| For prefabricated concrete structures connected by wet jointing,the interface between the old and new concrete is the weak layer of the structure and the interface debonding or peeling can affect the static and fatigue mechanical properties of the members.The traditional methods of detecting interface damage and fatigue damage have problems such as low sensitivity to the accumulation of micro-damage processes and difficulty in long-term monitoring,and a new index needs to be proposed.As a ferromagnetic material,reinforcement in reinforced concrete structures has piezomagnetic effect under different stresses.The feasibility of piezomagnetic signals for fatigue damage detection has been demonstrated in existing experimental studies and has been introduced into the fatigue performance testing of reinforced concrete structures.However,few studies have been conducted on the mechanism of the effect of debonding on prefabricated concrete structures,and rare research has been conducted on the mechanism and application of piezomagnetic signals in prefabricated concrete structures.In this paper,based on the test on new-old concrete interface and composite beams,a series of experimental and theoretical studies have been carried out for the stress-strain change law,damage mechanism,piezomagnetic signal change mechanism and theoretical model at the interface and member level.The details are as follows:(1)Static and cyclic loading tests were carried out on 64 sets of the specimens of new-old concrete to investigate the tensile and shear properties.Static tests were carried out to investigate the interface mechanical properties and the effect of interface shear keys under static loading.Fatigue tests were carried out to investigate the degradation of the shear and tensile properties of the interface under different load levels with and without shear keys.The results show that ultimate failure layer were all occurred at the new and old concrete interfaces both under static and cyclic loading,and the interfaces are weak surfaces.The interface shear key can effectively improve the shear and tensile static and fatigue performance of the interface.The S-N curves of the new-old concrete interfaces under cyclic loading were obtained by data fitting analysis.(2)Three different cross-sections of prefabricated concrete composited T-beams were designed,and four-point negative moment static and cyclic load were carried out on 15 specimens to investigate the static performance of the composite beams,the fatigue performance under different load levels,and the mechanism of the influence of the new-old concrete interfacial debonding on the member’s mechanical properties.It was found that under static load,the interface debonding reduced the synergistic performance of the precast beam and the cast-in-place layer,resulting in the yielding of the reinforcement in advance and causing premature member failure.Interfacial shear keys can limit and delay the development of interfacial peeling and improve the static performance of the member.Under cyclic loading,the interface peeling also causes stress redistribution in the cross-section,and the increase in the reinforcement’s stress amplitude will accelerate the fatigue fracture of the reinforcement.Compared to high load levels(70%-80%),shear keys can more effectively inhibit the generation and development of interfacial peeling and improve the fatigue performance of the member at low levels(50%-60%).(3)Based on the strip analysis method and segmental linear analysis,a whole process non-linear analysis calculation model considering different material damage is established.The model takes into account the stress redistribution caused by the gradual peeling of the new-old concrete interfaces.Based on the model,the stress and strain in the composite beam under each load level or after different loading cycles can be calculated,and the calculation of the fatigue damage and the prediction of the fatigue life can also be achieved.A comparison of the simulated and experimental values verifies the validity of the model.Meanwhile,the effect of interface debonding on the static and fatigue performance of the composite beams in the experimental study was again verified by the simulated model.(4)The piezomagnetic signal of the steel reinforcement of the interface specimens with shear keys under static and cyclic load was measured and analyzed.The relationship between the piezomagnetic signal and the stress and strain,and the evolution of the magnetic signal at each fatigue stage were investigated.The results show that the piezomagnetic signal has a similar trend to the reinforcement strain,and is more sensitive to the stress history,and also undergoes a "three-stage" development under cyclic loading.In the early and middle stages of fatigue,the magneto-mechanical and dislocation effects control the development of the magnetic signal,while at the end of fatigue the leakage effect causes distortion of the existing magnetic field.Based on the J-A model,the J-A-F model and the local magnetization equilibrium model,the development of the reinforcement magnetization at the new-old concrete interface under cyclic loading was simulated and the validity of the model was demonstrated.(5)The piezomagnetic signal of the composite beams under cyclic load was measured and analyzed.The mechanism of the magnetic signal evolution at different fatigue stages and the effect of interfacial debonding on the magnetic signal were explored based on the multiple indicator analysis.The analysis shows that the magnetic signal of reinforced concrete members under cyclic loading is also consistent with the "three stages" trend of the conventional fatigue damage index.The magneto-mechanical and dislocation effects control the variation of the magnetic signal in the first two stages,while the leakage effect controls the magnetic signal variation in the last fatigue stage.The interface debonding leads to stress redistribution and a sudden change in the microstructure of the steel bar,resulting in a mutation in the magnetic signal.Compared to conventional fatigue damage indicators,the piezomagnetic signal is more sensitive to the location of fatigue damage and the accumulation of micro-damage.(6)Based on the mechanical simulated model of the composite beam,and the simulated model of the magnetization of the reinforcement at the new-old concrete interface,the simulated model of the piezomagnetic signal at the member level was established.The comparison of the calculated results with the experimental results shows that the model can effectively simulate the development of the reinforcement magnetization in the composite beam under cyclic loading.Meanwhile,the damage accumulation formula is used to obtain the transition points at fatigue stage two based on the simulated and the calculated piezomagnetic signal,and a method for predicting the residual fatigue life of the member is proposed. 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