Study On The Fatigue Behavior Of CFRP Plate Strengthened Steel Structures And Its Design Method

Fiber reinforced polymer(FRP)composites have been widely used to strengthen and retrofit the concrete structures because of their unique properties such as high strength-to-weight ratio,favorable anti-fatigue and durability behavior and good design ability.In recent years,the use of FRP is attracting more attentions for strengthening and retrofitting the steel structures.Compared to the traditional strengthening method for the steel structures,the externally-bonded FRP technique possesses many advantages,and thus has a wide prospect for strengthening the steel structures.Fatigue failure is one of the main failure modes for the steel structures under cyclic loading.The use of FRP for enhancing the fatigue behavior of the steel structures will be an important application.Therefore,it is necessary to conduct a systematic study on the fatigue behavior of FRP-strengthened steel structures.However,the related studies are still limited,and many issues are needed to be further studied.Fatigue behavior of steel structures strengthened with carbon fiber reinforced polymer(CFRP)plates was investigated in this paper.The main research work and conclusions are listed as follows.(1)An experimental and theoretical study was conducted on the bond behavior between CFRP plates and steel substrates.The effects of the adhesive type,the adhesive thickness and the CFRP bond length on the bond behavior were evaluated.The three-dimensional digital image correlation(3D-DIC)technique was first used to measure the bond-slip behavior of the CFRP plate-to-steel bonded interfaces.The test results show that the bond-slip relationship can be simplified to a bilinear model or a trilinear model depending on the mechanical properties of the adhesive.Based on a theoretical analysis,the bilinear and trilinear bond-slip models were proposed and the key parameters of the bond-slip models were formulated.The proposed models were verified by comparing the predicted results to the test results and were shown to have reasonable accuracy.The comparisons to the two existing models demonstrated that the proposed models had an obvious improvement in terms of the accuracy and the applicability.Based on the proposed bond-slip models,the calculation method for the effective bond length and the ultimate load was also provided.(2)The fatigue behavior of CFRP-strengthened central-cracked and edge-cracked steel plates were experimentally investigated.Under the equivalent CFRP tensile stiffness,the effects of CFRP type and CFRP configuration on the fatigue repair effectiveness were evaluated.The test results showed that the use of CFRP could reduce the crack growth rate,increase the critical crack length and prolong the fatigue life of the cracked specimens.For the central-cracked specimens,the fatigue life of the strengthened specimens was 3.5-4.9 times that of the unstrengthened specimen.For the edge-cracked specimens,the fatigue life of the strengthened specimens was 6.9～11.3 times that of the unstrengthened specimen.Under the equivalent tensile stiffness,the CFRP plate was more effective than the CFRP sheet,and the CFRP configuration had an effect on the repair efficacy.(3)Fatigue tests were carried out to compare the fatigue repair effect of steel beams strengthened with the different FRP plates and the traditional welded steel plate under the equivalent tensile stiffness.The test results showed that FRP plate could increase the crack initiation life,reduce the crack growth rate and prolong the fatigue life more effectively than the welded steel plate.Under the equivalent tensile stiffness,the crack initiation life and fatigue life of the steel beams strengthened by the FRP plates were 2.00～3.25 times and 3.33～5.26 times that of the unstrengthened beam,respectively,whereas the crack initiation life and fatigue life of the steel beam strengthened by the welded steel plate was only 1.70 and 1.74 times that of the unstrengthened beam,respectively.Moreover,compared to the welded steel plate,the use of FRP plates could reduce the residual deflection and stiffness decay,improve the failure mode of the steel beam.(4)The "Three Dimensional Solid-Spring" finite element(FE)model was developed to simulate the CFRP strengthened steel plate and steel beam.Base on the virtual crack closure method,the stress intensity factors of the specimens were calculated and analyzed.The strengthening mechanism of the CFRP plate to enhance the fatigue behavior of the steel structures was revealed,including the reduction of the stress in the steel and the constraint to the crack opening.Using the FE method,the effect of the CFRP on the stress concentration factors of the steel beams was also analyzed.Based on the linear elastic fracture mechanics theory,the predictive method for the fatigue crack propagation of CFRP-strengthened cracked steel plates was established,and the reasonable accuracy was verified by comparing the predictive results and the test results.Further,the predictive method for the fatigue crack propagation of CFRP-strengthened cracked steel beams was developed,which could simultaneously consider the growth of the main crack,secondary crack and web crack.The developed method can predict the crack growth curves and crack growth life with reasonable accuracy.Moreover,the comparative studies show the crack growth life can also be approximately predicted by the simplified method assuming the cracks at the two sides of the tensile flange propagate symmetrically.(5)The influential parameters of the fatigue repair effectiveness of CFRP-strengthened steel structures were systematically investigated by conducting a series of the numerical analyses on the CFRP plate-strengthened cracked steel plates.The effects of nine parameters on the stress intensity factor and fatigue life were discussed,including the initial crack length,CFRP thickness,CFRP elastic modulus,CFRP width,CFRP length,adhesive thickness,adhesive shear modulus,local debonding,and single/double-sided repairing.Furthermore,the effect of CFRP configurations on the fatigue repair effectiveness was investigated.Based on the analysis results,the tiled CFRP configuration is recommended when the fatigue crack of the steel member is similar to the central crack,and the stacked CFRP configuration is recommended for the steel member with a single-edge crack.(6)Based on the theoretical and numerical studies,the calculation formulas for the stress intensity factor of the CFRP-strengthened central-cracked and double-edge-cracked steel plates were developed,respectively.Further,by simplifying the CFRP-strenthened tensile flange to the CFRP-strengthened steel plate,a simplified calculation method for the stress intensity factor of the CFRP-strengthened double-edge-cracked steel beams was proposed.Summarizing the existing studies,the simple predictive method for the fatigue life of CFRP-strengthened steel structures was presented,and the design procedures for the fatigue strengthening of the damaged steel structures using the CFRP plates were provided.