The Study Of Magnetic Memory Testing Signal Based On The Bending Test Of The Corrugated Web Steel Box Girder

Metal magnetic memory testing technology is a new nondestructive testing technology which can be used to detect the early damage of ferromagnetic components.Since coming into being more than ten years,it has been widely concerned by scholars in the world.However,the test results and the evaluation criteria of the technology are still in the stage of improvement,and one of the important reasons is the wide application of ferromagnetic materials,resulting in a large number of research areas.Therefore,this nondestructive testing is applied the special bridge steel commonly used in the field of bridge steel structure,through the study of bridge steel plate specimen tensile test,the applicability of the technique in the field of bridge steel structure is tested.Through the emphatically research on the bending test of the steel bridge steel box girder with corrugated webs,the variation of magnetic memory signal under different stress and strain is analyzed,and the magnetic memory signal from top to bottom direction under different loads of corrugated web surface is also analyzed,the magnetic properties associated with the mechanical properties of corrugated steel webs are gotten.The main work are as follows:(1)By the Q345qC plate tensile test,the applicability of the metal magnetic memory testing principle in the field of bridge steel structure is verified,and it is showed that in the specimen fracture position,normal magnetic field intensity Hp(y)is crossing zero and gradient K has extreme point,and magnetic memory signal value changes sign;In the elastic stage,the curve of the magnetic memory signal is approximately linear,and the gradient curve is approximately a line.In the plastic phase,the curve of the magnetic memory signal is nonlinear,and the gradient curve is nonlinear.(2)The variation of magnetic memory signals in loading stage of the steel box girder is studied by means of the bending test of the steel box girder with corrugated webs of Q345qC.It is showed the magnetic memory signal curve under the flange surface of the specimen was similar to "ladder",it was similar to bending moment diagram of specimens under vertical concentrated load,after the upper and lower flange yield,magnetic memory signal curve began to appear "zero",and with the load increaseing,magnetic field ―zero cross point ‖position gradually close to the maximum damage,failure and damage is the location of the maximum overlap,so it can regard flange magnetic memory signal curve " zero " position as dangerous moment and dangerous position criterion.While the magnetic memory signal under concentrated force loading position on the web surface has obvious "peak" phenomenon,it can try to determine the stress concentration zone of the use of Web "wave" position,the wave peak amplitude is higher,the greater the degree of stress concentration.(3)Magnetic force relationship by four point bending test of steel box girder can be found by the following features: in the elastic stage,the intensity of magnetic memory signal increases with the increase of strain,the magnetic memory signal and the strain curve of the slope increased gradually,magnetic memory signal with strain rate becomes larger;in the plastic stage,the magnetic memory signal strength still increases with the increase of strain,the relationship between magnetic memory signal and the strain curve of slope suddenly began to decrease,the change rate of magnetic memory the signal decreases with increasing strain.When the slope of relationship curve of the magnetic memory signal and the strain has a mutation point,the specimen is just yielding,and the detection characteristic can judge the specimen begin to enter the yield stage.(4)By studying magnetic memory characterization technique on the properties of corrugated steel web,it is found that on the corrugated web,with the increase of height,the magnetic memory signal reduces,and as load increases,the shear stress increases,the magnetic memory signal strength increases.The main factors that affect the rate of change of the magnetic memory signal along the direction of the calibration beam are as follows : in bending shear zone or pure bending area,the value of the load applied to specimen.The curve of average rate change of magnetic memory signals and load on the bending shear section of corrugated steel webs has distribution of a parabola approximation,and the regional average absolute value of the magnetic memory signal change rate increases initial and then decreases;the relation curve between webs of pure bending section of the regional average change rate of magnetic memory signals and load changes irregularly;The relationship of shear and regional average magnetic memory signal change rate was studied preliminarily,without considering the coupling effect of other factors associated with the magnetization of ferromagnetic materials,the regional average change rate of magnetic memory along the height direction and shear square are approximateily linear relation.