Study On Pulse Eddy Current Testing Of Small Diameter Tubes Based On Magnetic Conductor Ring Structure

Small diameter metal pipe is widely used in the power,aerospace,oil and other industries.The circumferential crack is a defect that can lead to fracture.Since the eddy current of conventional eddy current testing is parallel to the circumferential crack,the circumferential crack is easily missed.Other nondestructive testing methods are also not easy to detect circumferential crack of the small diameter tube due to its thin wall.The advanced non-destructive testing technology,such as array eddy current and rotating magnetic field scanning technology,is difficult to be widely used in practical inspection due to the high cost and complicated mechanical device.This paper aims to explore a low-cost detection technique for circumferential cracks.Based on the orthogonal symmetry between the magnetic field and the electric field,a magnetic conductor ring structure sensor is designed to form an electric field perpendicular to the circumferential crack.Based on this type of probe,the detection of circumferential crack defects in small diameter tubes is studied by using the pulsed eddy current detection technique developed in recent years.The detection platform is established first and then the method and mechanism of the pulse eddy current testing of the circumferential crack in the small diameter tube are studied in the probe structure,the excitation voltage and the excitation frequency through experiments combined with ANSYS finite element simulation.Three types of probe are designed and manufactured,single-layer single-chip dual-excitation,single-layer dual-chip dual-excitation and single-layer distributed dual-excitation.The effect of the probe structure on the detection effect is studied by experiment and the numerical simulation.The experimental results show that the single-chip probe has the best detection sensitivity to the circumferential crack of the small diameter tube,with the highest signal-to-noise ratio and the highest detection sensitivity for small size crack buried at the depth less than 0.5mm.The double-chip probe has a good detection sensitivity on long cracks at the depth greater than 0.7mm,but the detection of small-size cracks is susceptible to interference from the background signal.The detection sensitivity of distributed probe is the worst among these three.The simulation results agree with the experimental results.The effects of different amplitude of the excitation voltages on the detection are studied.The excitation voltage is set to 1V,2V,3V and 5V respectively,and three kinds of crack defects on the small diameter tube are tested.The results show that the detection sensitivity for detecting the deep crack increases gradually with the increaseof the excitation voltage.However,the differences of testing results for excitation 3V and 5V are not obvious when detecting shallow cracks.In terms of signal-to-noise ratio,the greater the excitation voltage,the higher the signal-to-noise ratio.Then,the simulation results show that the normalizations of the received signal at different excitation voltages are not obvious for small crack defects,which is consistent with the experimental results.The excitation frequency mainly affects the penetration depth of the pulsed eddy current testing.Experiments and simulations at 50 Hz,100Hz,200 Hz,300Hz,400 Hz,500Hz excitation frequency show that the amplitude of the magnetic field signal in the normal direction of the pipeline is slightly reduced after increasing the excitation frequency,but the signal-to-noise ratio of the signal is obviously improved and the testing sensitivity is enhanced.