| Metal materials are the most widely used engineering materials,and metal structural parts are also the main object of nondestructive testing.The common defects in metal structures are cracks,debonding and surface defects.Therefore,regular nondestructive testing of metal structures is very important,which can prevent accidents and cause significant economic losses.In this paper,four kinds of electromagnetic acoustic response detection schemes for metal plate cracks under electromagnetic loading are proposed,and the detection mechanism of four kinds of electromagnetic acoustic response is analyzed by finite element modeling.Firstly,this paper introduces the detection principle of four kinds of electromagnetic acoustic response,and establishes the three-dimensional finite element simulation model of the first kind of electromagnetic acoustic response.It analyzes the distribution of static magnetic field,eddy current and Lorentz force on the surface of metal specimens with and without cracks.It is concluded that the existence of cracks will lead to the uneven distribution of eddy current and Lorentz force,resulting in the uneven propagation of sound waves.Then,the changes of eddy current and Lorentz force at the crack tip and the amplitude of sound wave are analyzed.At the same time,a bin-to-bin distance(BTBD)histogram based on variance is proposed to judge the crack location of 7075 aluminum alloy specimens with cracks.The model of 7075 aluminum alloy specimen with crack is established by using finite element simulation software,and the simulation results are analyzed to verify the feasibility of this method for crack location detection.Then build an experimental platform to detect the crack of 7075 aluminum alloy specimen with crack,get the signals at different receiving positions,compare and analyze with the received signals on the no crack specimen,and finally calculate the crack position by using BTBD histogram.Then,the simulation model of the second kind of electromagnetic acoustic response is established by simulating the acoustic emission signal.The time-domain threshold is used to judge the starting time of acoustic emission signal,and the signal is time-frequency transformed.The starting time obtained in the time-frequency diagram is consistent with the starting time judged by the time-domain threshold,so as to determine the starting time of acoustic emission signal,so as to judge when the crack expands.For the third kind of electromagnetic sound,the closed crack is added on the basis of the second kind of electromagnetic acoustic response simulation model to obtain the electromagnetic acoustic emission signal containing high-order harmonics,and the effects of crack and closed crack parameters on the amplitude of harmonics are studied.Finally,according to the threshold method and time-frequency analysis method,it is preliminarily judged whether the ultrasonic wave makes the crack expand,that is,the starting time of the acoustic emission signal generated by the fourth type of electromagnetic acoustic response.The starting time of acoustic emission signal is calculated by using Hilbert Huang Akaike information criterion method(HHT-AIC).The signals containing cracks with different pointing angles are processed,and it is concluded that the crack pointing angle can be determined by HHT-AIC pick up method. |
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