| The problem of cable-stayed bridge detection,especially the nondestructive testing(NDT)of cable corrosion,has always been a difficult problem in Bridge Engineering,which needs to be solved urgently.The cable,as an important load-carrying member of the cable-stayed bridge,its health condition is directly relating to the safety and service life of the cable-stayed bridge.At present,the corrosion of cable has become the key factor affecting the health of the cable,hower the corrosion of the cable is concealing,and the conventional technology is difficult to detect.The galvanized steel strand cable,as the common cable structure in cable-stayed bridge,the essence of its corrosion is that the galvanized steel strand occurring metal corrosion.After the corrosion of the steel strand,the local stress concentration and section loss defects will arise in the corrosion defect area.The steel strand,as a typical ferromagnetic component,its corrosion will form a spontaneous magnetic field leakage(SMFL)on the corrosion defect surface.The metal magnetic memory(MMM)technology,as a weak magnetic flux leakage nondestructive testing technology(NDT),its essence is effective quantitative detection of material damage based on the SMFL effect related to the defect damage information of the ferromagnetic material surface.Based on the magnetic flux leakage detection theory of MMM,this paper has carried out the magnetic flux leakage test and finite element numerical simulation for the steel strand specimens.Through theoretical analysis,experimental research and simulation analysis,the distribution characteristics of corrosion defects magnetic flux leakage signals are analyzed.Some criterions judging the corrosion of steel strand specimens based on the characteristics of metal MMM signals are constructed.Then the purpose is to detect the corrosion degree of steel strand specimens quantitatively.The main research contents are as follows:(1)The principle of magnetic leakage detection of steel strand corrosion defect is explored.Combining the SMFL theory and magnetic charge theory of electromagnetic field,a magnetic dipole model for magnetic flux leakage detection of steel strand corrosion defects is proposed based on MMM technology,and the adaptive analysis of the parameters of the model is carried out.Then the effectiveness of the magnetic dipole model to detect corrosion defects is also studied.In order to explore the mechanism of corrosion defects magnetic memory leakage,the geomagnetic shielding test is carried out.The test results show that spontaneous magnetization is the premise for the formation of defects magnetic flux leakage,and corrosion is the cause of defects magnetic flux leakage of specimens with spontaneous magnetization.Besides,the test results also show that that the formation of corrosion defects magnetic flux leakage is not a direct effect of the geomagnetic field,and the geomagnetic field does not affect the corrosion defects magnetic leakage signals distribution of the specimens.These provide theoretical support and guidance for subsequent experimental research based on MMM technology.(2)The magnetic leakage detection test for corrosion defects of non-stress steel strand specimens is carried out,and the corrosion defects magnetic flux leakage signals of the non-stress steel strand specimens are collected via horizontal scanning and vertical lifting scanning of the three axis micro-magnetic detection system.In the process of scanning magnetic flux leakage signals,the influences of placement of specimens,Lift-off of magnetic sensor,corrosion width and depth of specimens,length of specimen on the distribution variation rule of corrosion defects magnetic flux leakage signals are investigated.Based on the influences,the magnetic memory leakage characteristic parameters corresponding to the corrosion defects information of the specimens are constructed.(3)The magnetic leakage detection test for corrosion defects of stress steel strand specimens is carried out,and the effect of the stress state on the magnetic signals on the surface of the specimens is explored.The distribution characteristics of corrosion defects magnetic flux leakage signals of the specimens under the holding state of force are preliminarily explored,and according to the distribution characteristics,some characteristic parameters of magnetic flux leakage signals such as normalized peak value of tangential component,normalized peak value of normal component and normalized peak-to-peak spacing of normal component are constructed.The test results show that the relationship between the characteristic parameters of the horizontal scanning corrosion defects magnetic flux leakage signals and the corrosion degree of the specimens presents the Boltzmann function distribution form.In addition,the relationship between the pull force and the normalized peaks of magnetic flux leakage signals of the stress specimens also presents the Boltzmann function distribution.As for the vertical lift off magnetic flux signal of the specimens,there is an intersection point between the normal components of the magnetic flux leakage signals at different horizontal intervals.Similarly,the relationship between the intersection point and the degree of rust and the tensile force of the specimen presents the Boltzmann function distribution form.Therefore,the Boltzmann function distribution relation can be used to invert and reconstruct the corrosion defects of the stress steel strand specimens,and then the purpose to detect the corrosion degree of steel strand specimens can be achieved accurately.(4)The correctness and rationality of the magnetic dipole model for magnetic flux leakage detection of steel strand corrosion defects were verified by finite element simulation.Simultaneously the feasibility of magnetic flux leakage test for corrosion defects is verified.Furthermore,the feasibility of MMM magnetic technology as a detection technique and method for corrosion defects of steel strand specimens is clarified. |
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