| Plate-like structures are widely used in various devices and structures.During service,these structures are subjected to external environmental and loading effects,which can cause early microscopic damage such as fatigue cracks and corrosion.With the accumulation of structural damage,catastrophic accidents can easily occur.Therefore,it is urgent to develop new methods for high-precision and high-resolution damage detection/structural health monitoring of such structures.Lamb waves,due to their advantages of wide detection range and sensitivity to different types of damage,have become one of the most promising damage detection/structural health monitoring technologies.Currently,the Lamb wave-based damage imaging detection method has been widely studied.However,the analysis of the damage information contained in propagating Lamb waves alone cannot achieve super-resolution imaging of sub-wavelength damage information carried by evanescent Lamb waves.Therefore,the complex dispersion characteristics of evanescent Lamb waves in plate-like structures,the propagation and conversion mechanism of evanescent Lamb waves,and the further extraction of sub-wavelength damage information for super-resolution imaging are studied,which provides a theoretical basis for the formation of highresolution damage detection/monitoring methods.The main research content and innovation points of this article are as follows:1.The complex reciprocity relationship and complex power flow theory of Lamb waves in plate structures were studied.Based on the propagation theory and complex dispersion curve of Lamb waves,the characteristics of propagating Lamb waves and non-propagating evanescent Lamb waves were investigated.Furthermore,the power of propagating and evanescent Lamb waves in the plate was studied,providing a theoretical basis for analyzing the generation of evanescent Lamb waves and their conversion into propagating Lamb waves in discontinuous plate structures.2.The generation and conversion characteristics of evanescent Lamb waves in discontinuous plate structures were studied.Firstly,based on the complex reciprocity relationship and orthogonal relationship of Lamb waves,a single-mode A1 evanescent Lamb wave and propagating Lamb wave were successfully excited in the plate structure,which was verified through finite element numerical simulation.Secondly,the propagation characteristics of A1 evanescent Lamb waves in the discontinuous interface plate structure and the conversion mechanism of evanescent Lamb waves into propagating Lamb waves were studied.Finally,the differences and conversion efficiency between different excitation modes were quantitatively studied from the perspective of complex power flow(energy).3.The complex dispersion curves of Lamb waves were studied based on the finite element eigenfrequency frequency method.Firstly,based on the eigenfrequency method etc,the dispersion relation of Lamb waves that generate superposition in the irreducible Brillouin zone is obtained.Then the wave number of Lamb wave is obtained by using displacement expansion method and Fourier transform for each mode.Further,the out of plane displacement amplitude information corresponding to each mode is obtained,and finally the complex dispersion curve of the plate structure containing the out of plane amplitude information is obtained.On this basis,the complex dispersion curve of the discontinuous plate structure is studied,so that the mechanism of the propagation and transformation of evanescent Lamb wave propagation can be analyzed.In summary,the study of the propagation and transformation mechanism of the evanescent Lamb wave in plate structures provides a theoretical foundation for the development of highresolution damage detection/monitoring methods.The investigation of Lamb wave complex dispersion curves in plate structures based on the finite element eigenfrequency method can offer a new approach for selecting Lamb wave detection frequencies and analyzing vibration modes in damage detection.Furthermore,this method can be applied to complex periodic structures and has practical value for Lamb wave research in such structures. |
