Study On Zero Group Velocity Temperature Correlation And Energy Conversion Of S₀ Mode Of Lamb Wave In Thin Plate

Laser ultrasonic nondestructive testing technology can be used to detect plate materials in high temperature environment and defective plate materials.In this paper,the influence of temperature on zero-group-velocity（ZGV）Lamb wave in aluminum alloy plate and the energy conversion of S₀ mode in defective plate are studied from numerical calculation and experiment.In this paper,aluminum alloy plate is regarded as a periodic element structure and one of the elements is obtained,the dispersion curve of Lamb wave in aluminum alloy plate is calculated by finite element method,and the reason for the existence of ZGV Lamb wave is explained.Then,the influence of temperature on the frequency of ZGV Lamb wave in aluminum alloy plate is theoretically analyzed,and the reasons are found as follows:the thickness d,poisson’s ratio?and shear wave velocity _Sc of aluminum alloy plate are changed by temperature,so that the frequency of ZGV Lamb is changed.An experimental system is established for the all-optical detection and excitation of S₁-ZGV mode on the upper surface of aluminum alloy sheet at different temperatures using a pulsed laser based on Doppler vibrometer.The relation between frequency change of S₁-ZGV mode and temperature is obtained.When the temperature increases from 20℃to 370℃,the experimental results show that the frequency of S₁-ZGVmode is reduced by 10.4%,the theoretical calculation results show that the frequency of the S₁-ZGV mode is reduced by10.9%.The experimental results agree well with the theoretical ones.Theoretically,the influences of thickness d,poisson’s ratio?and shear wave velocity _Sc on the frequency of ZGV Lamb wave are analyzed respectively.The results show that when the temperature increases from 20℃to 370℃,the shear wave velocity has the greatest influence on the frequency of ZGV Lamb wave,while the thickness has the least influence on the frequency of ZGV Lamb wave.The model of single nondispersive S₀ mode induced by the action of equivalent impulse force on the side of defective thin aluminum plate is established.The time domain waveform detected at the position between the side of force source action and defect shows that the normal displacement of reflection A₀ mode converted by S₀ mode at the defect（S₀-r A₀）is larger than the normal displacement of incident S₀ mode.The energy of the S₀-r A₀ mode normal component,incident S₀ normal component and tangential component at the defect is further calculated.The results show that the S₀-r A₀ mode normal component at the defect is mainly converted from the incident S₀ tangential component.The research results of this paper can provide some reference and basis for the related research in the future.