Study On Interfacial Debonding Mechanism And Fatigue Behaviour Of Notched Steel Beams Strengthened With Prestressed CFRP Plate Subjected To Hygrothermal Environment

Prestressed Carbon Fiber Reinforced Polymer(CFRP),as an active strengthening technology,can effectively inhibit the expansion of fatigue cracks in steel structures,and increase the bearing capacity and fatigue performance,and improve CFRP plate stress hysteresis and CFRP utilization.For the prestressed CFRP plate reinforced steel structure,the intact bonding interface is a prerequisite for maintaining normal prestress and inhibiting crack propagation.The alternation of humidity and temperature in the southern coastal areas would cause the deterioration of the adhesive and the weakening of the interface bonding.Besides,the overloaded fatigue would exacerbate the damage of the adhesive interface,which would affect the bonding behavior and fatigue performance of the reinforced component.This paper takes the notched steel beam strengthened with prestressed CFRP plate as the research object,and focus on the effects of hygrothermal environment and fatigue damage,the interface debonding failure mechanism,the prestress loss of the CFRP plate,the bending capacity of the reinforced steel beam,the interface fatigue crack propagation and fatigue life were studied by theoretically and experimentally.Specific research contents and results include:First,considering the effect of prestress,the interfacial stress distribution,the stress intensity factor at the notch tip of the steel beam,and the interface energy release rate were studied,and the theoretical calculation formulas of interface debonding and crack propagation failure are derived.The calculation results show that the prestressed CFRP strengthening can significantly reduce the interface principal stress,the notch tip stress intensity factor,and the interface energy release rate.In addition,the parameter analysis shows that the initial crack length and the thickness of the bonding layer have a significant effect on the interface stress distribution,and the crack stress intensity factor and the interface energy release rate both increase with the increase of the initial crack length.The mid-long-term prestress loss of CFRP plate under hygrothermal environment was monitored by fiber grating sensor.The results showed that the prestress loss of CFRP plate mainly occurred within 7 days after prestress release,and then the prestress level remained basically stable;after 78 days,the prestress of CFRP in the laboratory environment and the hygrothermal environment only decreased by 1.64%and 2.65%,respectively.It shows that the prestressed anchorage developed in this paper can effectively restrain CFRP plate,and the hygrothermal environment causes a degradation in bonding performance and increases the prestress loss.The flexural performance of reinforced steel beams in hygrothermal environment was studied,to investigate the effects of prestressing,hygrothermal environment on bearing capacity,failure mode,CFRP utilization rate,etc.and verified by the finite element analysis based on the mixed cohesion model in hygrothermal environment.The test results show that:prestressing can increase the interface cracking load and ultimate load of the strengthened steel beam,but the stiffness increase is not obvious;the hygrothermal environment reduces the cracking load of the reinforced specimen,but has little effect on the ultimate load;The failure mode of the prestressed reinforced specimen is the mixed failure including the CFRP plate extraction from the anchor and the fracture of the CFRP plate;the prestress strengthening greatly improves the utilization rate of the CFRP plate.The deviation of the crack load and corresponding displacement obtained by finite element simulation and test result are within 12%,which verifies the reliability and effectiveness of the mixed cohesion model based on the traction separation rule under environmental influence.Moreover,the interface performance and flexural bearing capacity of reinforced steel beams after overload fatigue damage were studied.The test results show that:prestressed reinforcement can significantly reduce the fatigue damage at the crack tip of the steel beam;Overload fatigue loading have a significant effect on the interface bonding performance at the notch location,thereby greatly reducing the interface cracking load;the combined effect of overload fatigue and hygrothermal environment can further reduce the interface cracking load,but have slightly effect on the ultimate load,indicating that the combined effect on the bonding layer appears mainly around the notch.Finally,the fatigue performance of the reinforced steel beam after hygrothermal environment was studied.The results show that under fatigue load,the crack tip of the steel beam expands first,and then the bonding interface starts to debonding from the notch;prestressed CFRP strengthening greatly increases the fatigue cycles of initiation of steel beam crack and the interface crack,and slows the steel beam notch and interface crack growth rate and increases the fatigue life of the reinforced steel beam;however,the effect of the hygrothermal environment significantly reduces the no-cracking life and crack propagation life of the bonded interface.From the test results,the S-N curve based on the maximum principal stress of the bonded interface at the notch and the no-cracking life of the interface is obtained,which can predict the fatigue life for the same kind of reinforced structure.Besides,based on Paris Law of energy release rate,the formula for predicting the interface crack growth life in laboratory environment and hygrothermal is proposed.In sum,this paper derived the theoretical formula of interface debonding of prestressed CFRP reinforced notched steel beams,and constructed a finite element model in hygrothermal environment.The interface bond degradation under overloaded fatigue damage and hygrothermal environment were revealed,a method for predicting fatigue life of steel beams under reinforcement was proposed.It will provide theoretical basis and experimental data for the design and construction specifications of prestressed CFRP strengthening steel structures,and provide important references for engineering applications such as strengthening for steel,bridges and crane beams.