Study On Lamb Wave Mode Selection Based On Electromagnetic Ultrasonic Transducer

With the rapid development of industry,aluminum and its alloys are widely used in industrial production due to their advantages of low density,corrosion resistance,strength,good mechanical properties and easy processing.The structure of these products will change and be damaged with the increase of the use time and the neglect of the initial manufacture,so the nondestructive testing of the metal sheet is required.Electromagnetic ultrasonic sensor detection technology is a relatively new ultrasonic detection method,compared with the traditional piezoelectric ultrasonic,electromagnetic ultrasonic sensor technology has the advantages of noncontact,no coupling agent and so on,is widely used in the plate defect detection.In this paper,the electromagnetic ultrasonic Lamb wave of the modal characteristics and Lamb wave interaction with defects and mode conversion phenomena are studied,using ANSYS simulation method,the modal and inspire a single S0 A0 mode to eliminate the influence of the modal characteristics of aluminum plate detection,on this basis,by stimulating single Lamb wave interaction with defects,further analysis the influencing factors of the modal conversion.In this paper,ANSYS finite element simulation analysis is used to study the excitation method of a single mode of electromagnetic ultrasonic transducer under the condition that there are only A0 and S0 in the low-frequency thickness product.Lorentz force is applied on the upper and lower surfaces of the aluminum plate respectively to excite the single Lamb wave mode.The results show that the single mode can be generated by controlling the Lorentz force applied on the upper and lower surface of the aluminum plate.The single S0 mode can be excited by applying the same in-plane Lorentz force or the opposite out-plane Lorentz force on the upper and lower surface of the aluminum plate respectively,and the single S0 mode excited by applying the same in-plane Lorentz force has the best effect.A single A0 mode can be excited by applying opposite inner lorentz force on the upper and lower surface of aluminum plate or the same outward Lorentz force.The modal transformation phenomenon of Lamb wave and defect is also studied in this paper.Under the excitation of single S0 and A0 modes of Lamb wave,the relationship between Lamb wave and defect shape,defect position change(in the direction of thickness)and defect depth change was studied.The results show that the modal transformation is affected by the shape,position and depth of the defect,and the degree of modal transformation is inversely proportional to the degree of symmetry of the defect on the horizontal center line of the plate.In addition,the modal transformation of triangular prism defects in plates with fixed bottom,fixed height and different top angles is studied.The simulation results show that the degree of modal transformation is affected by the degree of left-right symmetry of the defect,and the two are inversely proportional.In this paper,the excitation of single Lamb wave is realized by controlling the lorentz force applied on the upper and lower surface of aluminum plate,so as to eliminate the negative influence brought by the multimode of Lamb wave on the detection results and improve the accuracy of detection.The study on the modal conversion rule of the interaction between Lamb wave and defect provides support for the analysis of received signal,which is of far-reaching significance for the application of electromagnetic ultrasonic transducer in engineering.