Research On Imaging Of Damage In Isotropic Plate Using Tfm Based On Frequency-domain Inverse Scattering Model Of Lamb Wave

Ultrasonic Lamb wave detection technology has the advantages of long propagation distance along the structure,small attenuation and suitable for large range detection,so it has great potential for the detection and safety evaluation of large metal plate structure damage.A variety of imaging methods based on Lamb waves,including offset,time reversal and phased array,have been able to locate and image damage in plate-like structures.But the existing imaging methods are often based on the array single transmitter and multiple receiver detection mode,so the acquisition of array signals can only provide limited information about the space of damage,resulting in limited accuracy of imaging results.And many imaging methods ignore the interaction between incident wave and damage only according to the imaging index of reflection wave amplitude,so it is difficult to reflect the physical characteristics of damage scattering and further affect the imaging accuracy.Aiming at the above two problems,this paper adopts the Total Focusing Method(TFM)based on full matrix acquisition(multiple transmitter and multiple receiver detection mode),so that each element in the array is excited and received to collect more spatial wave field information and improve the accuracy of damage imaging.Taking the reflectivity which includes physical parameter of damage characteristic as the imaging index,the damage inverse scattering model is established,and the imaging detection accuracy of damage in plate structure is further improved.The work is as follows:(1)On the basis of ultrasonic Lamb wave theory,the multi-mode and dispersion characteristics of Lamb wave are analyzed,and the dispersion equation in isotropic plate structure is deduced.To analyze the lamb wave structure which is in isotropic plate and the central frequency and mode of excitation signal are determined in this paper.Array beam virtual focusing theory is introduced to provide a theoretical basis for subsequent TFM imaging.(2)The frequency domain TFM imaging method(including dispersion relation)is developed based on TFM imaging method and frequency domain virtual focusing theory,which avoids the inherent dispersion characteristics of ultrasonic Lamb and improves the accuracy and imaging accuracy of damage location.The relationship between incident sound field and scattered sound field is analyzed based on the whole propagation process of sound wave from driving array to damage and returning to sensor array after damage.The scattering model of received displacement about the physical parameters(reflectivity)of damaged surface is established by reciprocity theorem.Then inverse Fourier transform is used to solve reflectivity and develop inverse scattering TFM imaging method in frequency domain.(3)The numerical model of wave excitation,propagation and interaction with damage in Lamb structure containing single or double damage metal plate is established based on COMSOL finite element simulation software.Damage scattering signal is extracted by active Lamb wave time domain filter without reference signal.The frequency domain inverse scattering TFM method was used to perform damage imaging and the imaging results were evaluated in terms of damage localization accuracy and array performance index(API).(4)To construct an experimental platform for damage detection of aluminum plate structure,and to investigate the quality improvement effect of inverse scattering TFM imaging method in frequency domain.Experimental results show that the TFM imaging method based on full matrix acquisition makes full use of each element in the array,so that each element is excited to receive the information containing damage characteristics in many directions,and the imaging accuracy is improved.A dispersion relation is included in the calculation,which can avoid the dispersion characteristics of Lamb waves and improve the accuracy of damage location and imaging.The damage inverse scattering model with damage surface reflectivity as imaging index can further characterize the damage and improve the imaging accuracy.If the detection environment is single damage,The inverse scattering TFM method in frequency domain reduces the API value of damage imaging by 9.75(the effective area of damage is reduced by 44.06%);Compared with the phased array beamforming damage imaging method,Damage imaging API value decreased by11.29(damage effective area decreased by 47.70%).For the detection of multiple environmental damage,Frequency domain inverse scattering TFM method still maintains high precision,compared to the time domain TFM,the API values of the two damage imaging were respectively reduced by 9.73 and 10.31(the effective area of the damage was respectively reduced by 44.07% and 46.54%);Relative to the array beamforming method,API values of both damage imaging were respectively reduced by 11.23 and 7.54(the effective area of damage was respectively reduced by 47.63% and 38.91%).According to the above results,the frequencydomain inverse scattering TFM is more accurate.A frequency domain inverse scattering TFM imaging method developed in this paper which achieves imaging detection of damage in isotropic plate structures.According to the results,it is expected to provide a new method for the study of damage detection in composite materials,and also to provide a theoretical and methodological basis for practical engineering application based on ultrasonic Lamb damage detection.