Mechanism Study On Magnetic Memory Effect Of Ferromagnetic Material

Metal magnetic memory testing is a new non-destructive testing method, whichcan be applied to the early diagnosis of defect. Due to applied load, the scatteringmagnetic field would occur on the surface of ferromagnetic materials. Its tangentialcomponent Hp(x) exhibits a peak and normal component Hp(y) changes its polarity inthe maximum stress-concentration position.Tension-tension fatigue tests of notched specimens made of Q235B steel wereconducted by square wave load and on-line magnetic memory signals on the surfaceof specimens were measured under different fatigue cycles using magnetic memorydetector. The results show that magnetic memory signals along the notch of untreatedsamples distribute randomly in the initial stage of fatigue. After5000cycles, magneticmemory signal curves transform into a curve of the waveform with onepeak-to-trough. Magnetic memory signal curves tend to be stable before the cracksinitiated, but diverge after macroscopic crack appeared. In the last stage, magneticmemory signal densities increase with increasing cycles. Based onmagneto-mechanical effect, the variation of magnetic memory signals in the initialstage of fatigue can be well explained.Different loads were applied to the specimen of API5L X52pipeline steel and itswelding sample. TSC-2M-8type metal magnetic memory device was used to detectthe normal component Hp(y) and tangential component Hp(x) of the magneticmemory signal online to study the relationship between stress and magnetic field. Theresults show that there was a large initial magnetic memory signal without loading. Inthe initial stage of the elastic loading, Hp(y) curve rotated clockwise and Hp(x)decreased. After the initial magnetic memory signals were eliminated, and compositedthe two component, the value of the magnetic memory signal increased. Loaded to140MPa, Hp (y) and|Hp(x)|changed little. When large plastic deformation occurred,Hp(y) curve rotated counterclockwise with the increase of the plastic deformation,while|Hp(x)|decreased. Loaded to plastic stage, magnetic memory signals detectedunload was different from detected loading.After welded, there was a large residual stress in the weld joint of API5L X52pipeline steel, which resulted in distortion of magnetic memory signal. When there was a large defect in the weld joint, it can be well predicted the final fracture locationwith the extreme points of differential value of the normal component and thetangential component of the magnetic memory signal. But if the defect was little, theextreme points of differential value of the normal component and the tangentialcomponent of the magnetic memory signal cannot predict the fracture locationsuccessfully.