| Adhesively bonded CFRP strengthened-steel structure technology has broad application prospects in the field of civil engineering.On the one hand,the original structure transfers part of the load(including live load)to the CFRP through the bonding layer,thereby reducing the stress level of the original structure.On the other hand,due to the CFRP has high fatigue strength,and the expansion of cracks in the original structure can be restricted by the action of the bonding layer to achieve the effect of reinforcement and repair.Both functions need to transfer loads,the interface layer between CFRP and steel is the weak part of the composite structure in the process of transferring loads.Regardless of the presence or absence of defects in the steel structure,the interfacial bond behavior between CFRP and steel is one of the key issues of this technology.This paper focused on the key issue,and conducted in-depth research on the preparation of high-performance adhesives,the stress distribution of the bonding interface,and the influence of external factors(temperature,humidity,and fatigue load)on the interfacial bond behavior.The main research work is listed as follows:(1)Based on the self-developed epoxy adhesive,the effect of the type and amount of the curing agent and nanomaterials on the basic mechanical properties of the epoxy adhesive at room temperature was obtained,and the mechanism of the nanomaterials reinforced and toughened epoxy adhesive was revealed.The basic blending ratio of high-performance nanomaterial epoxy adhesive was proposed,and the types and reasonable amount of curing agent and nanomaterial were clarified.The results showed that the condensation amine 105,polyetheramine D230,and condensation amine 105 and amino-terminated polyoxypropylene D230 in a mass ratio of 1:2 were the recommended choice of the type of curing agent at room temperature.The addition of graphene oxide and functionalized carbon nanotubes mainly improved the tensile strength of the adhesive,and the addition of functionalized nano-SiO2 mainly improved the fracture toughness and shear strength of the adhesive.With the increased of amount of nano-SiO2,the stress-strain relationship of the adhesive was changed from linear to nonlinear,and the strain energy was increased by 292.10%.Microstructure analysis showed that the addition of nano-materials can suppress the crack propagation in the adhesive matrix and increase the roughness of the cross section.In addition,the addition of nano-SiO2 formed dense plastic voids in the section,and greatly improved the toughness of the adhesive.(2)Using different types and amounts of curing agent and different amounts of nano-SiO2 adhesives,the shear test of CFRP plate/steel double lap joint after curing at room temperature was conducted.The effects of curing agent and nano-SiO2 on the interfacial bond behavior of lap joints were studied.Using condensation amine 105 as a curing agent,the incorporation of nano-SiO2 can significantly increased the ultimate shear strength and effective bonding length of lap joints,and increased the strain on the CFRP surface and the peak value of interfacial shear stress.The bond-slip curves of lap joints with nano-SiO2 content of Owt.%and 0.5wt.%were bilinear triangle models.The bond-slip curve of the lap joint with a nano-SiO2 content of 1wt.%was a trilinear trapezoidal model,which included three development stages:"linear growth","yield platform" and "falling section".The toughness of the bonding interface was greatly improved.(3)Aiming at the difficulty of balancing heat resistance and toughness with a single curing agent,and the effects of the two kinds of curing agents(condensation amine 105 with excellent heat resistance and amino-terminated polyoxypropylene D230 with excellent toughness)mixed on basic mechanical properties of nano-SiO2 epoxy adhesives and bond behavior of lap joints at elevated temperatures(70?)was studied.The mixing ratio of curing agent that meets the CFRP reinforced steel bridge at elevated temperature was obtained.The results showed that the blending ratio of the two curing agents(condensation amine 105 and amino-terminated polyoxypropylene D230)was 1:2,and the recommended preferred curing condition was 90?for 2 hours.The tensile strength and strain energy of the nano-SiO2 epoxy adhesive with the recommended ratio and the recommended curing conditions between ambient temperature 20?-70? were much better than the commercial adhesives,and the elongation at break was also much better than most commonly used commercial adhesives.CFRP plate/steel plate lap joint based on the recommended ratio and recommended curing process of nano-SiO2 adhesive had much better load-bearing capacity and interfacial fracture energy than commercial adhesives.(4)The temperature of steel bridge can be as high as 60? in summer,and the environmental temperature has a significant effect on the mechanical properties of adhesives.Based on the two recommended mixing ratios of nano-SiO2 epoxy adhesives(Mixing ratio 1:the amount of nano-SiO2 was 1.0wt.%,curing agent was condensation amine 105,curing at room temperature;Mixing ratio 2:the amount of nano-SiO2 was 0.5wt.%,curing agent was a mixture of condensation amine 105 and amino-terminated polyoxypropylene D230 in a ratio of 1:2,curing at 90? for 2h),the quasi-static tensile test of the adhesive colloids and the shear test of the CFRP plate/steel plate lap joints were carried out at different ambient temperatures.The influence mechanism of ambient temperature on the mechanical properties of nano-SiO2 epoxy adhesives and the interfacial bond behavior of CFRP/steel lap joints was obtained.By linearly fitting the ultimate bearing capacity and the parameters of bond-slip constitutive of the lap joints at different ambient temperatures,the theoretical prediction model of its ultimate bearing capacity and the bond-slip constitutive at different temperatures was established.(5)Because the steel structure bridge is exposed to the natural environment,often in the hot and humid,sea water and other harsh environment,and long-term experience of vehicle load.Through the constant temperature water bath test of the adhesive colloids and the CFRP plate/steel plate lap joints,the effect of wet and hot conditions on the mechanical properties of the adhesive colloids and the CFRP plate/steel plate interfaces were clarified.The shear fatigue test of the CFRP plate/steel plate interface under different fatigue load amplitudes was carried out,the damage evolution law of the overlap interface was revealed,and the damage model of the interfacial bonding stiffness was established.(6)Based on the bond-slip constitutive of CFRP/steel interface,the theoretical analysis model of interfacial stress was established,and the calculation formulas of interfacial slip,shear stress and CFRP tensile stress were derived.Based on the analytical solution,the influences of the bonding length,the elastic modulus and thickness of CFRP plate and the elastic modulus and thickness of steel plate on the bond behaviors of the lap joints were analyzed.Through the finite element analysis,the damage development process of the lapped interface was obtained,and the influence of the ambient temperature on the bond behavior was simulated. |
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