| External bonding of carbon fiber reinforced polymer(CFRP)composites has become more and more widely used for strengthening old RC bridges all over the world since the 1990 s.For cracking RC bridge structure strengthened with FRP,fatigue cracks in RC structure keep on growing and fatigue life could be dropped due to repeated traffic loading.On the other hand,fatigue life of RC bridge could drop also due to the severe servicing environment such as hot-wet environment.Therefore,it is necessary to discover the effect mechanism on fatigue crack propagation behavior of RC members strengthened with carbon fiber reinforced polymer(CFRP)under hot-wet environment.In this paper,a new type of FRP sheet called "carbon fiber laminate(CFL)" was used to strengthened the RC beam.Taking this as the research object and by combining Digital Image Correlation method,the testing method to observe fatigue crack propagation of RC beam based on DICM and finite element method to calculate stress intensity factor of RC beam was studied.The main research contents and conclusions are as follows:(1)The study on testing method and description method of concete fracture property.The study results show that: 1)the technology to observe concrete crack tip based on DIC method is effective and feasible;2)the calculation of concrete fracture energy based on P~CMOD curve is more accurate than that based on P~? curve;3)the method to determine initial fracture toughness,unstable fracture toughness and cohesive fracture toughness based on Double-K model is effective and feasible;4)in finite element model of concrete,application of bilinear constitutive relation to model cracking behavior of concrete is more accurate than application of linear constitutive relation.(2)The study on fatigue crack propagation behavior of concrete.The study results show that: 1)it is effective and feasible to observe the whole process of initial cracking,crack propagation and fracture in notched concrete specimen.The a~N curve can be obtained accurately;2)the dispersion of da/dN data can be reduced using logistic function to fitting a~N curve;3)fatigue crack propagation curves of concrete show a decelerate/down-toaccelerate/up trend.In the dropping period,?a can be used to describe da/dN.In the rising period ?K is instead.(3)The study on stress intensity factor using finite element method.The study results show that: 1)finite element model of RC beam strengthened with CFL was built in concrete plastic damage model which describe tensile and compressive behavior of concrete.Also,environmental bond-slip relation between CFL and concrete was considered,which can model cracking behavior of RC beam under 3-p-b loads accurately;2)analytical solution of interface stress in RC strengthened with FRP under hot-wet environment was derivated.The analytical solution results agree well with the results of the finite element results.3)stress intensity factor considering bond-slip model under hot-wet environment is bigger than that at room temperature.The impact of temperature is greater than that of humidity.(4)The study on fatigue crack propagation law of RC beam strengthened with FRP.The study results show that: 1)The fatigue crack propagation experiment under environment and fatigue load coupling were carried out based on hot-wet environment simulation and control device which was developed by our research group.DIC method was applied to observed the whole process from cracking to failure;2)a~N curves of RC beams strengthened with CFL under hot-wet environment can be obtained using DIC method in low-load level loading.3)In the initiation stage of crack,the fatigue crack propagation law can be described using piecewise function of K and a;In the stable stage of crack,the fatigue crack propagation law can be described using Paris law;4)the fatigue crack propagation rate increased with environmental temperature increasing.But humidity has no obvious influence on fatigue crack propagation rate. |
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