Experimental Study And Theoretical Analysis Of Fatigue Performance Of Carbon Fiber Cable

With the development of modern bridge technology,cable-stayed bridges and suspension bridges have become the main structural forms of long-span bridges.Both of these structures have the advantages of large span and light weight.But at the same time there are their own shortcomings,such as complex cable-stayed bridge design and calculation,construction technology requirements,such as complex connection structures.The suspension bridge has poor vibration resistance,and the cable body cannot be replaced during service.Bridge engineers at home and abroad continue to promote the further growth of span,so that bridge engineers are faced with the problem of traditional steel cables:(1)Great self-weight;(2)Low service life;(3)High maintenance cost and construction difficulty.Based on the above mentioned problems,CFRP(Carbon Fiber Reinforced Polymer/Plastic),a new material which can substitute steel for the cable,can solve the above problems very well.FRP composite materials has the advantages of light weight,high strength,nonmagnetic,corrosion resistance,fatigue resistance and other excellent properties.Current applications of FRP in civil engineering field can be used for both the structure of strengthening and repairing,and it can also be applied to large span bridge in giving full play to the advantages of the material performance,especially used for cable-stayed bridge and suspension bridge.In this paper,experimental research and theoretical analysis are combined to study the fatigue performance of CFRP wedge-type cables.The main research contents and results are as follows:(1)Through analytical models,the preliminary design of wedge-type cable can be finished,and then through theoretical analysis and finite element simulation,from the perspectives of anchor-ring angle,pre-tightening force,contact surface friction coefficient and other aspects,we can explore the effects of these parameters on the anchoring performance of CFRP wedge-type anchorage,and determine the optimization of the anchorage.In addition,through static tensile tests of the new CFRP cables,the anchorage performance of the new type of anchorage can be studied,showing a good agreement with experimental results.(2)Based on the tensile fatigue tests of carbon fiber composite material,the degradation law of its strength and stiffness in the process of fatigue loading is discussed,and based on this law,a normalized fatigue model is proposed suitable for both the residual stiffness and strength.In this paper,five kinds of stress amplitude fatigue loads have been conducted to investigate the degradation law of its strength and stiffness,and the experimental data are compared with the prediction results of the proposed model to verify the accuracy of the proposed model.The experimental and analytical results show that the model is suitable for predicting the residual strength and stiffness of carbon fiber composite cable under different fatigue loads.In the process of fatigue loading,the stiffness and strength of carbon fiber composite cable degenerate and both show a three-stage development trend.It is also found that the larger the fatigue stress amplitude is,the more obvious the stiffness or strength degradation is and the more unobvious the three-stage boundary is.(3)In order to investigate the damage development regularity of composite material subjected to fatigue loading,this paper proposes a normalized phenomenological cumulative damage model based on the residual strength.In this model,it is assumed that cumulative damage and stress level have a linear relationship.Based on the linear relationship,damage curves of untested stress levels can be derived from that of tested stress levels.Fatigue tests of CFRP tendons are conducted,and experimental data of GFRP laminates from references are adopted to validate the reliability of the proposed damage model.Besides,this paper investigates the influence of stress level on the damage evolution of composite materials.As the stress level increases,boundaries between adjacent stages of damage curves become unobvious.Furthermore,this model is extended to predict fatigue lives of composite materials subjected to multi-level fatigue loading.Predictions of proposed model have good agreements with the experimental results of two-level fatigue tests,especially in the case of low-high stress tests.(4)In this paper,static load tests and fatigue performance tests are carried out on the three-hole 8mm CFRP wedge-type group cables.The main research contents are as follows: 1.static load tests of anchor group: design of loading device,description of failure phenomenon in static load test,and comparison of anchorage performance between single anchor and group anchor;Experimental results show that the wedge-type group cables meet the requirements of static and fatigue tests well;2.Group anchor fatigue tests: the determination of fatigue load level,the description of fatigue test failure phenomenon,the exploration of the causes of insufficient life,the impact of fatigue load on the overall residual strength and single residual strength,and the exploration of the applicability of fatigue damage model in chapter 4.