Research On Eddy Current Non-destructive Testing Method Based On Phase Trajectory Analysis

Electromagnetic eddy current nondestructive testing technology by applying alternating current to the electromagnetic coil on the magnetic field,and then the measured eddy current produces alternating conductor,obtained characteristic parameters of the conductor to be measured by detecting the eddy current field.Lift off noise and skin effect are the main problems in electromagnetic eddy current nondestructive testing technology.Based on the equivalent transformer model of electromagnetic eddy current nondestructive testing system,the theoretical analysis proves that the phase of the excitation current is not related to the coupling coefficient in the equivalent transformer model.In the equivalent transformer model of the electromagnetic eddy current nondestructive testing system,the excitation current represents the magnetic flux density on the material surface,and the lift off height is an important factor to influence the coupling coefficient.Therefore,it is concluded that the phase information of the magnetic flux density is not affected by the lift off height.The simulation research is carried out on the basis of theoretical analysis,the simulation results show that the measured specific position of the metal material on the surface of the detected method to the magnetic flux density BZ and lift-off height showed a good linear features,and is also the method to the extreme point of the magnetic flux density,magnetic flux density in the proof of the equivalent transformer model data and theory analysis of this point are approximate optimal.The BZ phase trajectory of the extreme points pmax at different lift-off is drawn,and the slope represents the phase information of BZ.Simulation studies on defect free surface,surface defect,sub surface defect and internal defect material are carried out.The results showed that the method can effectively identify the defect information in different locations.Further,the method is extended to the case of the most frequency excitation,and the results show that the frequency response characteristics of the phase trajectory can effectively distinguish the depth information of the tested material defects.Finally,the experimental system which is suitable for the phase trajectory is designed,and the correctness of the theoretical analysis is verified by experiments.