Analysis On The Main Fatigue Crack Propagation Law And Fatigue Life Of RC Beams Strengthened With Pre-tensioned Prestressed CFL

Reinforced concrete(RC)structure strengthened by prestressed Carbon Fibre Reinforced Polymer(CFRP)has been widely used in civil engineering project in recent years,because it can play the high strength of CFRP,close the original crack,limit the new crack initiation and unload the main reinforcement.However,the research on the fatigue performance of RC structure strengthened by prestressed CFRP is still not enough.Therefore,the study of the fatigue life and the main fatigue crack propagation law of RC structure strengthened with prestressed CFRP sheets has great engineering application value.Thus,this paper carry out the theoretical and experimental study of fatigue life and crack growth behavior under constant amplitude fatigue loads of RC beams strengthened with prestressed Carbon Fiber Laminate(CFL)which is a new type of CFRP sheet invented by our research team.The main contents and conclusions in this study are as follows:(1)The finite element model of RC beam reinforced with CFL with the mode-I crack submitting the 3-point bending load,which is used to calculate the J-integral of the main crack tip of the prestressed strengthened beam,is established by using the finite element(FE)software ABAQUS.And the calculation results of the finite element model in this paper are in agreement with the theoretical calculation.The results of parametric analysis show that the elastic modulus of concrete,the sectional area of CFL and the prestress level have great influence on the J integral.(2)Prestress is applied to the strengthened beam of CFL by Pre-tensioning Method,and the loss of prestress is collected by the strain gauge.The test results show that the loss of prestress mainly comes from five phases including stretching process,pasting process,curing process,release process and long-term loss,and the total prestress loss rate is 13.85%.(3)The fatigue test of the RC beam strengthened by the prestressed CFL under constant amplitude fatigue load is carried out,and the fatigue life and fatigue failure mechanism of the strengthened beam are analysed.According to the experimental results,the S-N curve of the strengthened beam under the constant amplitude fatigue load is obtained.Combined with the experimental data of our research team,the formula of fatigue life of the strengthened beamwith the influence of prestress is derived.It's convenient to estimate the fatigue limit and the fatigue life of strengthened beam under given fatigue load and prestress level by using this formula.(4)The test of the main fatigue crack propagation of the strengthened beam is carried out.The digital image correlation method(DICM)is used to measure the height of the main fatigue crack,and the a~N curve of the reinforced beam under the constant amplitude fatigue load is given.Combined with the J integral calculated by the finite element model,the Paris semi-empirical formula of the propagation ratio of the main fatigue crack in the stable propagation phase is fitted.By using this formula,the fatigue life of stable propagation phase can be predicted conveniently.And the approximate fatigue life of the strengthened beam can also be obtained.