Analysis Of Ultrasonic Lamb Wave Properties And Research On Detection And Imaging Method For Plate Structure Damage

Plate metal structure is widely used in industrial and civil facilities fields.In the long-term service,the service environment will make the plate structure aging,reduce its bearing capacity and service life for the different degrees of damage.Microcosmic damage and macroscopic damage are two evolution processes of plate structure damage.The nondestructive damage detection and evaluation of structures need to be carried out by the ultrasonic or other ways.The microcosmic damage of structure occurs due to external forces and other factors,and then gradually forms macroscopic damage until structural failure.Nonlinear ultrasound is sensitive for detection and evaluation of early stage damages such as closed microcracks,while linear ultrasound is suitable for detection and imaging of macro damages.Lamb wave as a guided wave has the advantages of long propagation distance and low attenuation,which is suitable for the damage detection of plate structures.However,due to its dispersive and multi-mode characteristics,it is difficult to study the propagation mechanism of the interaction between Lamb wave and damage,and it is impossible to use a unified model to describe its physical process.Moreover,the multi-mode lamb signal wave packet is easy to be overlapped and deformed,and it is difficult to extract damage characteristics information,then affect the damage location and imaging results.Therefore,through theoretical model analysis,simulation and experimental method,taking the isotropic aluminum plate commonly used in engineering as the test object,this paper studies the influence of plate structure damage on the linear and nonlinear characteristics of Lamb wave propagation,mode conversion characteristics,and effective detection methods for damage location and imaging.The main research contents are as follows:(1)The nonlinear ultrasonic effect of closed microcracks in plates is studied.The relationship between the ultrasonic nonlinear coefficient and the size of closed microcracks is derived through analyzing the contact nonlinear ultrasonic theoretical model.On this basis,the finite element mesh model of closed microcracks in aluminum plate is established by means of finite element modeling and simulation analysis.By analyzing the dispersion curve of the material's nonlinear matching,the S1 mode Lamb wave is excited in the plate by the wave structure loading method,and analysed the second harmonic component in the received signal.The cumulative effect of second harmonic amplitude varying with propagation distance is obtained by simulation calculation.And the influence of closed microcracks with different length and width on the nonlinear coefficient is discussed.The validity of the theoretical model is verified.(2)The effects of macro cracks on the modes and characteristic parameters of linear Lamb waves are studied.By means of finite element modeling and simulation analysis,the finite element mesh models of various macro cracks in aluminum plate are established.The single S0 or A0 mode Lamb wave is excited in the plate by the wave structure loading method.The propagation time and amplitude of each wave packet in the received signal are analyzed to study the phenomenon of mode transformation and the relationship between crack size and signal amplitude.The simulation results show that there is mode conversion when the single mode Lamb wave interacts with an asymmetric crack,and no mode conversion with a symmetric crack.The energy of reflected wave increases with the increase of the depth and width of crack,and the energy of transmitted wave decreases correspondingly.The size and inclined angle of crack can be quantitatively estimated by the fitting function from the energies of reflected and transmitted waves.The variation of crack depth,width and angle is approximately linear with the amplitudes of reflected and transmitted wave packets.A fitting function is constructed by using the amplitude ratio of reflected wave packet or transmitted wave packet to total wave packet,which can be used to estimate the crack depth,width and angle.(3)The Lamb wave detection and imaging method based on the Compressive Sensing is studied.In this method,the plate structure is meshed,and the over complete dictionary is constructed based on the scattering signal of Lamb wave when there is damage in each grid.The measurement signal vector is composed of cascaded differential scattered signals of multiple transmit-receive sensor pairs,and sparse representation method is used to update the dictionary and reconstruct the signal,so as to locate and image the damage.An adaptive sparse reconstruction(ASR)algorithm is proposed to solve the problems that the number of damages is unknown.The greedy algorithm algorithm is used to select the atoms that may contain damage,and take the estimated value of signal sparsity as the initial step size of the algorithm.The step size is gradually adjusted by the energy difference between adjacent reconstruction signals to construct a new dictionary.Then the new dictionary is reconstructed by the basis pursuit denoising algorithm for damage localization and imaging.Simulation analysis and experimental verification results show that the ASR algorithm has good performance in the reconstruction efficiency,damage identification rate and reconstruction residual under different signal-to-noise ratios,which is able to achieve the localization and imaging of multiple damages with less artifacts.(4)The damage detection and imaging technology of plate structure using Lamb wave time reversal method is studied,and a damage probability imaging algorithm based on Lamb wave time reversal without reference signal(BFTRPI)is proposed.A transmitter-receiver network with sparse distribution is constructed on the plate,and the received signal from each sensing path is reversed and focused by the time reversal method.A damage index is calculated by the difference between the focused signal and excited signal.And a damage probability distribution function is defined by the damage index and the relative distance between the damage and the sensing path.The damage location is estimated by the focused signal from all sensing paths after damage probability weighted.The experimental results show that the BFTRPI algorithm can effectively extract the location,size and other information of damage,improve the accuracy of damage location and imaging quality,and its performance is better than the existing similar imaging methods.