Analysis Of Magneto Acoustic Emission Exciter And Research On Feature Extraction Of Main Frequency Energy

This paper is supported by the National key Research and Development Project(No.2016YFF0203000).The National Natural Science Foundation(No.51675258).Ferromagnetic metal materials are widely used in large-scale bridge construction,hoisting machinery,petrochemical equipment,power station and other industrial fields.However,long-term online service will lead to stress concentration,plastic deformation,fatigue and creep damage.The existence of these hidden damage will threaten the healthy development of national economy and the safety of people’s life all the time.Non-destructive testing technology can be used to evaluate the surface or interior damage of metal materials without damaging the current and future performance of materials.With the development of electromagnetic theory,the technology of Magneto Acoustic Emission(MAE)has been further studied and promoted.Because it is very sensitive to the microstructure and stress level of metal materials,MAE has great potential in the application of early damage detection of ferromagnetic metal materials.In the existing MAE detection system,the excitation voltage acts on both ends of the coil to generate alternating magnetic field,and then magnetizes ferromagnetic materials to generate MAE signal.According to the MAE generation mechanism,the larger the excitation voltage is,the more MAE signals will be generated,and the more abundant the magnetic characteristic information of materials will be obtained.The larger the power amplifier magnification,the higher the magnetization voltage generated,and on-site testing usually requires a portable testing device,and the resulting impact is the higher the volume and cost of the testing instrument.In order to further solve the problem of miniaturization and portability of MAE testing instruments and accelerate the engineering application process of MAE testing technology.So it is very important to improve the excitation ability of MAE.The main contents of this paper are as follows:According to the generation principle of Magneto Acoustic Emission detection mechanism,two different types of exciters are simulated by ANSYS Maxwell software.First,according to the ampere loop theorem,the number of ampere turns of the u-yoke and the coil remains unchanged.By using the single variable method,the finite element simulation is carried out for the pole spacing,pole thickness and winding mode of different coils of the yoke,and the changes of the above factors are analyzed to influence the magnetization effect of Q235 sample.Finally,according to the simulation results,on the basis of the optimal size of the U-shaped yoke,keep the excitation frequency unchanged.By changing the coil turns,find out the coil turns that make the best magnetization effect of Q235 sample,and further improve the excitation ability of the U-shaped magnetization device.When Q235 sample is rod sample,another method is needed to excite the sample.Usually,for this kind of small rod-shaped sample,the multi turn excitation coil is wound on the plastic pipe made of polyethylene to form a device similar to the electrified solenoid.The magnetization of the whole sample will be affected by the difference of coil inner diameter and length.Therefore,it is necessary to combine the finite element simulation analysis to find the most suitable coil size by changing the coil inner diameter and length.After the coil size is determined,find out the coil turns with the best magnetization effect in the same way to further improve the excitation ability of the solenoid.MAE signal is similar to acoustic emission signal,but its amplitude is low,so it is easy to be annihilated by noise in field engineering application,so the extracted signal can not reflect the current micro state of materials.The Generalized S-transform has the characteristics of good time-frequency resolution and noise reduction,and it is introduced into the feature extraction of the static tensile signal of Q235 sample.The maximum value of main frequency band energy sequence is introduced as a new characteristic parameter.The results show that the new feature parameters can avoid the influence of noise band and extract the features of MAE signals under different stress levels.Finally,the newly proposed characteristic parameters are applied to evaluate the low-cycle fatigue life of Q235 sample.It is found that the maximum value of the energy sequence of the main frequency band gradually decreases with the increase of the cycle number,which has a good correspondence with the fatigue life of the Q235 sample.