Computational Characterization of Adhesive Layer Properties Using Guided Waves in Bonded Plates

This research develops a guided waves technique to nondestructively characterize the stiffness properties of bonded engineering components. This study first quantifies the influence of the relevant adhesive layer properties—Young’s modulus, Poisson’s ratio and bond thickness—on the dispersion curves of a two-layer bonded system, an aluminum plate with an adhesive tape layer bonded to its lower surface. Both a commercial finite element (FE) code (ABAQUS/Explicit) and the global matrix method (GMM) are used to determine the dispersion relationships of this bonded plate system in the form of frequency-wavenumber and slowness-frequency relations. These dispersion curves are then used to determine a set of adhesive tape parameter sensitive points, whose frequency coordinates represent the solution criteria for a proposed inversion procedure. This inversion is based on the GMM and assumes the three adhesive tape properties are unknown. The performance of this inversion procedure depends on the number of input time-domain signals; it is possible to solve the inverse problem for all three of the unknown adhesive tape properties if multiple input signals are known.     

An Intelligent Methodology for Railways Monitoring Using Ultrasonic Guided Waves

It is far from trivial to inspect railways for defections. In particular, for the foot area of the rail non destructive testing methods are known to be difficult to apply. In this paper, an ultrasonic guided wave method is considered along with classification methods for automated rail foot defect detection. In effect, given a set of gathered ultrasonic signals, multiple features are extracted from time-, frequency- and time–frequency domains. Next, a robust feature selection method is performed, to collect a small set of complementary features. The classification task is accomplished by means of a kernel-based support vector machine. To demonstrate the performance capabilities of our approach, an extensive experimental setup is designed under representative environmental and operational conditions. The sensitivity and the resolution of the proposed defect detection system are reported. A study on the influence of rail fastening on the proposed method is also reported where robust defect detection rates, greater than 93 %, are achieved assuming that a compact feature subset is considered. However, it is evident in experiments that even in the case of large defects, changes in the environmental conditions (temperature and humidity) increase the interpretation of the acquired signals, thus making the detection task more difficult.     

Monitoring Early Age Properties of Cementitious Material Using Ultrasonic Guided Waves in Embedded Rebar

The evaluation of early age properties of concrete is critical for ensuring construction quality. This paper presents a sensing method to use ultrasonic guided waves in a rebar for monitoring the early age properties of cementitious material. An EMAT sensor was used to excite the longitudinal mode L(0,1) wave in a rebar embedded in cement/mortar, and an ultrasonic transducer was used for receiving the echo signals. Guided wave dispersion curves were developed to select appropriate frequency range. The leakage attenuations of the L(0,1) mode wave from the rebar to the surrounding cement materials were continuously monitored for the first 10 h. The evolution of the shear wave velocity was also monitored simultaneously. The leakage attenuation from experimental measurements was compared with the theory-predicted attenuation in both time and frequency domains, and showed good agreement. Experiments were performed on three cement paste samples and three mortar samples. The results indicated that attenuation is nearly linearly related to the shear wave velocity, and shear wave velocity is linearly related to the penetration resistance (ASTM C403) in logarithmic scale. These results suggest that mechanical properties and hardening process of cement materials can be monitored by using the ultrasonic guided waves in a rebar.     

Detection of Axial Cracks in Pipes Using Focused Guided Waves

The implementation of synthetic guided wave focusing to locate axially aligned defects in pipes has been investigated. Results from both finite element computer models and experiments on real pipes are presented and the data show good agreement. Focusing is necessary to improve the reflection coefficient from axially aligned defects, as the signals are very weak. The Common Source Method (CSM) of synthetic focusing has been applied which makes it possible to apply focusing via post processing to previously collected data. The dependence of reflection coefficient on crack length was measured for both through thickness and part depth axially aligned defects, at a range of frequencies, using the torsional guided wave family. The results show that the reflection coefficient is approximately doubled when focusing is employed, compared to the sensitivity for unfocused fundamental torsional waves. However the sensitivity is still very low, so in practise this approach could only be used to find severe defects.     

