Experimental Investigation Of Reference-free Damage Diagnosis Of Metal Sheet Based On Active Lamb Wave

Active Lamb wave based structural health monitoring (SHM) has received agreat deal of attention and has been an attractive topic of nondestructive testing(NDT) due to its sensitivity to cracks, holes and delamination in thin plates.Conventional Lamb wave based methods compare baseline signals with those current· signals so that the changes form baseline can be attributed to damages,which is susceptible to varying operational and environmental conditions of plates.A new Lamb wave methodology to detect crack in a thin plate is studied, and it isbased on mode conversion without using prior baseline signals. Numerical andexperimental results are presented to demonstrate the applicability of the proposedtechnique. Main contents as follows:The propagation dispersive equtions of Lamb wave and the modeling of PZTactuating and sensing are analyzed. The effect of PZT poling directionality on Lambwave mode are discussed. A5-cycle sinusoidal narrowband toneburst modulated byHanning window and focused on150kHz frequency was used as an input signal forfurther study in this research. The optimized layout of PZTs and the optimaldistance between patches are discussed.The propagation of Lamb waves in aluminum plates is studied by finiteelement method using ABAQUS. A2-D model of plane strain with displacementsimulations of actuators and sensors were constructed. Selective generation of eitherS0or A0mode is achieved by simultaneous excitation of the two PZTs with in-phaseor out-of-phase input signals. The mode conversion of Lamb waves due to a crackwas analyzed. Pre-set different depths of notches are induced to the aluminumplates as damages, and the effectiveness of the reference-free crack detectionmethod is proved by.numerical studies.A data acquisition system based on virtual instrument is constructed, includingan arbitray waveform generator and a data acquisition unit. Original signals aredenoised by wavelet decomposition and reconstruction. Experimental studies onexcitation and acquisition of Lamb waves are carried. Then the PZT wafers areexcited by input signals modulated by different window function, and the receptionsignals are analyzed in time domain and frequency domain by comparing intensity and band width of the signals. The effectiveness of reference-free crack detectionmethod is validated experimentally by introducing notches on the aluminum plates.