Time-reversal Damage Detection In F-k Domain Using Laser Ultrasonic Lamb Wave Based On Multiple Frequency Bands And Temporal Filtering Method

Ultrasonic Lamb wave detection technology is widely used in the damage testing of metal plate,because it has advantages of wide spread distance,small energy attenuation,which is suitable for wide range detection.However,when using piezoelectric transducers to motivate and pick up ultrasonic Lamb wave signals containing damage scattered signals,the spatial resolution is limited by the distribution scale of the transducer array,which will affect testing results of the structural damage.Laser ultrasonic detection technology is based on the thermoplastic effect to generate Lamb wave in plate structure,so that the signal can be excited in the form of absolute non-contact,making it a hot topic in the field of non-destructive testing.In this paper,a new time reversal method in frequency and wavenumber domain(f-k)for damage detection of aluminium plate structure by simulation and experiment is then studied.The main works is as follows:Firstly,the background and significance of laser ultrasonic testing techniques is introduced and the development trend of laser ultrasonic Lamb wave detection technology is discussed.Through theoretical analysis of ultrasonic Lamb wave,the propagation characteristics and dispersion relation of Lamb wave are clarified.Some methods of signal procession are investigated and discussed.Finally,the research direction and content of the project are determined.Secondly,the laser ultrasonic model of three-dimensional damaged plate are constructed using COMSOL finite element analysis software.Based on the theory of delay and sum,an array beamforming damage imaging method was established to achieve rapid location of damage.Since the array beamforming imaging precision and range are general,thus,a time reversal method in f-k domain based on the temporal filtering method with frequency spectral decomposition is proposed.Continuous wavelet transform is applied to extract a narrow frequency band information to eliminate the effect of strong dispersion caused by the wide frequency band of laser pulse signal,then the adaptive temporal filtering method is adopted to extract the scattering signal without reference signal.By investigating the frequency response function and back propagation process during the time reversal process,the time reversal focusing characteristics of the narrow band and broadband signal are verified.Furthermore,based on the f-k time reversal imaging method,the reconstruction of incident wavefield and scattered wavefield are reconstructed to compensate for the dispersion effect,and the cross-correlation imaging condition is introduced to identify the damage boundary precisely in the plate structure.Then,the imaging results of each narrowband frequency components from the laser ultrasonic broadband signal are added up to improve the accuracy of damage imaging and the signal-to-noise ratio.Thirdly,a fully non-contact laser ultrasonic detection system is designed by combining YAG pulsed laser with Scanning Laser Doppler Vibrometry(SLDV),which can receive signals with high space resolution.The f-k TRM damage imaging technique is used to visualize the measured signal to reconstruct the back propagation process of Lamb wave.The cross-correlation imaging condition is introduced to image the through holes of different sizes,and the localized normalized amplitudes are also used to discuss the gradient trend.At the same time,the imaging results of double damages of through holes and cracks are given experimentally to verify the effectivenesss of this method in identifying the scattering boundary of multiple small damages.The results are compared to verify that the temporal filtering method with frequency spectral decomposition and the f-k TRM damage imaging technique can accurately and effectively locate the damage in the aluminium plate and recognize the damage boundary of different sizes and different types of damages,which has the potency to get wider applications in engineering detection practice.The last part summarizes the research work in this paper and point out further research directions.