Long Range Detection of Defects in Composite Plates Using Lamb Waves Generated and Detected by Ultrasonic Phased Array Probes

This article presents a technique for the generation and detection of Lamb waves guided along large plate-like structures made from various types of materials (metal, polymer, fibre-reinforced composite, etc.). A multi-element matrix ultrasonic probe is driven using the well-known phased array principle, for launching and detecting pure Lamb modes in/from specific directions along the plate, which are arbitrary for isotropic materials and limited to specific directions for anisotropic materials, e.g. principal directions or directions for which both phase and group velocities are collinear. The probe is gel-coupled to the tested specimen and allows quick inspection of large area from its fixed position, even of zones with limited access. The technique, which takes into account the frequency dispersive effects, is different than SHM-like (Structural Health Monitoring) inspection, since all transmitting or receiving elements are grouped together in a localized area defined by the active surface of the probe, and not permanently attached to the tested structure. The use of a multi-element probe, for long range Lamb waves-based inspection, is also distinctive from that usually performed, which consists of very local inspection of a material by steering the ultrasonic beam below and nearby the probe. A prototype is presented, as well as measurements of its performances in terms of modal selectivity and directivity. Finally the detection and localisation of a through-thickness hole in a large aluminium plate, of a delamination-like defect in a carbon epoxy composite plate and of an impact damage on a stiffened composite curved plate are shown.     

基于多个控制力作用下结构动力特性的损伤定位

用状态反馈控制的方法有目的地对结构进行极点配置，得到所需的受控结构的特征频率和特征振型，并利用这些数据，运用信息融合技术对由不同识别方法得到的损伤定位结果进行融合。本文还讨论了反馈控制力个数和位置对损伤定位结果的影响。数值仿真结果表明，使用受控结构的动力特性数据有可能提高损伤识别指标对损伤的敏感度，反馈控制力的个数和位置对损伤识别结果有影响，使用信息融合技术将采用不同损伤识别方法得到的识别结果进行融合，能进一步提高损伤定位的准确性。     

Estimation of Thermal Contact Resistance Using Ultrasonic Waves

In this paper, numerical simulations of both the three-dimensional heat conduction and two-dimensional elastic wave propagation at the interface of contact solids have been carried out. Numerical results of heat conduction simulations show that both the true contact area and thermal contact conductance increase linearly with an increase in the contact pressure. Numerical results of the ultrasonic wave propagation show that the intensity of a transmitted wave is very weak but depends clearly on the contact pressure. On the other hand, the intensity of reflected wave amounts to more than 99% of the standard reflected wave that results from the case of one cylindrical specimen without contact. However, the intensity of the modified reflected wave defined by the difference between the reflected wave and standard reflected wave shows the same tendency as that of the transmitted wave. The intensities of both transmitted and modified reflected waves could be expressed by the same power function of the contact pressure. By comparing the results of heat conduction with those of ultrasonic propagation calculations, a power functional correlation between the thermal contact conductance and transmitted or modified reflected intensity has been obtained. Using this correlation, it will be possible to estimate the thermal contact conductance between two solids through measuring the intensity of either reflected or transmitted ultrasonic waves.Paper presented at the Seventh Asian Thermophysical Properties Conference, August 23–28, 2004, Hefei and Huangshan, Anhui, P. R. China.     

Research on Damage Localization of Plate-like Structure Using Mode Acoustic Emission and Matching Pursuit

Based on mode acoustic emission theory,the paper analyses the acoustic emission analog signal of thin steel plate using matching pursuit,then obtains the characteristics interpretation of the different frequency signal energy concentration degree; Combined with four-point arc positioning method,the papers researches the damage localization of the plate-like structure. Simulation experiment shows that this method can accurately detect and locate the damage. This can provide data support for further imaging research based on time reverse theory.     

利用模态参数进行弹性薄板的损伤识别

以一四边简支矩形弹性薄板为研究对象，通过数值计算得到板损伤前后的多阶模态，进而利用板损伤前后的位移振型比及柔度差来进行板的损伤识别研究。结果表明一阶振型比及柔度差均可用于结构损伤的检测与定位，并能大致判断损伤的程度。其中又以一阶位移振型比的精度为优。     

Temperature Independent Damage Detection in Plates Using Redundant Signal Measurements

