Research On Crack Detection Performance Of Flexible Koch Differential Pick-up Fractal Eddy Current Probe

As the beginning of eddy current testing system,the performance of eddy current sensor is closely related to the reliability of testing results.When detecting short cracks parallel to the direction of eddy current,the detection effect of eddy current probe with traditional coil structure decreases.Therefore,the Koch fractal eddy current probe based on fractal self-similarity theory is proposed.The conditioning effect of the Koch fractal eddy current probe on the eddy current in the space of the tested component makes its detection performance of short cracks in all directions more consistent.In order to study the crack detection performance of the Koch fractal eddy current probe and improve the crack detection performance of the probe.This paper proposes a Koch differential pick-up fractal eddy current probe,builds the equivalent circuit model of the probe,and analyzes the working principle of the probe.The crack detection performance of the probe at different lift-off distances and the detection performance of the internal crack are also studied.Finally,a method of covering the magnetic film is proposed to improve the crack detection performance of the probe.The main research contents of this paper include:（1）A flexible Koch fractal eddy current probe with multi-turn differential pick-up is designed.From the perspective of equivalent circuit and two-terminal network,the equivalent circuit model of the probe is established,and the expression of the probe output signal is derived respectively.The results show that the equivalent cross-impedanceZ_req derived from the equivalent circuit model and the R parameter equation coefficient R₂₁ of the derived two-terminal network model can represent the crack signal of the probe,and its size is related withω,M₁₃,M_32l,M_32r,R₃L₃.（2）Through finite element analysis and test methods,the evolution law of probe crack detection performance with increasing lift-off distance under different excitation frequencies and the maximum lift-off distance when the probe can detect cracks are studied.The results show that the increase of lift-off distances results in the reduction of the similarity between the eddy current distribution shape and the excitation coil shape,and the distribution area diverges to all sides,as well as the weakening o f the output signal strength.Under the excitation frequency of 10 k Hz,20 k Hz,50 k Hz,100k Hz,200 k Hz,500 k Hz and 1000 k Hz,the maximum lift-off distance when the real part of the probe output signal can detect cracks is 5.0 mm,7.0 mm,8.0 mm,8.0 mm,8.0 mm,6.5 mm and 4.0 mm respectively,and the imaginary part is 6.5 mm,6.5 mm,7.5 mm,5.5 mm,8.0 mm,6.5 mm and 6.5 mm respectively.（3）Through finite element analysis and experiment methods,the evolution of eddy current on the surface of the experiment block and at different depth profiles below the surface,as well as the detection effect of the probe on the pre-cracked experiment block covered with different thickness of aluminum plate are studied.The results of finite element analysis show that with the increase of profile depth,the eddy current density in profile layer decreases and the concentration of eddy current distribution decreases.The difference between the eddy current distribution shape and the eddy current shape on the surface of the experiment block is becoming greater and greater.The shape evolves from snowflake to circular,which is called"skin effect of excitation source information".The experiment results show that when the probe detects the prefabricated crack specimen covered with different thickness of aluminum plate,the output signal strength of the probe decreases with the increase of the thickness of the cover.When the excitation frequency is 10 k Hz,the maximum thickness of the aluminum plate covered when the probe can detect the crack is 2.0 mm.（4）A method of covering magnetic film on the excitation coil of the probe is proposed to improve the crack detection performance of the probe,and the method is verified by finite element analysis and experiment.The finite element results show that the signal strength of the probe can be improved by covering the magnetic film.For a3 mm short crack,the peak value of the real part and the imaginary part of the crack signal detected by the thin film probe increased by 96.11%and 243.11%respectively compared with that detected by the thin film probe.For a 15 mm long crack,the peak value of real part and imaginary part increased by 57.87%and 150.65%respectively.The experiment results show that under the excitation frequency of 20 k Hz,50 k Hz,100 k Hz and 1000 k Hz,the crack detection performance of the probe can be improved by covering the magnetic film.The growth rate of the output signal of the film probe for different cracks is 10.44%,9.63%,22.88% and 35.71% respectively.