Experimental Study On Detecting Cracks Of Steel Box Girders Based On Metal Magnetic Memory

The steel box girder has the advantages of light weight,high strength,convenient transportation and installation.It is one of the first choices for the main girder structure of long-span bridges,which has been widely used at home and abroad.However,varying degrees of cracks have been appeared in the steel box girder bridges with the service time goes on.So it is urgent to detect the cracking problems of steel box girder ficiently and accurately.The existing non-destructive testing technology can effectively find the macrocracks,but it still has difficulties in finding of the early cracks,while the metal magnetic memory(MMM)testing is expected to discover the early damage detection.Based on the theory of MMM,this article will do the experimental study on detecting the cracks of the steel box girder based on the mental memory,including the theoretical study,the experimental simulation and the simulation research.The main research contents are as follows:(1)Carry out the no loading self-magnetic flux leakage(SMFL)signals detection test on the cracked steel plate,the T-type welded steel plate and the steel box girder.as Analysis the cracks' orientation,detection paths,lift-off heights and the dimension and their influences on the signal.Ultimately,variation characteristics of signals around cracks were founded.?DFor the tangential component of signals Bx and By,maximum value appeared in the central of the crack.While for the normal component Bz,the maximum and minimum values anti-symmetrically appeared at the ends of the crack and over zero value appeared in the central position.In addition,for the gradient of Bz,the maximum value appeared in the central of the crack.?With the increasing length and width of the crack,the Maximally Stable Extremal Region(MSER)of Bx and By increase,but the maximum value decrease.However,the MESR and extremes of Bz and its gradient all increase.?With the depth of the crack increasing,the MESR and extremes of Bx,By and Bz all increase.(2)Carry out the loading SMFL signals detection test on the cracked steel box girder Combine with ABAQUS finite element simulation of steel box girder,using the digital speckle system and the strain gauge acquisition system to obtain the change of stresses during the loading process.Meanwhile,via the magnetic signal monitoring system,the synchronous signals of the relevant regions are collected.Ultimately,acquire the relationship between the early stress and SMFL signals in the early crack.As forcurve's relation between stress and signals,an obvious inflection point appeared when the stress ranged from 30 MPa to 50 MPa,which meant the intensity of magnetic signals would increase with the stress increasing.When the stress exceeded 30 MPa?50 MPa,the magnetic signals would decrease with the stress increasing.In the early period of the elastic stage,in a certain range of loads the local magnetic signals would be enhanced in order to overcome the instability of the initial state.However,in the later period of the elastic stage,loads had generated the stresses which would weaken the intensity of magnetic signals.Eventually,it was close to hysteresis state,and the signals would gradually decrease.(3)Based on the COMSOL finite element software,the simulation model of the cracked steel plate was established.Then the simulation results of SMFL signals with different dimensions and heights of cracks are extracted.Meanwhile,the simulation will be compared with the test and the magnetic dipole model.The result shows that the simulation is effective.The research results in this paper can lay a good foundation for the subsequent research on the detection technology of the cracked steel box girder based on the metal magnetic memory.