This paper describes a method to detect notch like damages in plates using piezoelectric transducers. The method does not use prior baseline data for damage detection. A single pair of piezoelectric wafer transducers made of Lead Zirconate Titanate (PZT) is attached back to back on the opposite sides of a plate and are used for simultaneous actuation and sensing. A notch, which is a sudden change in thickness of the plate, leads to mode conversion of Lamb waves. The mode converted wave component in the measured signal is then separated from the other Lamb wave mode components using polarization characteristics of the piezoelectric wafer transducers. The damage index is a function of the amplitude of this mode converted component of the signal. In real world situations, the damage index will not be exactly zero due to inaccuracy in transducer collocation and non-uniformity in their bonding conditions. Therefore, a (non-zero) threshold for the damage index needs to be established to avoid false alarms. True to the spirit of baseline-free damage detection, this threshold is computed from the signals acquired only from the current state of the structure. This is achieved by using redundancy in signal measurements. Since the method detects damages without having to rely on baseline data, environmental variations like temperature change do not affect its performance. Results from numerical simulations as well as experiments on aluminum specimens are provided to demonstrate the effectiveness of the method described above.     

用概率社会网络进行结构损伤位置识别

在不计测量误差情况下,神经网络能够成功地识别损伤位置及其程度,但在测量噪声影响下,神经网络的损伤识别效果则比较差,考虑到基于多变量模式分类的概率神经网络具有处理受噪声污染的测试数据的能力,本文将可能的损伤位置作为模式类,利用概率神经网络的分类能力来识别结构的损,地对两个算例,一个六层框架和一个两层框架进行数值模拟分析,并将概率神经网络与BP网络进行了比较,结果表明,概率神经网络具有更好的识别效果,是一种很有潜力的结构损伤位置识别方法。     

Guided Ultrasonic Wave Imaging for Immersed Plates Based on Wavelet Transform and Probabilistic Analysis

We propose a nondestructive evaluation method for immersed structures based on the propagation of ultrasonic waves induced by means of laser pulses and detected with an array of immersion transducers. In the study presented in this article, a laser operating at 532 nm is employed to excite leaky guided waves on an aluminum plate immersed in water. An array of immersion transducers is used to record the waves radiating from the laser-illuminated point. The detected signals are processed with an imaging algorithm based on continuous wavelet transform and probabilistic analysis to localize the presence of artificial defects machined in the plate. With respect to the existing imaging methods for plates, the proposed algorithm is pseudo baseline free, because it does not require data recorded from a pristine plate, but it requires that only a portion of the plate is free from defects. We find that the proposed algorithm enables the detection of surface cracks. Advantages and limitations of the algorithm for the nondestructive evaluation of underwater structures are discussed.     

Nondestructive Characterization of Planar Defects Using Laser-Generated Ultrasonic Shear Waves

This paper introduces a laser scanning technique to characterize internal planar defects in a specimen with parallel boundaries. Based on the principles of laser-based ultrasonic shear waves and shadowing, a procedure to determine flaw location, size, and orientation is described. The key feature of this scheme is the use of an optimum wave propagation angle where the maximum shear wave propagates. The feasibility of the approach is evidenced by testing specimens with various controlled and natural internal flaws. The experimental results are promising, in that the flaw characteristics can be determined with good accuracy. It is found that the scheme is especially useful for characterizing transverse-type flaws. The limitations of the technique are also addressed.     

Calculation of Nonlinear Waves Guided by Optical Fibres Using the Resonance Technique

Abstract

A recently developed, simple, and easily programmable method based on a resonance technique for the analysis of nonlinear guided modes for planar geometry has been extended to include fibre configurations. We consider here nonlinear media of the general type (non-Kerr media) including the more complicated but realistic case of saturable media. In the present paper, as an example of the proposed methodology, the following configuration is treated: a circular optical fibre consisting of a linear core bounded by a nonlinear cladding medium. The numerical results obtained by the method, correspond to the ‘weak guidance’ approximation, including both field distribution and power calculations.     

层状板中Lamb波的频散特性研究

利用矩阵递推方法，建立了层状板中Ｌａｍｂ波的特征方程以及相应的位移和应力分布计算公式，由此分析了双层板、软夹层板和硬夹层板中Ｌａｍｂ波的频散特性，特别指出软夹层的存在对Ｌａｍｂ波的频散特性有显著影响。该结论对工程测试分析有一定指导意义。     

Damage Localization for CFRP-Debonding Defects Using Piezoelectric SHM Techniques

