Study On The Propagation Characteristics Of Laser-induced Lamb Waves In (Quasi-) Periodic Structure Plates

The Laser-generated Lamb waves have been widely used in the material defect detection and performance evaluation with the benefit of non-contact excitation, broadband, multi-mode and low transmission energy consumption. Recently, the properties of band gaps, defect states and negative refractive for Lamb waves in the periodic elastic composite structure have been given more attention. It has great potential application in designing high performance microstructure acoustic filter, resonating cavity and confocal imaging device. In this paper, the properties of band gaps and defect states for Laser-generated Lamb waves propagating in the (quasi-)periodic elastic composite plates are studied by numerical simulation and experimental detection.A quasi-one-dimensional two-component periodic plate is constructed by embedding a row of circular air holes in a pure plate with the axis of periodicity parallel to the surfaces of the plate. In this structure, obvious lamb wave band gaps are found by numerical calculation and then detected based on the laser ultrasonic technique in the subsequent experiment. The further study find that abundant Lamb wave partial band gaps can be obtained in a periodic plate with double layer rectangular air holes lining on both sides of its mirror plane. Moreover, when the Laser-generated Lamb waves propagate in a periodic composite plate which is established by embedding a row of air holes periodically in the matrix components on one side of a two-component periodic plate, the number and width of the Lamb wave band gaps increase significantly because that the scattering effect becomes more significant. The results also show that, the width and position of the Lamb wave band gaps are closely related to the structure parameters of the periodic plates described above. In particular, when the Laser-generated Lamb waves propagate in a multiple heterostructures which consist of several pieces of periodic plate with different period, the band gaps of different components overlap together and more wider and more abundant band gaps can be obtained in the same frequency ranges.An asymmetric periodic plate is constructed by engraving a series of rectangular grooves in the pure plate surface. The influence of the structure parameters on the Lamb wave band gap structure is studied by numerical simulation and experimental method. The study find that Lamb wave band gap reflects the local resonance properties when the groove depth reaches a certain value. In particular, when the rectangular grooves have a double-layer structure, the coupling effect between resonance mode and plate mode is gradually strengthened with the increase of the width difference between two layers. Thus the local resonance characteristic of Lamb wave band gaps become more obvious. The band gap shifts to the lower frequency. Some new band gaps occur in the low frequency range.An one-dimensional quasi-periodic composite plate containing one cavity defect is designed. A band splitting due to the resonant tunneling effect for the Laser-generated Lamb wave propagating in the structure is discovered. The crucial parameter, i.e., the ratio of the length of cavity defect to the lattice spacing is presented for the decision of the number and location of the resonance peaks. The further study find that when Laser-generated Lamb waves propagate in a one-dimensional quasi-periodic plate containing two or more cavity defects, the number of the resonance peaks in the first band gap is equal to the sum of the number of the resonance peaks when these cavity defects exist alone, which shows a perfect additive effect. Based on this, the physical mechanisms of the multi-splitting and self-similarity of band gap structures in one-dimensional quasi-periodic plates of Cantor series are proposed..The results of this paper may provide theoretical and experimental basis for the transmission properties of Laser-generated Lamb waves propagating in the periodic elastic composite structure, and may accelerate the further design, development and application of various Lamb wave functional devices.