Papers by Keyword: Lamb Wave

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Authors: Ya Jie Sun, Yong Hong Zhang, Cheng Shan Qian, Zi Jia Zhang, Qi Wang
Abstract: The Lamb wave phased array theory is utilized to analysis the damage scattered signals to identify the structure defect. The damage scattered signals are gotten by comparing the health signals with the damage signals. Phased array theory based structural health monitoring can scan the structure in a certain range of 0°~180° by controlling the time delay of the excitation signals and the response signals. The processed signals in monitoring direction is shown on 3-D imaging to express the damage location in the structure. The method is verified by the experiment on the composite structure and the result shows that the Lamb wave phased array method can detect the damage in the composite structure and the 3-D image clearly displays the structure damage location.
Authors: Konstantin Jonas Schubert, Axel Siegfried Herrmann
Abstract: One of the largest issues remaining on the way to in situ Structural Health Monitoring of composite structures using Lamb waves is the impact that non-damaging factors like temperature changes and humidity absorption have on most measurement strategies. While some of these tasks have been successfully conquered, others, especially related to slowly developing influences like humidity absorption or mechanical ageing, remain challenging. In this paper, a method to approach this problem for a Lamb-wave based passive impact detection system is presented. Passive approaches use the waves generated by the impact event itself to both localize said event and evaluate whether it was large enough to damage the structure. For this, the impacts energy has to be estimated from sensors detecting the Lamb waves. The problem provided by changing conditions within the material is that the locally measurable wave amplitude due to an impact event of a certain energy is altered if the material properties change. This might happen due to temperature changes, mechanical loads, humidity absorption, fluid loads and other factors. The main idea of the presented approach is to mix a passive and an active system. Piezoelectric elements are used to generate Lamb waves to obtain the attenuation coefficients of the material before and after hot/wet-conditioning. These coefficients are then used to estimate the impact energy from passive sensor responses. Both the approach and experimental validation performed with low velocity impacts from an impact hammer are presented to show the ability to correctly calculate impact forces after conditioning.
Authors: Usik Lee, Yong Ju Jun, Il Wook Park
Abstract: A computer-aided time reversal process (CTRP) is proposed for the structural damage diagnosis. In the standard time reversal process (STRP), both forward and backward processes are conducted by the measurement. However, in the proposed CTRP, the forward process is conducted by the measurement while the backward process is conducted by the computation. As a benefit of the computer-based backward process, the unwanted Lamb wave modes which do not carry damage information can be readily removed from the signals reconstructed at the input excitation point. The reconstructed signals refined in such a way can make the signal processing for the damage diagnosis more efficient and easier.
Authors: Chen Ning Cai, Gang Yan
Abstract: This paper presents a genetic algorithm (GA)-based approach for localizing damage in plate-like structure. Diagnostic Lamb wave is excited into structure before and after damage to obtain scattered wave that contain characteristic information of the damage. After the time-of-flight (ToF) of the scattered wave in each actuator-sensor path is measured by continuous wavelet transform (CWT), a GA optimization procedure is developed to adaptively identify the location of damage without knowing the material properties a priori. It is achieved by minimizing the difference between the measured and calculated ToF of the scattered Lamb wave. Experimental study for an aluminium plate is conducted to validate the proposed damage localization approach.
Authors: Hua Zhang, Y.C. Xiao, Ming Li, G.Q. Liu, Jie Zhang
Abstract: L shaped braided composites under fatigue load was researched, using piezoelectric sensors and active Lamb wave monitoring technology. HHT transform based signal envelope obtaining method and the damage index method based on signal energy change were used to monitor the damage extending of the braided composites.
Authors: Shang Chen Fu, Zhen Jian Lv, Ding Ma, Li Hua Shi
Abstract: The use of Lamb waves for structural health monitoring (SHM) has complicated by its multi-mode character and dispersion effect, which impacts the damage positioning and high-resolution imaging. The group velocity dispersion curves of Lamb waves can be employed to warp the frequency axis, and then to establish warped frequency transform (WFT) to process Lamb waves. In this paper, received signals are directly compensated with warped frequency transform to suppress dispersion, and a new imaging method is proposed based on warped frequency transform. The propagation of Lamb waves in damaged aluminum plate is simulated by finite element software ABAQUS, results show that warped frequency transform can effectively compensate dispersive wave-packets, and high-resolution damage imaging can be obtained by the proposed method.
Authors: Nai Zhi Zhao, Shi Yan
Abstract: Damage detection in composite materials can be divided into active and passive approaches. The active approach is usually based on various non-destructive techniques utilizing actuators and/or receivers. Simple laser scans, revealing the change in Lamb wave response amplitudes, have been used to locate delamination and estimate its severity in a composite plate. The validity of the proposed method is demonstrated through experimental studies in which input signals exerted at piezoelectric (PZT) patches on a composite plate are successfully reconstructed by using the time reversal method. The ultimate goal of this study is to develop a reference-free damage diagnosis technique based on the time reversal process so that defects can be identified without relying on any past baseline data.
Authors: Yu Liu, Zheng Li, Ke Zhuang Gong, Xian Yue Su
Abstract: This paper describes a quantitative damage identification method for CF/EP composite laminates based on Lamb waves excited by distributed PZT wafers. The fundamental symmetric mode S0 is considered to detect defects (hole) in the plate. The Morlet wavelet and the cross-correlation analysis are introduced as signal processing tools for determining the time-of-flight (ToF) of Lamb wave. Considering the difference of Lamb wave velocities in different directions in a composite plate, the relationship of Lamb wave velocity in a unidirectional fibre reinforced laminate is studied and validated experimentally and numerically. In addition, a defect identification approach is revealed based on a regular arrangement of PZT wafers. Then, on the basis of the relationship of the wave velocity and the ToF, the location of a hole is identified by proposed method. Results demonstrate that the method is feasible in quantitative diagnosis of composite structures.
Authors: Wieslaw Jerzy Staszewski, Ruztamreen bin Jenal, Andrzej Klepka, Mariusz Szwedo, Tadeusz Uhl
Abstract: Non-contact optical/laser measuring techniques are very attractive in many engineering applications. The paper demonstrates examples related to structural health monitoring. Various methods based on strain, vibration and ultrasound measurements are presented together with relevant references. Applications examples utilise in-plane and out-of-plane measurements taken by 1-D and 3-D laser Doppler vibrometers.
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