Experimental Study On Fatigue Performance Of Reinforced Concrete Structures Based On Piezomagnetism Effect

With the wide use of high-strength concrete and high-strength steel bar,the development of reinforced concrete structures is light-weight,high-strength and large-span.The limit state design theory usually leads them to high stress service condition.The stress amplitudes of steel bar and concrete are much closer to the fatigue stress amplitude,the fatigue property of concrete structure is increasingly important to its safety and durability.Based on the piezomagnetism effect of steel bars,the fatigue damage of reinforced concrete structures and the evolution of piezomagnetic signals are studied in this thesis.The main contents are as follows:(1)Based on the Jiles-Atherton(J-A)magnetic mechanical effect model of elastic and plastic deformation,the pinning coefficient considering the initial dislocation and dislocation development is put forward according to the characteristics of the piezo-effect of the deformed steel bar.The J-A model is established and the numerical simulation is carried out for early and mid fatigue stages of deformed steel bar.The magnetic dipole model is used to analyze the affect on piezomagnetic signals of the leakage magnetic field of macroscopic fatigue crack at the late fatigue stage.The microscopic analysis and experimental results have demonstrated that it's feasible when applying the piezomagnetic effect to the fatigue study of reinforced concrete structures.(2)Through static and fatigue-piezomagnetic tests of HRB400 standard specimens,the relationship between stress,strain,initial damage position,fatigue fracture position and piezomagnetic signals is analyzed to investigate the fatigue properties of steel bar under cyclic loading.The evolution of the parameters such as time-varying and hysteresis curve reflects the fatigue damage development of steel bars,the fatigue damage variable is defined based on the characteristic points of the piezomagnetism signal which is in accordance with the three-stage fatigue principle.The fatigue failure criteria and life prediction method are established.(3)Through static and fatigue-piezomagnetic tests of HRB400 ribbed steel specimens,the relationship between stress,strain,and piezomagnetic signals is analyzed which is different with standard specimen due to the surface shape.According to the time-varying curve and the hysteresis curve,the characteristic points,amplitude and extreme ratio of the piezomagnetic signal are three stages with the fatigue process.Based on the variation of the pinning coefficient of the ribbed steel bar,the damage mechanism of the deformed steel are put forward and the distortion of piezomagnetic signal in late stage can help warning fatigue failure.The fatigue damage variables are defined based on the characteristic points and the amplitude of piezomagnetic signals,and the fatigue failure criterion and life prediction method of the ribbed steel bars are established.(4)The relationship of bonding stress,slip,and piezomagnetic signals is analyzed by static and cyclic loading tests of the pullout specimens for the local bond slip and the pre-gap beams for the bond slip between cracks.It is found that changes of the interface piezomagnetic signals at the initial cycles is the most significant,and the curve shape keep stable after several cycles.The evolution features of piezomagnetic signals of the two types of specimens are consistent with the interface damage mechanism,which can reflect the principle of interface performance degradation between the steel bar and the concrete.(5)Through the constant amplitude fatigue test of reinforced concrete beams,the relationship between load level,strain,deflection and piezomagnetic signal is analyzed.The results show that the piezomagnetic signal is sensitive to the cracking of beams,especially for initial cracking.It indicates that crack will lead to the sudden change of stress state of steel bar.According to the time-varying curve and the hysteresis curve,the characteristic points and amplitudes of the piezomagnetic signals are three-stage changing with the fatigue process and the distortion of piezomagnetic signal in late stage can help warning fatigue failure.The fatigue failure position and failure mode of the reinforced concrete beam can be preliminary judged by comparing the amplitude of the normal and tangential magnetic signals.(6)The fatigue damage mechanism of reinforced concrete beams is analyzed by fracture surface analysis and section state analysis.The degradation of interface behavior of reinforced concrete leads to complex stress and multi-stage stress of steel bar,and its fatigue life is significantly lower than that under axial tensile.The steel bar's stress state changing process with different fatigue failure modes is different.The piezomagnetic signal of the steel bar not only reflects the process of stress redistribution and interface degradation due to the fatigue crack propagation of the concrete beam,but also relates to the fatigue fracture position of the steel.The fatigue damage variables are defined based on the piezomagnetic parameters,and the fatigue failure criterion and life prediction method of reinforced concrete flexural members are established.