Research On Laser-excited Zero Group Velocity Lamb Waves In Liquid-loaded Thin Plates

Plate-like structure is a common material structure which is widely used in the production and living areas.As an important category of ultrasonic wave in solid,ultrasonic Lamb wave is used for nondestructive testing of plate structure.When there is a liquid load,the propagation of Lamb waves in the plate is complicated.In this paper,the cause of the zero group velocity Lamb wave,the effect of liquid loading and the nature of liquid loading change on zero group velocity of Lamb wave have carried on the theoretical analysis and experimental verification.First,by using the finite element method,we have calculated the Lamb wave dispersion characteristic curve in the free board.Through analyzing the calculation result,it illuminates that the two modes which are close to the cut-off frequency have caused the S1 backward wave and forward wave coupled to make the group velocity vanish.Secondly,in order to obtain the Lamb wave dispersion characteristic curve in the liquid load plate,the theoretical model of Lamb wave in the liquid load plate is established based on the wave equation.The sheet as made up by countless same unit structure of periodic structure,we selected one of the unit structure,using the finite element method to get the Lamb wave dispersion characteristic curve,and calculated the Lamb wave spectrum to verify the reliability of the results.Through the analysis,we expounded that:when liquid loading appears,the zero group velocity points will change.In this paper,the pulse laser is used to excite the zero group velocity in the free aluminum plate and the liquid load aluminum plate.The experimental results show that the existence of the liquid load will change the frequency position of the zero group velocity points.This result provides a basis for further study.Finally,we changed the properties of fluid loading and found that when there is an asymmetric liquid load,the symmetrical of the distribution of the particle displacement in the plate is changed at the frequency of the zero group velocity point.When the thickness of the liquid layer increases,the Lamb wave in the plate has the tendency to generate more zero group velocity points.When the density of liquid is increased,the coupling effect between the liquid longitudinal wave and the Lamb wave in the plate increases,and the larger the velocity of the liquid longitudinal wave,the less the influence of the change of the velocity point of the Lamb wave zero group in the plate.The results of this paper provide theoretical basis and reference for the detection of liquid loading sheet.