Research On The External Magnetic Field Impact Ferromagnetic Material Metal Magnetic Memory Testing

Micro and macro stress concentration area is the main site of metal components andmachine parts where is occur damage.Conventional NDT methods has important significancein defecting of detection pipeline and ferromagnetic material specimen, etc., but can not beeffective in preventing of sudden accident which is caused by a variety of micro stressconcentration region. MMM detection method which is developed in recent years is throughthe early diagnosis to ferromagnetic material in order to prevent.However, due to the technicaltheory is not perfect, and many other factors,the outside influence of magnetic field on toexternal the metal magnetic memory testing in engineering applications can not yet bequantitative assessed, so the studying of the method of the influence of the external magneticfield on the magnetic memory testing is necessary.This paper is study the energy balance theory, electromagnetic theory and J-Amagneto-mechanical effect theory, using different theories to analyze the influence of theexternal magnetic field on the test results during the magnetic memory testing process,thusproving the external magnetic field may mask Magnetic Memory Signal.By steel bars, colddrawn rods and other representative ferromagnetic material as a test specimen, using anexternal magnetic field to simulate the permanent magnet field, make the metal magneticmemory testing of the existence of stress steel and cold drawn rods in the external magneticfield.Test results show that the stress concentration area of magnetic memory signal disappearsunder the influence of an external magnetic field in a ferromagnetic material elasticstage,under the magnetic field after repeated force magnetic memory signal can bere-identification.Plastic stage stress concentration area of magnetic memory signal does notoccur significant changes in the external magnetic field. According to the theoretical analysisand experimental test results, combined MMM magnetic domain wall aggregation model,ferromagnetic material under the action of external magnetic field in the magnetization, whichinternal magnetic domain will occur deflection or displacement to form residual stress. In theelastic stage due to the applied stress relatively residual stresses is small, stress concentrationarea of magnetic memory signal is easily overshadowed by the external magnetic field signal,resulting in magnetic memory signal disappears. Under the magnetic field environment therepeated force will increase applied stress to offset the residual stress., the magnetic Memory Signal resurfaced. Due to the applied stress is much greater than the residual stress which iscaused by the external magnetic field in plastic stage, it will not have a significant impact.According to the theoretical analysis and experimental test results show, during metalmagnetic memory testing method when applied stress is small in the external magnetic field,elastic stage stress concentration area magnetic memory signal is canceled. When the appliedstress increases, the effect of the influence of the external magnetic field reduce. Externalmagnetic field does not have a significant effect on the plastic stage MMM signals. Therefore,MMM is an effective and reliable method for early diagnosis which has general applicability.