Research On Lamb Wave Time Reversal Technique And Its Applications In Structural Health Monitoring

Lamb wave based active diagnosis method is one of the most effective Structural Health Monitoring (SHM) techniques due to its sensitivity to small defects in plate-like structures, such as crack and composite delamination. However, the analysis and interpretation of Lamb waves are complicated due to their dispersive and multimodal natures. Because of the complicated structures, especially the composite, it is difficult to analyze the structural character and its damage. There also existed some problems in the applications of the Lamb wave based diagnosis method, including low signal-to-noise rate, simple damage information detection and low monitoring effective. Time reversal focalization of Lamb wave is researched in this paper. Improvements have been done for the Lamb wave based active diagnosis method based on the time reversal technique. Main contents as follows: Firstly, it was summarized on research background and significance of SHM and Lamb wave based active diagnosis method. And research direction was confirmed;Secondly, the propagation dispersive equations of Lamb wave and the modeling of PZT actuating and sensing have been analyzed using elasticity theory. After that, the principle of time reversal theory was discussed, and the processes of Lamb wave time reversal focusing with PZT elements were analyzed and researched experimentally;Thirdly, directly time reversal imaging technology was proposed for active Lamb wave method based on time reversal focusing. This method was compared with synthetic time-reversal imaging method in detail. Wave rebuilding method was proposed to overcome Lamb wave modes confusing which caused low precision of the damage image. In the presented method, narrower pulse was used to rebuild the sensing signals;Fourthly, signal enhancing methods based on time reversal theory were researched and proposed for signal peak value based and damage scattering signal based damage monitoring methods. Implementary details about the methods were given and validated by experiments.Fifthly, because active Lamb wave monitoring method based on reference signal had low practicability, baseline free damage imaging method was researched and proposed. New double elements PZT array and time window function were presented to intercept structural inner scattering signals from sensing. Synthetic time-reversal imaging method was adopted to detect and show the damages. After that, baseline free damage monitoring for composite plate with stiffeners was presented.Sixthly, during the fatigue tests of UAV wing box structure, application research works of Lamb wave based active diagnosis method were done on the box. PZT layer was designed to normalize the electric character, wiring and setting of the PZT array. Time reversal focusing method for signal-to-noise improvement was validated in the bolts loosing monitoring experiments.Lastly, summary of the whole works was drawn. The problems meted in the research and the continuing directions were presented.