| Aluminum plate is one of the important industrial raw materitals,but it is easy to produce defects in the process of production,which needs to be tested using Non-destructive testing to detect the defective products in time.Electromagnetic acoustic SH(Shear Horizontal)guided wave detection technology has great advantages in the detection of aluminum plate defects with its non-contact and simple dispersion characteristics.However,there are some shortcomings on the theoretical studies of SH guided wave electromagnetic acoustic transducer(EMAT).The acoustic field distribution is not clear,and it is difficult to solve by analytical method,which leads to difficulty in guiding the design of EMAT.Besides,the conventional methods can only find the approximate location of the defect,but cannot ascertain the type and size of the defect.In addition,SH guided wave EMAT has low energy conversion efficiency,which further limits the promotion and application of the technology.In this paper,to solve the problems above,the aluminum plate defect detection technology of electromagnetic acoustic SH guided wave will be studied.To solve the problem that the acoustic field distribution is not clear and it is difficult to solve by analytical method,a three-dimensional frequency domain FE model of SH guided wave EMAT is proposed,and the acoustic field direction angle is 16.7°.On this basis,the influence of EMAT geometrical parameters such as coil diameter,magnet spacing and number on the acoustic filed directivity is further studied,which is convenient to determine the optimal parameters,so as to guide the design of EMAT.To solve the problem that the conventional methods cannot realize the qualitative and quantitative detection of defects,the circumferential scatter diagram obtained by experiment is compared with the simulation results,and then the defect type and size are determined according to the similarity of the circumferential scatter diagram.For this purpose,a 3-D time domain simulation model of electromagnetic acoustic SH guided wave is proposed.Through the simulation,the circumferential scatter diagrams of SH guided wave with through-hole and groove defects are obtained.The simulation results show that when the size of the defect increases,the energy of the reflected wave increases and the transmitted wave energy decreases.To make up the low energy conversion efficiency of SH guided wave EMAT,an electromagnetic acoustic SH guided wave detection system is designed.A transmitting circuit is designed to output a Tone Burst signal with a maxium power of 2.5k W for exciting the SH guided wave EMAT.And the upper computer software is designed to realize the pulse parameter control.A four-channel receiving circuit is designed to amplify and filter the receiving signal for the convenience of result analysis,and the maximum amplification gain is up to 80 d B.Besides,the transmitting and receiving SH guided wave EMATs are designed for defect detection.An experimental platform of electromagnetic acoustic SH guided wave is set up,and the through-hole defects with diameter of 2mm and 6mm,and groove defects with length of 6mm and 15 mm are used to conduct the experiments.12-channel receiving EMAT is used to obtain the circumferential scatter diagrams,by compare it with the circumferential scatter diagrams of the simulation,and the type and size of the defect corresponding to the simulated circumferential scatter diagram most similar to the experimental results are the detected defect information.The defect information determined by the experiment is consistent with the theoretical value of the defect,which indicates that the method proposed this paper can realize the qualitative and quantitative detection of aluminum plate through-hole and groove defects. |
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