Study On Fatigue Crack Growth Behavior Of Corroded Reinforced Concrete Structure On Weak Magnetic Effect

The widely use of high-strength concrete and steel bars in engineering structures,the adoption of limit state design theory,and the long-term overloading would result in high working stress conditions of reinforced concrete（RC）structures such as bridges,crane beams,floor slabs,offshore platforms,and so on.The fatigue damage caused by the long-term action of repeated loads should reduce the bearing capacity and safety performance of RC structures.Under the action of the coastal corrosive environment,corrosion pits in steel bar would be formed at the transverse cracks of concrete members,which will accelerate the initiation of steel fatigue cracks and thus reduce the fatigue performance of the members.Fatigue fracture of steel bars at corrosion pits should lead to sudden failure of components,which is extremely harmful.Therefore,it is quite critical to explore the fatigue crack propagation law of corroded steel bars under fatigue for the fatigue performance analysis of RC structures.In this paper,the characterization method of the crack growth behavior of the corroded rebar in concrete is studied on the piezomagnetic effect of the steel bar and the distribution rule of the surface magnetic field near the defect.Finally,the calculation method of fatigue crack propagation of steel bars in actual corroded RC structures based on METS-Fatigue method and using weak magnetic signals is presented.The major contents of this article are as follows:（1）Based upon energy conservation law,the uniformity of characterizing steel fatigue damage based on fracture mechanics and weak magnetic effects（piezomagnetic effect and residual magnetic field distribution）was studied.Quantitative relationships exist between crack growth rate and crack length and weak magnetic signal;Based on the magnetic dipole model,the magnetic field distribution on the defect surface was numerically calculated,and the relationship between its stress and the defect size was analyzed;The force-magnetic coupling effect and the magnetic dipole model were verified by numerical simulations of standard compact tensile（CT）specimens and pit-corroded ribbed steel bars under tensile stress,as well as experimental studies on the evolution law of magnetic field of pit-corroded rebars subjected to uniaxial tension.（2）Through the high-cycle fatigue crack growth test of HRB400E steel bar CT specimen,the crack growth parameters C and m of the material under different stress ratios were obtained,and the evolution law of piezomagnetic signal and surface residual magnetic field distribution in the stage of stable crack propagation were analyzed.The results show that the weak magnetic signal can reflect the process of crack propagation and the change of crack length;Based on the experimental results,the quantitative relationships between the stress intensity factorΔK and the amplitude of the tangential magnetic induction intensityΔB_t,the crack length a and the maximum gradient value of normal residual magnetic field distribution G_max were established.It was verified by experiments,and the differences between the crack growth curve predicted by the piezomagnetic method and the fracture mechanics method and the experimental results were compared.（3）The high-cycle fatigue-weak magnetic field tests of HRB400 steel bar standard specimens prefabricated with a semi-ellipsoid pit were carried out.The fatigue crack growth behavior of pit-corroded steel bars was predicted based on Fracture mechanics-based fatigue crack growth model.The amplitude of the tangential magnetic induction intensity-stress hysteresis curve and the maximum gradient of the normal component of the residual magnetic field distribution all exhibited a three-stage evolution law.The influence of stress ratio and crater parameters on the evolution law of piezomagnetic signal and residual magnetic field distribution was analyzed;During the fatigue crack propagation of pit-corroded steel bars,the stress intensity factorΔK and the tangential compressive magnetic field hysteresis curve variationΔB_t,the crack length a and the maximum gradient value of normal residual magnetic field distribution G_max all showed a good linear relationship.Finally,the evaluation of fatigue crack growth of pit-corroded steel bars based on the piezomagnetic method was given and then compared with the prediction results of fracture mechanics.（4）Through the high-cycle fatigue-weak magnetic test of pit-corroded HRB400steel bars in concrete beams and in air,the law of weak magnetic signal changes in the fatigue process of pit-corroded steel bars in air and concrete beams is analyzed.The change of weak magnetic signal can reflect the development of fatigue damage and fatigue fracture failure,and the characteristics of weak magnetic signal can reveal the stress concentration and fatigue damage characteristics of pitted steel bars in concrete;Based on the experimental results,the quantitative relationships between the variation of the normal piezomagnetic hysteresis curve and the stress intensity factor in the crack tip were established for pit-corroded steel bar inside RC beams.A method for quantitatively evaluating the fatigue crack growth process of pit-corroded steel bars in RC beams based on weak magnetic signals is presented and compared with the crack growth behavior predicted by fracture mechanics.（5）The similarity of steel corrosion pits and cracks was analyzed through literature research.Considering the life of the equivalent crack initiation stage at the pit,the fatigue life of the pit-corroded steel standard specimen is predicted based on the equivalent surface defect size model.The equivalent surface defect size of pit-corroded HRB400 steel bars in air/RC beams were obtained by model regression analysis.The quantitative relationships between the equivalent surface defect size and the maximum cross-section corrosion rate and the pit depth-to-width ratio were obtained by surface fitting.（6）Based on the METS method for predicting the durability life of RC structures,METS-Fatigue method was established to evaluate the fatigue life of the research object based on the results of indoor accelerated corrosion fatigue tests.Suggestions on the method and parameter selection for statistical data analysis of engineering structure and indoor corrosion coupling test are given.The statistical data of engineering structure and indoor corrosion coupling test method and parameter selection were analyzed.The limit state equation is established with the crack size as the key damage index,and the evolution of the corrosion pit and fatigue crack size of rebar in actual RC structure under the action of corrosion fatigue with the service time was analyzed based on the linear elastic fracture mechanics method.Taking the weak magnetic index as the nondestructive evaluation method,based on the METS-Fatigue method,the process and steps of evaluating the crack propagation behavior of corroded steel bars in actual concrete bridges and predicting the remaining fatigue life were given.