Numerical And Experimental Investigation Of Multimode Lamb Wave Propagation Properties

Lamb wave sensors are widely used in ultrasonic nondestructive testing of infinite plates because of their advantages of fast propagation speed,long propagation distance,high sensitivity and high detection efficiency.However,Lamb wave has the characteristics of dispersion and multi-mode in the propagation process.At least two modes of Lamb wave exist at the same excitation frequency.The group velocity and phase velocity of different modes are constantly changing within the excitation bandwidth,which makes the analysis and interpretation of Lamb wave propagation signals complicated.The lamb wave nondestructive testing usually excites the lamb wave of a single mode to study its propagation characteristics,but single mode damage detection has some limitations.In order to achieve more accurate detection of steel plate defects,,it is necessary to excite multi-mode Lamb waves to study their propagation characteristics.In this paper,numerical and experimental studies of the propagation characteristics of multi-mode Lamb waves are carried out as follows:Firstly,the theoretical propagation characteristics of Lamb waves are introduced.The propagation of Lamb wave in steel plate is simulated by finite element method,focusing on the influence of the parameters selection of the excitation signal on the propagation of Lamb waves.According to the numerical analysis results,the mode characteristics of lamb wave signal in plate under unilateral,symmetric and anti-symmetric excitation are investigated.The effects of different plate thickness and propagation distances on the group velocity dispersion curves are discussed.It is found that there is a certain error between the simulated group velocity and the theoretical group velocity.With the increase of excitation frequency,the amplitude of wave peaks of both symmetric and anti-symmetric modes is decreasing,and the amplitude magnitudes of the two modes are different at different frequency thickness.Then the excitation frequency is increased to study the effect of different wave crest numbers on the propagating signal under high frequency excitation.The results show that the larger the wave crest number,the narrower the bandwidth of the excitation signal,the smaller the collected signal dispersion and the more complete the waveform information.At the same time,finite elements are used to simulate the excitation and propagation of multi-mode Lamb waves under high-frequency excitation.The S₀ mode can be directly separated at 2.5MHz with unilateral excitation and the characteristic information of the less mixed A₀ and A₁ mode signals can be extracted.According to the amplitude change characteristics of different modes,it is found that the smaller the group velocity,the faster the amplitude decay rate.The consistency of the measured and simulated signal variation was verified by using the measured S₀ mode Lamb wave signal.Compared with the measured and simulated group velocity,the measured group velocity is closer to the theoretical group velocity.At last,for defect the steel plate detection,a finite element model of the defective steel plate was established.The influence of different locations of defect on the received signal is discussed.The reflection and mode conversion characteristics of symmetric and anti-symmetric modes at the defect are analyzed.The relationship between the anti-symmetric mode Lamb wave defects response signal and different defect types,different buried defect depths,different defect directions and different defect sizes under different excitation modes is investigated.The study found that the response of the anti-symmetric mode is more sensitive to the defects than the symmetric mode excitation,which provides a theoretical basis for the defect detection of the measured multi-mode Lamb wave.