| In recent years,fiber reinforced polymer(FRP)composites,and especially for carbon fiber reinforced polymer(CFRP)composites,have been widely used in the strengthening engineering of steel structures for its high strength-to-weight,excellent corrosion resistance,favorable anti-fatigue and convenient construction.The reinforcement effect and failure mechanism of CFRP reinforced steel structures have the close relationship with the bond behavior of CFRP-steel interface,meanwhile,fatigue failure is one of the main failure modes for the steel structures under cyclic loading.Therefore,it is necessary to make a comprehensive study on the bond behavior of CFRP-steel interface under static and fatigue loading.However,the related studies,and especially for the research on interfacial bond behavior under different curing conditions,are extremely limited.Accordingly,in this paper,taking the effects of different factors(including the curing temperature)into account,the bond behavior of CFRP-steel interface under static and fatigue loading were investigated using the methods of experiments and theoretical derivation.The main contents and achievements of this paper are listed as follows.(1)Using double-shear specimens,the experimental was conducted on the bond behavior of CFRP sheet-steel interface under static loading.The effects of the CFRP bond length,the number of CFRP bond layer,the CFRP bond width,curing age and curing temperature on the bond behavior were evaluated.The test results show that the effects of CFRP bond length and curing age on the bearing capacity was not significant when CFRP bond length exceeded the effective bond length.The interfacial bearing capacity increased with the increment of the number of CFRP bond layer,CFRP bond width and curing temperature.An increase in either the number of CFRP bond layer,bond width or the curing temperature can improve the ductility of specimens.The interfacial bond stiffness increased with the increment of the curing age.The effective bond length increased with an increase in the number of CFRP bond layer and CFRP bond width,but decreased with the increment of the curing age.(2)The calculation model of the interfacial shear stress of CFRP sheet-steel was provided based on the elastic theory analysis.On this basis,according to the experimental data,the bond-slip relationship curve of CFRP sheet-steel interface was obtained and the effects of different factors on the bond-slip relationship were discussed.Based on the test results and relevant research results,considering multi-factors influence,the Popovics model,simplified bilinear model and three straight lines model(for curing temperature exceeded 40?and reached 50?)of the bond-slip relationship were proposed.The key parameters in the model were analyzed quantitatively,and a comparison of the test results and calculated results indicated that the proposed models were shown to have reasonable accuracy.(3)A series of CFRP sheet-to-steel double-shear specimens were tested under fatigue loading by considering two key parameters—namely,the upper bound value and the lower bound value of the fatigue loading.The test results show that the fatigue life decreased with an increase in the upper bound value of the fatigue loading and increased with an increase in either the lower bound value of the fatigue loading or the curing temperature.The effect of the lower bound value of the fatigue loading on the effective force transmission length was not significant and the effective force transmission length increased with the increment of the upper bound value of the fatigue loading,decreased with an increase in curing temperature.But there was an increasing trend of the effective force transmission length with an increase in curing temperature when the curing temperature exceeded 40?.(4)The CFRP sheet-steel interface fatigue crack propagation behavior was investigated based on the results of fatigue tests.The test results showed that the crack propagation approximately present two stages.The first stage was a period of stable crack development.This stage can nearly reach to 95% of the fatigue life.The second stage was the debonding failure period and can reach to about 5% of the fatigue life.The crack development rate increased with the increment of the upper bound value of the fatigue loading and decreased with an increase in either the lower bound value of the fatigue loading or the curing temperature.The relationship between the stress ratio,standardized stress amplitude,stress level and the crack development rate were evaluated.On this basis,according to the experimental data and relevant research,taking the stress level as a parameter,the calculation model of the crack development rate with reference to the form of the Paris Law was put forward.(5)The effects of the fatigue stress level and the curing temperature on fatigue life were discussed quantitatively and the prediction method for fatigue lives of CFRP-steel interface was proposed.Based on the test results,the empirical formula for ?S-N relation was obtained considering the curing temperature.The empirical formula for ?S-N relation was verified by comparing the test results and was shown to have better accuracy.(6)The evolution law for the bond-slip relationship of CFRP sheet-steel interface under fatigue loading was investigated considering the stress level and the curing temperature.The results showed that fatigue loading leaded to the degradation of bond-slip relationship of CFRP-steel interface and this degradation was reflected in degradation of the shear strength and the stiffness of the ascending portion of the curve.The shear strength decreased with the increment of the number of cycles and the rate of the degradation increased with an increase in the stress level,but decreased with an increase in the curing temperature.Based on the test results,the degradation model of the interfacial maximum shear stress was proposed.The slip corresponding to the maximum shear stress increased slightly with the increment of the number of cycles,and the increasing amplitude was very small.Accordingly,the degradation model of the bond-slip relationship of the CFRP sheet-steel interface was proposed considering the effects of the stress level and the curing temperature.The stiffness of the ascending portion of the bond-slip curve kept degenerating with the number of cycles.The rate of this degradation increased with the increment of the stress level and decreased with the increment of the curing temperature.The degradation model of the stiffness of the ascending portion was proposed considering based on the test results.The fatigue damage was described quantitatively by using the stiffness of the ascending portion and the fatigue cumulative damage model was obtained according to the test results. |
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