| Carbon fiber-reinforced polymer composites(CFRP)have shown broad application prospects in the maintenance and reinforcement of engineering structures because of their excellent properties such as high strength,light weight,superior durability,and great corrosion resistance.Compared with concrete structures,the studies on CFRP-strengthened steel structures are limited.The key problem of fatigue strengthening of steel structures with CFRP is the interfacial bonding between CFRP and the strengthened structure and their fatigue performance,as well as the influence of harsh environment(e.g.,high temperature).In this paper,the bond behavior and fatigue performance of CFRP-strengthened damaged steel components are studied by means of experimental,theoretical,and numerical studies,considering the effect of temperature.(1)The bond behavior between CFRP laminate and steel at room temperature are investigated.The mechanical behavior of CFRP-steel lap joints with different epoxy adhesives and CFRP materials were studied experimentally.The effects of material properties of adhesive and CFRP on the failure mode,bond-slip constitutive,and bonding strength were analyzed,and the bond strength calculation method was discussed and improved.The relationship between the interfacial bond strength and the mechanical properties of the adhesive was examined based on the experiment results in this paper and the existing research database.The interfacial bonding mechanical behavior of CFRP-steel was numerically studied based on the cohesive zone model,and the effect of peeling stress on the interfacial bond strength was discussed.(2)The mechanical performance of epoxy adhesive and CFRP-steel bonding interface at elevated temperatures was studied.The high-temperature mechanical properties of four typical epoxy adhesive castings were studied by dynamic mechanical analysis(DMA)and quasi-static tensile test.Two types of adhesives with high glass transition temperature and tensile strength at high temperatures were selected to fabricate CFRP lamina-steel double-lap specimens.The mechanical properties of these specimens at different temperatures(25?-70?)were studied via tensile tests and digital image correlation(DIC)technology,and the bond strength,joint stiffness,failure mode,and interfacial damage mechanisms were obtained.The degradation law of mechanical properties of bulk adhesive at high temperature were analyzed,as well as its relationship with the mechanical degradations of lap joints.(3)The strength and toughness of the bonding interface between CFRP lamina and steel with a novel film adhesive were carried out at room and elevated temperatures.The thermal mechanical behavior of the film adhesive was investigated by DMA.The tensile shear tests of the film-adhesive CFRP-steel double-lap specimens with different bond lengths were carried out at room and elevated temperatures,and the failure mode,load-displacement response,CFRP surface strain and interfacial bond stress distributions were obtained.The bond-slip relationship of the film-adhesive bonding interface at different temperatures was established and compared with those of the traditional paste-adhesive bonding interfaces.Based on the test data and collected data,the commonly used bond strength models are discussed,and the bond strength models are proposed for different failure modes.The novel film adhesive was verified to have significant advantages in strength,toughness and high-temperature resistance for bonding CFRP to steel.(4)Experimental study on fatigue behavior of CFRP-strengthened single-cracked steel plates under tensile cyclic loads was carried out at different temperatures.The thermo-mechanical properties of the bulk adhesive specimens with different curing procedures were studied via DMA,and the optimal curing procedure of the epoxy adhesive was obtained.The fatigue tests of pure steel plate(un-strengthened)and CFRP-strengthened cracked steel plates were carried out at different temperatures.In the tests,the "beach marking" and "back-face strain" techniques were utilized to monitor the propagation of fatigue crack in steel plates and damage within bonding interfaces.The failure mode,fatigue crack growth,fatigue life enhancement factor,and interface damage evolution of CFRP-strengthened damage steel plate at different temperatures were obtained.(5)The fatigue life prediction theory and design method of CFRP-strengthened single-cracked steel components are studied.The calculation method of interfacial bond stress within CFRP-steel bonding interface is analyzed.The calculation method of the stress intensity factor at the crack tip of CFRP-strengthened cracked steel plate is proposed based on the linear elastic fracture mechanics,and the modified Newman model for fatigue life prediction of CFRP-strengthened damaged steel components is proposed.Compared with the fatigue test results in(4),the applicability of classical Paris model and modified Newman model for fatigue life prediction of CFRP-strengthened damaged steel components is discussed.Based on the verified theoretical model,the effects of ambient temperature and key design parameters on the fatigue crack growth and interfacial bond stress distribution of CFRP-strengthened steel components are analyzed,and the corresponding fatigue design criteria are proposed.(6)The fatigue performance of diaphragm cutout and CFRP reinforcement in steel box girders were studied.The fatigue performance of smooth diaphragm cutouts in a steel box girder is studied through full-scale fatigue test and theoretical analysis.The thermal residual stress distribution of cutout and its effect on fatigue performance are revealed by thermal-elastoplastic finite element simulation.The applicability of nominal stress method and hot-spot stress method for fatigue evaluation of cutout details is studied,and the fatigue grade and corresponding evaluation methods of the smooth diaphragm cutout details are determined.The fatigue performance of cutouts with initial geometric defects is studied experimentally,and then they were strengthened by externally bonded CFRP laminas.The fatigue properties of the strengthened cutouts and CFRP reinforcement systems were tested and analyzed,and some valuable conclusions were achieved. |
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