Testing Technology Of Ferromagnetic Material Based On Characteristic Permeability Differential

Characteristic differential permeability testing technology is an emerging electromagnetic NDT methods. By analyzing the maximum value of ferromagnetic specimens’ differential permeability, the technology can figure out the specimens’ stress concentration, degree of fatigue damage and microscopic structural changes.Thus the technology can provide early diagnosis of the specimens’ injury, as well as life assessment of the specimens. So with the technique it is possible to prevent accidents caused by ferromagnetic components’ failure, which has a high value.This work describes the research status of the technology, and explains the detection mechanism based on Magneto-mechanical Effect theory. A two-dimensional testing simulation model has been set up by using ANSYS finite element simulation software. Build a testing platform and complete the ferromagnetic specimens’ stress concentration and fatigue tests.A linear relationship exists between the stress and initial differential permeability in ferromagnetic materials. The detection signal u can directly reflect the differential permeability of the specimens. With ANSYS simulation and experimental study,optimum excitation frequencies presents in the lower frequency band(about 100 Hz),and the excitation voltage is from 1Vpp to 3Vpp. A second-order band-pass filter is added to the output of the probe for filtering out interference in complex electromagnetic and mechanical testing environment. After true RMS Value transform module, data acquisition card and A/D transform, the detection signal has been sent to host computer for a continuous display. In the tensile testing, when stress or residual stress increases linearly, differential permeability value has a linear reduction which is up to 140 m V. In fatigue testing, it was found that the differential permeability in fatigue zone is less than non-fatigued areas, and the maximum difference is 52 m V.The results show that the technology can sensitively detect internal stress concentration and fatigue damage in ferromagnetic material.