Nonlinear Lamb Wave Harmonic Feature Based Research On The Identification Of Cracks In Plate Structures

Most of the main load-bearing parts such as wings and fuselage are made of plate structure,which is very prone to fatigue cracks during service.If the fatigue cracks are not detected in time,they will expand rapidly under external loads,leading to structural failure and thus causing catastrophic accidents.The scattering signal is extremely weak when using the traditional linear ultrasonic Lamb wave detection,especially when the incident wave direction is the same as the crack direction,it is almost impossible to extract an effective scattering wave packet,which makes it difficult to achieve accurate crack identification.The opening and closing contacts of the two contact interfaces of the breathing crack produce nonlinear modulation effects on Lamb wave propagation,and nonlinear phenomena such as subharmonics and higher harmonics will appear.This nonlinear phenomenon has received wide attention,and its nonlinear harmonic characteristics are gradually utilized for damage detection.In the crack detection process,ultrasonic Lamb waves have complex dispersion characteristics,and the crack and depth directions are generally at different angles,and crack detection and imaging identification become more difficult.In this paper,from the harmonic characteristics of nonlinear Lamb waves acting with breathing cracks,finite element simulation and experiments are used to carry out the study of nonlinear Lamb wave propagation characteristics in aluminum plates containing breathing cracks,and explore the RAPID laminar imaging aluminum plate crack identification method based on nonlinear harmonic coefficients to provide the basic theory for crack detection of plate structures.In this paper,the research focuses on three aspects:（1）Theoretical analysis of the dispersion characteristics of Lamb waves in thin plate structures;based on the theory of bilinear stiffness intrinsic model,the acoustic nonlinearity phenomenon of crack contact is studied considering the rough interface characteristics of breathing cracks,the quantitative relationship between the microscopic interface parameters of breathing cracks and the effective tensile and compressive moduli of cracked solids is established,the nonlinear Lamb wave harmonic coefficients caused by breathing cracks are derived,and the The conditions for the generation of nonlinear Lamb wave second harmonics were analyzed.（2）ABAQUS software was used to establish a finite element simulation model of a three-dimensional aluminum plate containing breathing cracks,and the transient dynamics analysis of ultrasonic Lamb wave propagation in a plate containing breathing cracks was carried out to explore the effects of crack length,depth and incident angle of the excitation wave on the scattering characteristics of nonlinear Lamb wave harmonics;the study investigated the scattering of nonlinear Lamb wave harmonics at different excitation frequencies,incident angles and tilt angles of the circumferential The directional distribution law of the harmonic coefficients of Lamb waves with different excitation frequencies,incidence angles and tilt angles was studied.The results show that the nonlinear Lamb wave interacts with the respiratory crack and undergoes mode conversion,and the nonlinear harmonic coefficient is proportional to the crack length and inversely proportional to the crack depth;the nonlinear harmonic coefficients show an obvious beam directivity pattern on the circumferential scattering.（3）Using the scattering distribution law of nonlinear harmonic coefficients,the research of RAPID laminar imaging crack identification method based on nonlinear harmonic coefficients was carried out,and the influence of circular,rectangular and linear piezoelectric sensor arrays on the crack identification accuracy was investigated.Experiments on RAPID tomography imaging of cracks based on circular sensor arrays were carried out.The results show that the use of nonlinear harmonic coefficients as a characteristic parameter to measure the SDC_tr’value of signal difference coefficient in RAPID tomography can better characterize the distribution probability of cracks and realize the reconstruction and identification of cracks in plate structures,among which the accuracy of crack identification is higher for the circular sensor array.