Baseline-free Lamb Waves Damage Identification Technique For Plate-like Structures Based On Beamforming

Lamb waves have been utilized for structural health monitoring (SHM) and nondestructive testing (NDT) of plate-like structures due to they can propagate over long distance with little amplitude attenuation and detect large areas of structures, meanwhile, they are also sensitive to small defects, such as hole, crack and composite delamination. Damage scattering signal features including both the temporal information and signal energy are usually utilized for damage detection in plate-like structures. The conventional Lamb wave-based damage detection techniques often require a baseline data measured in pristine condition to extract damage scattering signals. However, there are many significant technical challenges to realizing these techniques, in the presence of varying environmental and operational conditions of the system. As a consequence, baseline-free Lamb waves damage identification technique has bacome a hot research topic in structural health monitoring. To overcome the problems associated with baseline comparison, a baseline-free Lamb wave damage localization and imaging method based on beamforming technique is proposed in the article. Time window functions and a distributed active sensor network are used to separate the damage scattering signals from the sensing signals. Main work of this paper is as follows:Firstly, the dispersion characteristic of Lamb wave is introduced. Moreover, the phase velocity and group velocity dispersion curves of an aluminum plate are plotted by the calculated results of the Rayleigh-Lamb frequency equation. A 3-D finite element mode is built to simulate Lamb wave in an aluminium plate using the commercial package ABAQUS/Explicit. Selective excitation of Lamb wave fundamental SO and AO modes is analyzed. The interaction of the SO and AO mode with damages (e.g. hole and crack) are investigated. The principle of beamforming algorithm is introduced and the robustness of beamforming, such as noise immunity and fault tolerance ability of velocity error, is investigated through a finite element mode of an aluminium plate with a square sensor array.Secondly, a baseline-free Lamb wave damage identification method based on beamforming is proposed. Time window functions and an active sensor network in conformity to a pulse-echo configuration are used to separate the damage scattered signals from the sensing signals. Then, a beamforming technique based on the delay-and-sum algorithm is applied to diagnose the hole damage of an aluminium plate through localization and imaging. The validity of the proposed baseline-free technique is demonstrated by numerical simulation and experiments. What's more, beamforming imaging technique is used to locate crack position accurately and estimate crack orientation qualitatively. The effectiveness of this technique is demonstrated by predicting orientation-specific damage cases including a horizontal crack, a vertical crack and a diagonal crack by numerical simulation. A damage size characterization algorithm, relying on temporal information of the scattering signal obtained by time reversal theory and Hilbert transform, is used to estimate the size of hole damage in an aluminium plate.Finally, a baseline-free Lamb wave damage identification method based on probability is proposed. Time window functions and a distributed active sensor network are used to separate the damage scattered signals. Continuous wavelet transform is used to calculate the time of flight of damage signal waves. A probability-based hyperbola diagnostic imaging method using cumulative distribution function is developed to diagnose damage through localization and imaging. Results verified by numerical simulation and experiments show that the proposed method can identify damage effectively.