变形力学”理论,采用勒让德正交多项式展开法,推导了在水平和垂直方向施加初应力时,沿纤维增强复合板的非主对称轴方向传播的声弹Lamb波的波动方程,并对波动方程进行数值求解。为了验证方法的准确性,将该文方法求解的各向同性材料的相速度频散曲线与Disperse?软件的计算结果进行比较,两者吻合良好。以单层单向纤维增强复合材料板为例,计算了无初应力状态下的波结构应力曲线,并与应力自由边界初始条件的一致性进行了比较。研究了水平和垂直方向初应力效应对Lamb波频散曲线的影响。针对声弹效应较为敏感的Lamb波A₀模态,着重分析了初应力效应对波结构位移分布曲线的影响。     

基于多重信号分类算法的复合材料冲击定位

为了提高复合材料结构冲击定位的精度和实时性, 将阵列信号处理技术引入到结构健康监测领域, 提出了利用小波变换和多重信号分类算法实现复合材料结构冲击定位的新方法: 通过小波变换提取冲击响应信号某一窄带频率成分, 运用多重信号分类(MUSIC) 算法实现冲击源到达方向的估计; 根据Lamb 波传播特性, 用小波变换求出某一中心频率下的对称模式和反对称模式的Lamb 波到达同一传感器的时间差, 结合对称模式和反对称模式Lamb 的速度差就可以估计出冲击源到达传感器的距离, 实现冲击定位。对玻璃纤维/环氧树脂复合材料层合板和碳纤维/双马树脂基复合材料层合板2 种试件的实验均表明该方法能快速、精确地识别出冲击源位置。      

基于相移光纤光栅传感器的碳纤维增强树脂复合材料基体裂纹超声探伤

复合材料内部的微小裂纹常会引起后续严重的破坏,因此需要对其进行检测。然而超声探伤复合材料基体裂纹非常困难。本文搭建了一个具有高灵敏度、大带宽的相移光纤光栅超声传感系统,利用此系统探测了在正交铺层碳纤维增强树脂复合材料板中传播的Lamb波。对Lamb波进行数据处理发现,随着三点弯曲实验产生的基体裂纹个数增加,Lamb波的幅值和频谱峰值线性减少。通过和传统压电传感器比较表明,相移光纤光栅传感器测得的Lamb波信号随复合材料基体裂纹数的增加其幅值具有更高的下降速率,表明相移光纤光栅传感器更适合于复合材料基体裂纹的超声探伤。研究表明,新开发的传感系统能够探测到中心频率为300 kHz的微弱超声信号,并能够对碳纤维增强树脂复合材料板中微小基体裂纹个数进行精确评估。      

基于多尺度数值模型的复合材料各向异性热膨胀系数预测

依据复合材料内部纤维在基体内的排布规律及层合板铺层特性,基于多尺度方法,建立单层板和层合板代表性体积单元(RVE)模型,施加相应的边界条件,预测单层板的热膨胀系数和工程常数,进而预测复合材料层合板各向异性的等效热膨胀系数。通过与实验数据对比发现,基于正六边形单层板RVE模型预测的热膨胀系数,相比理论预测值,整体更接近实验值,其中预测的单向T300/5208碳纤维增强环氧树脂基复合材料、P75/934碳纤维增强环氧树脂基复合材料和C6000/Pi碳纤维增强环氧树脂基复合材料的横向热膨胀系数与实验结果的误差分别只有3%、1%和2%;采用单层板RVE预测的单向ECR/Derakane 510C玻璃纤维增强乙烯基酯树脂基复合材料的工程常数与实验值最大相差7.5%;层合板RVE模型预测的正交AS4/8552碳纤维增强环氧树脂基复合材料厚度方向的热膨胀系数与实验结果误差可以忽略,只有0.08%。最后以大型复合结构常用的正交铺层结构为研究对象,基于给出的单层板和层合板RVE模型预测了不同铺层复合材料烟道层合板的等效热膨胀系数,环向铺层比例对厚度方向的热膨胀系数影响较小。      

Generation and reception of ultrasonic guided waves in composite plates using conformable piezoelectric transmitters and optical-fiber detectors

