Study On Magnetic Memory Effect Of Corrugated Inclined Web Steel Box Girder Under Bending Test

Metal magnetic memory(MMM)detection is a new nondestructive testing technology.Compared with conventional NDT,it can detect the damage such as dislocation,slip and distortion caused by stress concentration.The bridge steel structure not only suffers static load,dynamic load and earthquake,but also the influence of heavy environment such as rain and snow,hurricane and so on.Under the influence of these factors,the micro defects in the bridge steel structure will accumulate,the bridge structure will be damage after the defect accumulates to a certain extent and even cause more serious harm.Using metal magnetic memory testing technology to detect early damage of bridge steel structure,we can detect damage location in advance and avoid hazards.In this paper,the changing law of magnetic signal and magnetic signal gradient on the surface of upper and lower flange and web plate in four point bending test of corrugated inclined web steel box girder and the relationship between magnetic signal and stress-strain are studied.The main tasks are as follows:(1)Through the four point bending test of corrugated inclined web steel box girder,the variation of magnetic signal on the surface of steel box girder is studied.The following conclusions are drawn: the magnetic signals at the stress concentration on the upper flange and the web appear "extreme value",the magnetic signal gradient value has “peak-trough” and “zero crossing” phenomenon.The magnetic signal in the stress concentration region of the lower flange appears “peak-trough” and “zero crossing” phenomenon,while the magnetic signal gradient value shows the phenomenon of “two peak clips and one valley” or “two valleys and a peak”.The location of stress concentration can be better judged by these characteristics.(2)The change rule of magnetic signals during the whole loading process is found:In the elastic phase,the magnetic signal on the surface of the steel box beam shows a consistent variation with the increasing load,and the change law of magnetic signal is greatly influenced by the initial leakage magnetic field in the early stage,and then the changing law of magnetic signal will gradually become consistent,and the value of the magnetic signal number increases sharply and changes the law of change at the yield.At the plastic stage,the variation of magnetic signal is the same as the yield,but the magnetic signal value of the upper flange increases with the increase of the load,but the absolute value of the magnetic signal of the web and the lower flange decreases with the increase of the load.(3)The correspondence between stress,strain,and magnetic signals in the loading process is found:The absolute value of the magnetic signal of the upper flange increases monotonously with the increase of stress,but the change rate of magnetic signal varies in each stage,and the change rate of magnetic signal will appear“inflection point” at the yield,the absolute values of the magnetic signals of the web and the lower flange increase with increasing stress and then decrease with increasing stress.When the stress reaches the yield,the magnetic signal shows and “extreme point” and the rate of change of the magnetic signal also increases sharply,and the“extreme point” will appear in the correspondence between the strain and the absolute value of the magnetic signal.Then,the “extreme point” of the magnetic signal can be used to determine the yield state of the test piece.