Structure Design And Optimization Of Electromagnetic Acoustic Transducer For Aluminum Plate Defect Detection

With the large-scale construction of infrastructure,aluminum plate is more and more widely used in national defense and industry by virtue of its excellent characteristics.In the production and use of aluminum sheet,affected by initial defects,environmental erosion and external factors,it is easy to lead to the destruction of components or defects,thus affecting the mechanical strength and stability of the whole structure,so how to effectively detect the quality of aluminum plate is extremely significant.Electromagnetic acoustic testing(EMAT)is a newly-developing nondestructive testing technology,which excites acoustic waves in the structure through electromagnetic coupling.Compared with the traditional piezoelectric acoustic testing technology,EMAT has the superiorities of non-contact,various kinds of excitation waveform,etc.The application of EMAT in industrial nondestructive testing is limited by its low efficiency and complex energy transducer.To solve these problems,a new type of EMAT is designed and optimized in this thesis,aiming at improving the amplitude of acoustic echo signal,and it is applied to the positioning and quantitative analysis of different defect types and sizes.The main research contents of this thesis are as follows:Firstly,the finite element model of the surface wave EMAT is established,and the distribution of magnetic induction intensity,induced eddy current and Lorentz force in the coupling process of electromagnetic and acoustic is analyzed.The time and frequency domain analysis of surface wave is carried out,and the propagation characteristics and the optimum detection frequency are obtained.The parameters of EMAT are optimized by orthogonal test.The optimal number of excitation cycles is analyzed and obtained.To solve the problem of weak surface wave echo signal,a soft magnetic alloy strip is proposed to optimize the magnetic circuit of surface wave EMAT.The simulation results show that the addition of soft magnetic alloy strip can effectively improve the magnetic circuit of EMAT and enhance the signal intensity of surface wave.Then,considering the Halbach array principle of permanent magnet has a ascendant “unilateral effect”,it can concentrate the magnetic line on one side of the magnet and weaken the magnetic field strength on the other side,but the research about the Halbach array principle of permanent magnet structure applied to excite electromagnetic acoustic surface wave is still lacking.Therefore,in this thesis,the EMAT structure of Halbach array permanent magnet and parallel split coil is designed,so that the excited surface wave signal intensity can be significantly improved.Finally,based on the above work,four kinds of aluminum plate defect models are established based on the common size and shape of aluminum plate defects.In the finite element simulation model,the influence of different factors of defects on the excited acoustic wave,peak value and the surface stress distribution of aluminum plate is compared and analyzed.The influence law of different factors on acoustic reflection and transmission coefficient is summarized,and the determination method of surface and near-surface defects of aluminum plate is obtained based on acoustic signal reflection and transmission coefficient.