Experimental Study On MFL Inspection Magnetized By Coil

Magnetic flux leakage inspection is one of the most common nondestructive techniquesused to detect crack-like defects in ferromagnetic materials. Magnetization is the importantfirst step in MFL inspections. The detectability of a flaw by using this method largely dependson the magnetic flux density passing through a specimen to be examined or on the intensity ofmagnetic field acting in/on the specimen. When the magnetization is not strong enough tobring Magnetic-field distortion on the surface, even high sensitivity probes cannot acquire aMFL signal. In MFL Inspection, as the thickness of the ferromagnetic material to bedetected or the buried depth increases, flaws are more and more difficult to be found. For thisreason, saturation magnetization becomes especially important in MFL inspections.In order to search the ultimate, the detect ability was studied experimentally on a thicksteel plate with blind holes in different depth. Signals under different magnetizations weregiven by means of increasing exciting current step by step.We found that, distance from theblind holes which were detectable to the surface increased with increasing magnetization.High SNR MFL Signal was realized in the test of ф2mm×20mm blind holes whose depth wasup to70mm on conditional that the magnetization was approaching to saturation, which couldprovide guidance for thick wall steel tube.On the other side, we studied the MFL signals under large lift-off by the action of intensemagnetization, which provided guidance to the measurement of Magnetic flux leakage signalsin non-contact test. We found in our trials that signals were detected in the test of0.5mm×0.5mm artificial groove under12mm lift-off, and the lift-off competence improved assizes of the imperfections increased.In order to discuss the diffusing characteristics of leakage magnetic field between theflaw and the sensor, analyzed the test data above.It was shown that, signal intensity frominternal defect was much more stronger than that from external defect of the same size, oncondition that kept the same distance between the flaw and the sensor. Therefore, attenuationof Magnetic-field distortion in ferromagnetic material is slower than in the air. For this reason,we did more further experiments, identified that rate of descent of Magnetic-field distortion would slow when covered the surface of standard specimen with ferromagnetic material. Thus,we could increase the distance between probes and ferromagnetic material to be detected bythis way, at the same time maintain the current level of signal intensity or even improve thesignal to noise ratio.