| Fiber Reinforced Polymer (FRP) becomes more and more widely applied in structure repairing engineering in recent years, because of their merit of high-strength, high-elastic modulus, convenient alternative and corrosive protection. The fatigue and durability property of structure strengthened with FRP is concerned. The crack propagation of reinforced concrete (RC) construction strengthened with FRP takes a dominant part of the whole fatigue life under cyclic loading. Therefore, study on crack propagation in RC construction strengthened with FRP under cyclic loading has important scientific significance. Furthermore, random loading is the most form of loading which is submitted by bridge structure in service, study on fatigue property and fatigue life prediction of strengthened structure under random loading has great engineering application value for determining repairing period and insuring safety of the structure.The research subject in this paper is RC beam strengthened with carbon fiber laminate (CFL) which is invented and developed by our research team. The fatigue crack propagation law of RC beam strengthened with CFL under cyclic loading is discussed. The fatigue property and fatigue life prediction of RC beam strengthened with CFL under random loading is researched. Main contents and conclusions in this paper are following:1) Based on virtual crack model and considering the influence of fracture process zone (FPZ) on fracture resistance of strengthened beam, the fracture resistance calculation formula and resistance curve of RC beam strengthened with CFL are proposed, and the influence factor is discussed. The calculation result shows that when crack relative height (α) approaches 0.7, the fracture resistance of strengthened beam increases rapidly. This result is corresponded with observation by test. Parameter analysis show that the influence of different steel and CFL contents on fracture resistance is more apparent.2) The fracture mechanics analysis model of RC beam strengthened with CFL with mode-I crack submitting 3-point bending loading is established. The stress intension factor (SIF) and stress nearby the crack tip is calculated by using popular finite element (FE) soft ANSYS, building 3-dimension model and calculating. The results show that with the increase ofα, SIF of crack tip first increase and then decrease. Whenαis 0.3, the SIF is maximum. Whenαapproaches 0.75, the SIF approaches zero.3) The experimental study of main crack propagation law of RC beam strengthened with CFL under cycle loading is carried out. The result shows that the crack propagation can be divided by three phase: crack initiation and quickly propagation (phase I), stable propagation (phase II) and unstable propagation (phase III). The fatigue life of stable propagation takes the domain part of whole fatigue life (>95%). Based on experiment results, a~N curve is obtained.4) Based on a~N experimental curve, the semi-experimental formula of crack propagation ratio of phase II is proposed. In this formula, SIF of main crack (mode-I crack) is calculated by the former formula which is proposed in chapter two of this paper. By using the semi-experience formula, fatigue life of the phase II under fracture failure mode-I can be predicted conveniently. And the approximate fatigue life of strengthened beam can be obtained too.5) The calculation formula of shear stress near mid-span interface crack tip of strengthened beam is deduced based on elastic theory, and the close solution is obtained by boundary conditions. The result shows that with the increase of distance of origin away form mid-span interface crack tip, the interface shear stress rapidly decreases. The influence of interface crack length on shear stress is not significant.6) The experiment research of interface crack (mode-II crack) propagation is carried out, and a~N curve is obtained. Based on that, the semi-experience formula of interface crack propagation is proposed. In this formula, SIF of interface crack is determined by their periodicity characteristic and the formula of shear stress nearby interface crack tip. By using this formula, the fatigue life of RC beam strengthened with CFL under interface debonding mode can be predicted.7) The residual life prediction model and the residual strength prediction model are proposed, based on the principle of linear fatigue cumulate damage. The fatigue life of RC beam strengthened with CFL under random loading is estimated, and the theoretic S～N curve is obtained. By experiments of strengthened beam under random loading, the effect of prediction model is discussed. Based on result of experiments, the total damage of former principle is modified, and the amended residual life prediction model is proposed. According the results, the residual strength prediction model which considering initial strength and the amended residual life prediction model are more precise.
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