Study On Crack Opening Displacement Monitoring Of Cracked Steel Structure Strengthened By Smart Carbon Fiber Composite Plate (CFRP)

Bonded CFRP plates are commonly used to strengthen cracked steel structures,but it is still necessary to monitor the cracks after reinforcement to ensure safety.In view of the problem that the conventional methods cannot be used because the cracks are concealed by bonding CFRP reinforcement,the smart CFRP is proposed in this paper,and the fiber is embedded into CFRP plate by utilizing fiber sensing technology and high-performance carbon fiber composite material to integrate reinforcement and cracking monitoring.Firstly,the crack opening displacement monitoring method based on fiber optic strain in cracked area is studied theoretically and experimentally.Secondly,the bridge strain of CFRP reinforced cracked structure is theoretically,experimentally and simulated analyzed.The results show that the theory of fiber strain distribution and crack opening displacement agree well with the experiment,and it is feasible to monitor crack.The stress transfer mechanism of cracked steel structure strengthened by CFRP is revealed.The main work of this paper is as follows:(1)Aiming at how to monitor the crack by fiber strain in cracked area,the conversion formula of the relationship between the strain distribution of fiber in cracked area and crack opening displacement was established based on the existing strain transfer mechanism of surface bonded fiber.The elasticity and plasticity of the coating covering the optical fiber core and the Angle θ between the axial direction of the optical fiber and the crack length direction were considered.The results show that the plasticity of the fiber coating is inevitable,the plasticity length is related to the crack opening displacement,and θ has a certain effect on the fiber strain.Finally,a theoretical model of fiber strain inversion of crack opening displacement in the cracked region considering the elastoplasticity and the included Angle of fiber coating is obtained.(2)In view of the feasibility of monitoring the crack theory by optical fiber strain near the crack,a verification test was carried out on the pure bending steel members with prefabricated cracks as the object,and the distributed optical fiber sensor PPP-BOTDA was used to collect the optical fiber strain.The effects of measured and theoretical fiber strain distribution,crack opening displacement theory and measured values,θ on the fiber strain in the cracked region were compared.The results show that the theoretical results of fiber strain and crack opening displacement agree well with the measured results.The monitoring method proposed in this paper is correct and feasible.The method has good accuracy and sensitivity for θ less than 30°.(3)In view of the bridging strain field of CFRP bonded to the crack,the shear stress distribution of the elastic bonding layer under the action of load was firstly deduced theoretically,and the load transfer mechanism of cracked steel structure-CFRP was found out and the bridging strain was obtained.Secondly,the test of CFRP reinforced pure bending steel members was carried out,and the strain and crack opening displacement of CFRP plates were collected.Finally,establish the finite element model,the finite element model is validated with the test results,respectively analysis of elastic and plastic rubber adhesive layer shear stress.The CFRP slab strain and the effect of elastic modulus of carbon plate,carbon plate thickness,rubber thickness and shear modulus on the crack opening displacement.The results show that the load transfer of CFRP and cracked structure occurs at the end of the plate and the cracked area,and the cracking will increase the stress of the cracked CFRP plate.The thickness of the carbon plate,the elastic modulus of the carbon plate and the shear modulus of the adhesive layer have great effects on the crack opening displacement,while the thickness of the adhesive layer has little effect.The reinforcement can obviously reduce the crack opening displacement.Smart CFRP integrates reinforcement and monitoring,which provides theoretical basis and technical support for promoting the application of smart CFRP in engineering reinforcement and monitoring of cracking.