Rotating Focused Field Eddy-Current Sensing For Arbitrary Orientation Defects Detection

As one of the five conventional non-destructive testing technologies,eddy current testing is widely used in the detection of surface and subsurface defects of conductive specimens because of its advantages of non-contact,low requirements on the surface state of the specimen and fast detection speed.The eddy current sensor excites the induced eddy current in the specimen,and the existence of defects will disturb the distribution of eddy flow field,and then the subsurface produce the secondary magnetic field signal which can represent the defect information.In this process,the intensity and direction of the induced eddy current at the defect are the two main factors affecting the interaction between eddy current and defect.In engineering practice,the size of defects in the test pieces is different,and the trend of defects is difficult to predict.At this time,it is an effective way to avoid the defect missing inspection that the eddy current with high strength and all directions is excited in the specimen.Therefore,in this thesis,a rotating focused field eddy-current sensing technology is proposed to detect the defects in arbitrary orientation of carbon steel specimens.The main contents and conclusions are as follows:(1)A rotating focused field eddy-current sensing technology is proposed.This thesis introduces the 8-shaped focusing coil and the method of rotating field excitation,and describes the process of rotating focused field eddy current testing by using two pairs of 8-shaped coils.The distribution characteristics of eddy current field excited by single coil,focusing coil and rotating focused field probe are compared and analyzed by using ANSYS software,which confirm the formation of rotating focused field.Furthermore,the detection of defects with different orientations is simulated.The results show that the sensitivity of the rotating focused field probe to detect defects in arbitrary direction is the same,and signal-to-noise ratio is high,the signal amplitude is large,and it has the potential to distinguish the direction of defects.(2)A rotating focused field eddy-current testing probe is designed,and the influence of excitation parameters and geometric parameters of the probe on the detection signal are simulated and analyzed.Based on the skin depth and signal amplitude of detection for defects in Q235 steel plate under different frequencies,100 Hz is selected as the excitation frequency of the probe;the rotation characteristics of the excitation field are affected by the excitation phase difference,and the interaction between the excitation field and the defects in each direction is consistent when the phase difference is 90 degrees.The single parameter analysis of the lift-off height,center distance,coil height and opening angle of the probe coil shows that their size will affect the intensity and uniformity of the excitation field;for a given size of the probe coil,150 ° is selected as the coil opening angle.(3)The characteristics of the response signal of rotating focused field eddy-current testing probe for plate defects are studied.A defect size evaluation method is proposed,which takes the peak value of defect edge signal as the characteristic quantity to quantify the depth and width,and the distance between signal peaks as the characteristic quantity to quantify the length.This thesis discusses the problem that the sensitivity of the probe to the defects with different directions on the pipe wall is inconsistent;further analysis shows that when the ratio of the pipe diameter to the coil center distance is 25,the peak value of circumferential defect detection signal is 98.92% of that of axial defect detection signal,and the detection sensitivity can be considered to be the same.(4)A rotating focused field eddy-current testing probe with GMR sensor as receiving element is developed,and the eddy current testing system is built.The testing experiments are carried out on Q235 plate and pipe with different depth and different direction defects.The experimental results are in good agreement with the simulation results,which verifies the effectiveness and practicability of the proposed rotating focusing field eddy current testing technology.