This study employed a piezoelectric sensors-based structural health monitoring (SHM) technique to monitor debonding defects in real-time. A carbon fiber–reinforced polymer (CFRP) concrete beam specimen was fabricated, and the debonding conditions were inflicted in three successive steps. When the damage level was increased, the electromechanical impedance and guided wave signals were measured at each different damage level from the piezoelectric sensor array already surface mounted on the CFRP. A damage metric based on the root mean square deviation (RMSD) was investigated to quantify the variations in the signals between the intact and progressive damage conditions. To improve the performance of “debonding damage localization,” a new damage metric obtained by the superposition of the damage sensitive features extracted from both the impedance and guided wave signals was introduced. Polynomial curve fitting was performed using the superposed damage metric values. The location corresponding to the highest peak of the curve was determined. The location point was then compared with the actual inflicted damage points to confirm the effectiveness of the proposed damage localization technique. Further research issues are discussed for real-world implementation of the proposed approach using the above experimental results.     

Measurement of Interfacial Fracture Toughness of Surface Coatings Using Pulsed-Laser-Induced Ultrasonic Waves

This research develops a new technique for the measurement of interfacial fracture toughness of films/surface coatings using laser-induced ultrasonic waves. Using pulsed laser ablation on the bottom substrate surface, strong stress waves are generated leading to interfacial fractures and coating delamination. Simultaneously, a laser ultrasonic interferometer is used to measure the normal (out-of-plane) displacement of the top surface coating in order to detect coating delamination in a non-destructive manner. We can thus determine the critical laser energy for delamination, yielding the critical stress (that is, the interfacial strength). Subsequently, to examine the interfacial fracture toughness, additional pulsed laser irradiation is applied to a pre-delaminated specimen to show that the delamination area expands. This type of interfacial crack growth can be visualized using laser ultrasonic scanning. Furthermore, the calculation of elastic wave propagation was carried out using a finite-difference time-domain method) in order to accurately estimate the interfacial stress field. In this calculation, the stress distribution around the initial delamination is calculated to obtain the stress intensity factor. Based on the experimental and computational results, interfacial fracture toughness can be quantitatively evaluated. Since this technique relies on a two-laser system in a non-contact approach, it may be useful for a quantitative evaluation of adhesion/bonding quality (including both interfacial fracture strength and toughness) in various environments.     

Guided Wave Damage Characterization via Minimum Variance Imaging with a Distributed Array of Ultrasonic Sensors

Guided wave imaging with a distributed array of inexpensive transducers offers a fast and cost-efficient means for damage detection and localization in plate-like structures such as aircraft and spacecraft skins. As such, this technology is a natural choice for inclusion in condition-based maintenance and integrated structural health management programs. One of the implementation challenges results from the complex interaction of propagating ultrasonic waves with both the interrogation structure and potential defects or damage. For example, a guided ultrasonic wave interacts with a surface or sub-surface defect differently depending on the angle of incidence, defect size and orientation, excitation frequency, and guided wave mode. However, this complex interaction also provides a mechanism for guided wave imaging algorithms to perform damage characterization in addition to damage detection and localization. Damage characterization provides a mechanism to help discriminate actual damage (e.g. fatigue cracks) from benign changes, and can be used with crack propagation models to estimate remaining life. This work proposes the use of minimum variance imaging to perform damage detection, localization, and characterization. Scattering assumptions used to perform damage characterization are obtained through both analytical and finite element models. Experimental data from an in situ distributed array are used to demonstrate feasibility of this approach using a through-hole and two through-thickness notches of different orientations to simulate damage in an aluminum plate.     

Hamilton体系下的功能梯度电磁弹性固体内的导波研究

根据Hamilton原理建立了三维压电压磁动力学耦合系统的Hamilton对偶体系，将经典的弹性力学一类变量问题转化为二类变量，建立了Hamilton正则方程组，研究了功能梯度电磁材料（FGMM）板／管内的弹性导波的频散特性及波结构特征。结果表明：（1）压电效应提高了Lamb波的频率和波速，而磁效应则相反，压电效应对波动的影响远大于磁效应；它们而对SH波没有影响（厚度方向极化）。（2）短路及断路电学边界条件对SH波不发生任何影响（厚度方向极化），而短路对Lamb波的频率和波速有不同程度的降低（相同波数下）。（3）在波结构上，对平板而言，所谓的“对称”和“反对称”Lamb波由于材料的梯度特性而变得不再严格的关于中心线对称或反对称。对管而言，由于材料的非均匀分布导致存轴对称栩转波模态中出现了横截面翘曲现象．轴对称纵向波也出现厚度剪切应力。