The Defect Detection Of The 20# Steel Based On The Low Frequency Magnetic Flux Leakage Testing Technology

Electromagnetic nondestructive testing technology is based on the principle of electromagnetic field.It's widely applied in the inspection of special equipment.Currently,there are some kinds of electromagnetic testing technologies such as magnetic flux leakage testing technology,eddy current testing technology,magnetic memory testing technology.These technologies have its own advantages and limitations.In this paper,we carried out some works included theory analysis,numerical simulation and experiments to improve the magnetic flux leakage testing's ability of inspecting the buried defects.The main contents are depicted as follows:1.Some typical nondestructive testing technologies were introduced.And the principle of magnetic flux leakage testing technology was specifically presented.Besides,the brief introductions of magnetic flux leakage testing technology were given.2.Magnetic dipole model and differential equation model were built to explain the spatial magnetic field distribution when defects exist in the sample.In the magnetic dipole model,we used infinite magnetic dipole line model,infinite magnetic dipole ribbon model,finite magnetic dipole ribbon model and magnetic charge along the edge model to calculate the spatial magnetic field intensity.In the differential equation model,two differential equations were derived from the Maxwell Equations to analyze the distribution of spatial magnetic field intensity.The refraction of magnetic intensity in two different mediums and the skin depth were discussed.3.Based on the differential equation model,we use finite element method to gain the distribution of spatial magnetic field intensity.Two models called saturation magnetization and signal difference were built in the COMSOL Multiphysics software.Simulation results show that saturation magnetization method is capable of testing buried defects and signal difference method is not.The detectable buried depth can reach 11 mm where the thickness of 20# steel plate is 12 mm.As for the top surface defects,the saturation magnetization testing sensitivity is 66.95Gs/mm within the depth range of 8mm.There is 9Gs difference between defects existence and no defects existence within the depth range of 10 mm.4.The low frequency magnetic flux leakage testing sensor based on the saturation magnetization simulation model was made.The experiments about standard sample inspection in laboratory and three pipeline inspection in a chemical plant were carried out.The standard sample inspection results show that the 6mm buried depth defect can be detected in the 12 mm 20# steel plate.And an ultrasonic thickness gauge was used to validate the results obtained from three pipelines inspection by our sensor.The results are consistent with our sensor.