Research On Typical Structure Detection Technology Based On Time-reversal Lamb Waves

One of the most beneficial characteristics of ultrasonic guided waves is that theycan propagate a relatively long distance with low attenuation. Ultrasonic guidedtechniques have been widely used in damage detection of pipes and large plate-likestructures, and have become an effective detection method in engineering practice.However, guided waves still cannot replace conventional ultrasonic nondestructivetesting methods in some instances, which include the detection of small defects inthick-walled structures, and baseline-free defect detection, etc. In this dissertation, intwo types of experimental subjects including thick-walled pipes and composite plates,wave energy focusing, signal enhancement, and baseline-free defect detection andlocation are studied by the use of the combination of time-reversal theory andultrasonic Lamb waves.The main research contents are given as follows.(1) In view of inherent propagation characteristics of Lamb waves, such asdispersion and multimode, the interaction law of Lamb waves and time reversal isanalyzed. Moreover, boundary reflection and multipath effects of Lamb waves areinvestigated. Based on the theoretical and literature research, the effects of timereversal are summarized as focus and reconstruction, of which the differences andrelations are discussed. At last, the time-reversal based focusing method and thetime-reversal based reconstruction method are utilized in thick-walled pipeline andcomposite plates inspection respectively.(2) Combined with VISA function library, LabVIEW platform is used to developcontrolling software of a defect testing system which is composed of severalinstruments and one host computer. The software makes the instruments cooperatewith each other easily and achieves some functions including signal generation,acquisition, procession and storage, and as a result improves the efficiency ofultrasonic testing using time reversal method.(3) This dissertation proposes a multichannel time reversal focusing (MTRF)method based on circumferential Lamb waves, which is used to detect defects on bothinner and outer side of the thick-walled pipe. Due to the focusing effects of signal packets both within single channel and among multiple channels, the amplitude ofdefect scattered signal has been significantly improved. As a result, defect detection inthick-walled and large-diameter pipe has become much easier, especially for thesesmall ones on the inner surface of the pipe. Besides, another apparent focus takesplace before the defect scattered wave packet, which corresponds to signalpropagating through direct path. Accurate localization of defects has been achieved byusing the time information of the two focuses and group velocity of circumferentialLamb waves.(4) A rule about the focus position of time reversal signal is summarized, and anew method of calculating the time reversal damage index (DI) is developed. Theinfluence of parameter β in the weighted distribution function to the effective range ofsensing path is studied. For delamination in composite plate, the optimal β value isdetermined by experimental study. Based on the study of the conventional imagingalgorithm, the addition based imaging algorithm is improved, and a multiplicationbased time-reversal weighted distribution imaging method is proposed. Baseline freedetection and imaging of delamination at different locations is achieved by using thetwo imaging algorithm.