A condition monitoring nondestructive evaluation (NDE) system, combining the generation of ultrasonic Lamb waves in thin composite plates and their subsequent detection using an embedded optical fiber system is described. The acoustic source is of low profile with respect to the composite plate thickness, surface conformable, and able to efficiently launch a known Lamb wave mode, at operating frequencies between 100 and 500 kHz, over typical propagation distances of 100 to 500 mm. It incorporates both piezocomposite technology and interdigital design techniques to generate the fundamental symmetrical Lamb wave mode in both metallic and carbon-fiber composite plates. Linear systems and finite element modeling techniques have been used to evaluate the operation of the transducer structure, and this is supplemented by experimental verification of the simulated data. An optical fiber, either bonded to the surface or embedded across the length of the composite plate samples, is used to detect the propagating ultrasonic Lamb waves. Single mode silica fiber has been used in conjunction with a portable 633 nm Mach-Zehnder interferometer for signal demodulation and subsequent data acquisition. This hybrid system is shown to generate and detect the fundamental symmetrical Lamb wave (s(0)) in both carbon-fiber and glass-fiber reinforced composite plates. Importantly, the system signal-to-noise ratio (SNR) associated with the acoustic source compares favorably with s(0) Lamb wave generation using a conventional transducer and angled perspex wedge arrangement.     

Analysis of fracture process in single-edge-notched laminated composites based on the high amplitude acoustic emission events

The objective of this study is to analyze the fracture process of single-edge-notched (SEN) laminated composites with different lay-up configurations and different fiber composite systems based on the behavior of high amplitude acoustic emission (AE) signals. The classification of signal type according to the dominant frequency band and its magnitude via FFT, combined with the microscopic observations under reflection and transmission optical microscopy, have been also performed in order to support the effectiveness of the analysis based on the high amplitude AE signals. It has been shown that the behavior of high amplitude event rate describes well the fracture process in the SEN laminated composites with different fiber orientations; the direction of the main crack propagation in SEN laminated composites is decisively affected by the fiber orientation and is irrespective of the initial notch direction. Finally, the AE characteristics for SEN laminated composites have been summarized in association with the individual fracture process.     

Effect of moisture absorption on the mechanical behavior of carbon fiber/epoxy matrix composites

A carbon fiber/epoxy unidirectional laminated composite was exposed to a humid environment and the effect of moisture absorption on the mechanical properties and failure modes was investigated. The composites were exposed to three humidity conditions, namely, 25, 55, and 95 % at a constant temperature of 25 °C. The carbon fiber–epoxy laminated composites for two different carbon fiber surface treatments were used. The results showed that the mechanical properties differ considerably for each fiber surface treatment. The application of a coupling agent enhanced the fiber-matrix adhesion and reduced dependence of the properties on humidity. The damage mechanism observed at micromechanical level was correlated to acoustic emission signals from both laminated composites. The untreated carbon fiber failure mode was attributed to fiber-matrix interfacial failure and for the silane-treated carbon fiber reinforced epoxy laminate attributed to matrix yielding followed by fiber failure with no signs of fiber-matrix interface failure for moisture contents up to 1.89 %.     

密集型矩形阵列参数对激光Lamb波成像的影响分析

激光超声技术具有非接触、检测效率高等优点,在无损检测领域受到广泛关注;充分利用激光超声技术的高空间分辨率特性,结合密集型矩形阵列和激光Lamb波技术进行板中缺陷检测。采用连续小波变换对频带宽、时域分辨率低的激光Lamb波信号进行提取,得到特定频率下具有高时域分辨率的窄带信号;利用线性映射补偿技术消除所提取窄带信号中的频散,消除频散的信号用于缺陷成像;最后,结合幅值成像技术和符号相干因子成像技术对频散补偿后的信号进行处理,实现铝板中缺陷的成像和定位。在此基础上,进一步对不同的阵元数量和阵元间距对密集型矩形阵列指向性和缺陷成像质量的影响进行分析。当阵元数量为16,阵元间距为一个Lamb波波长时,主瓣宽度较窄且没有栅瓣出现,缺陷成像质量得到有效提高。     

Quantification of failure mechanisms in mode-I loading of fiber reinforced plastics utilizing acoustic emission analysis

Acoustic emission signals originating from interlaminar crack propagation in fiber reinforced composites were recorded during double cantilever beam testing. The acoustic emission signals detected during testing were analyzed by feature based pattern recognition techniques. In previous studies it was demonstrated that the presented approach for detection of distinct types of acoustic emission signals is suitable. The subsequent correlation of distinct acoustic emission signal types to microscopic failure mechanisms is based on two procedures. Firstly, the frequency of occurrence of the distinct signal types is correlated to different specimens’ fracture surface microstructure. Secondly, a comparison is made between experimental signals and signals resulting from finite element simulations based on a validated model for simulation of acoustic emission signals of typical failure mechanisms in fiber reinforced plastics. A distinction is made between fiber breakage, matrix cracking and interface failure. It is demonstrated, that the feature values extracted from simulated signals coincide well with those of experimental signals. As a result the applicability of the acoustic emission signal classification method for analysis of failure in carbon fiber and glass fiber reinforced plastics under mode-I loading conditions has been demonstrated. The quantification of matrix cracking, interfacial failure and fiber breakage was evaluated by interpretation of the obtained distributions of acoustic emission signals types in terms of fracture mechanics. The accumulated acoustic emission signal amplitudes show strong correlation to the mechanical properties of the specimens. Moreover, the changes in contribution to the different failure types explain the observed variation in failure behavior of the individual specimens quantitatively.     

Finite element modeling of lamb wave propagation in anisotropic hybrid materials

A finite element approach for modeling of acoustic emission sources and signal propagation in hybrid multi-layered plates is presented. Modeling results are validated by Laser vibrometer measurements and comparison to calculated dispersion curves. We investigate hybrid plates as typically found in composite pressure vessels, composed of fiber reinforced polymers with arbitrary stacking sequences and attached metal or polymer materials. Hybrid plate thickness, the ratio between anisotropic and isotropic materials and material properties are varied. Lamb-wave propagation in a geometry representative of a pressure vessel is modeled. It is demonstrated, that acoustic emission sources in multi-layered structures can cause Lamb-waves superimposed by guided waves within the individual layers.     

Propagation of Iamb waves in adhesively bonded multilayered media

The effect of introducing attenuation on Lamb wave dispersion curves is studied in this paper. Attenuation is introduced to a three-layered composite plate by an adhesive bond layer with viscous behavior. No changes are required to the transfer matrix formulation for the propagation of elastic waves. By introduction of a complex wavenumber, the model can be used to the propagation of attenuative Lamb waves. Numerical examples for a three-layered aluminium-epoxy-aluminium plate show that attenuation values of each mode in plates are related not only to attenuation, but also to the thickness of the bonded layer, which is in agreement with practical situations.     

Quantification of the Uncertainty of Pattern Recognition Approaches Applied to Acoustic Emission Signals

Acoustic emission analysis is a nondestructive technique frequently used to assess the integrity of fiber reinforced plastics. Pattern recognition techniques have shown great potential to identify microscopic failure mechanisms in plate-like structures. Because every assignment of an acoustic emission signal to a respective failure mechanism is possibly associated with an error, one key question is the reliability of the assignment method. It is useful to distinguish between the uncertainty of the assignment and the false assignment of an acoustic emission signal to a group of signals. The first is owed to statistical effects and the reliability of the classification method itself. The second is caused by false conclusions or disputable assumptions on the source mechanisms. The present study will focus on the first aspect. For this purpose, we propose a model based algorithm that estimates the uncertainty of a feature based pattern recognition approach based on cluster validity indices. Further, we demonstrate the application of the algorithm to experimental acoustic emission data obtained from a double cantilever beam specimens with unidirectional layup of carbon fiber reinforced polymer. Based on previous investigation we use a pattern recognition approach to distinguish between different failure mechanisms like matrix cracking, interfacial failure and fiber breakage based on the frequency features of the acoustic emission signals. We consider the influence of dispersion and attenuation effects during propagation of Lamb-waves on the extracted acoustic emission features. This is done by investigating the influence of source-sensor distance by test sources like pencil lead breaks and piezoelectric pulsers. Using the model based algorithm it is possible to calculate the uncertainty of the pattern recognition results as a function of source-sensor distance. It is found that dispersion effects of Lamb-waves do not seriously affect the distinction between microscopic failure mechanisms for source-sensor distances up to 375 mm. We demonstrate that the spatial distribution of acoustic emission sources has a larger impact on the uncertainty of assignment than the absolute source-sensor distance. Applying the proposed algorithm to the current experimental setup, we obtain an uncertainty of classification below 7 % for source-sensor distances below 375 mm. Attenuation is quantified to be 0.165 dB/mm for the A ₀-mode and 0.047 dB/mm for the S ₀-mode. Within the source-sensor distance of 375 mm this causes severe attenuation of the signal amplitude and thus prohibits detection of weak acoustic emission signals long before the uncertainty of the classification method reaches 10 %.     

Lamb waves in a thin isotropic layer between two anisotropic layers

Attenuative Lamb wave propagation in adhesively bonded anisotropic composite plates is introduced. The isotropic adhesive exhibits viscous behavior to stimulate the poor curing of the middle layer. Viscosity is assumed to vary linearly with frequency, implying that attenuation per wavelength is constant. Attenuation can be implemented in the analysis through modification of elastic properties of isotropic adhesive. The new properties become complex, but cause no further complications in the analysis. The characteristic equation is the same as that used for the elastic plate case, except that both real and imaginary parts of the wave number (i.e., the attenuation) must be computed. Based on the Lowe's solution in finding the complex roots of characteristic equation, the effect of longitudinal and shear attenuation coefficients of the middle adhesive layer on phase velocity dispersion curves and attenuation dispersion curves of Lamb waves propagating in bonded anisotropic composites is visualized numerically.