Stress Analysis Of Adhesive Layer In FRP Strengthened Beams

Fiber reinforced polymer/plastic(FRP)is widely used in the reinforcement and restoration of buildings and structures in the civil engineering because of its advantages,such as lighter weight and higher strength,stronger corrosion resistance and smaller impact on the surrounding environment,etc.The most common method for strengthening structures by FRP is bonding FRP sheets directly to the surface of structural elements with an epoxy matrix.Debonding failures often occurs at the interface between FRP and the original beam when using this strengthing method,leading to a waste of FRP strength.This damage is related to the stress distributions on the adherend-adhesive interfaces and in the adhesive layer,and it is of great theoretical value to analyze the stress of adhesive layer in FRP strengthened beam.Most of the existing theoretical researches on the interface stresses of adhesively-bonded FRP strengthed beam assumed that both the shear and normal stresses in the adhesive layer were invariant along its thickness.direction,however,the stresses distribution near the edge of the adhesive layer are complicated.The interface stresses obtained by the existing anal ytical models can not reflect the real situations of the stresses in the adhesive layer,and the stresses can not be measured through tests.Therefore,further studies on the stresses in the adhesive layer of the FRP strengthened beams is necessary.The following research results are obtained in this dissertation through theoretical deduction and numerical simulation :(1)An improved theoretical analytical model fot the stresses in the adhesive layer of the FRP strengthened beams was proposed.In the present model,the internal forces of adhesive layer are all considered by assuming the internal forces of adhesive layer satisfy the theory of Timoshenko beam.In the compatibility conditions along the adherend-adhesive interfaces,the displacements in the adhesive layer are fully included,among which the longitudinal shear displacement is based on Timoshenko beam theory,and the transverse deformation is obtained by two-dimensional elasticity theory.Additionally,two interface compliance coefficients are introduced so that the effect of interface stresses on local deformation is captured.Based on the equilibrium differential quations and displacements compatibility conditions,the coupled control differential equations of beams forces are eastablished,and the expressions of the internal stresses of the adhesive layer can be derived after solving the equations.(2)Using a simply supported reinforced concrete(RC)beam with a CFRP plate under uniformly distributed load(UDL)as illustrative example,a corresponding finite element model is established.To validate the present method,comparisons among the present solutions with the numerical results of finite element analysis and existing two-parameter model solutions are conducted.(3)Parametric studies of the RC beam strengthened by an external CFRP plate are conducted based on the analytical solution of the proposed model.The influences of the thickness and elastic modulus of the adhesive layer and FRP on the stress distributions along the longitudinal direction and the thicknes direction are discussed,and then the influence of the interface compliance is also studied.(4)The bilinear model is used as the constitutive relation of the adhesive layer and a corresponding two-parameter model is established.The elasto-plastic analytical solution of the interface stresses of the FRP-strengthened beam is obtained.The effect of the elastoplasticity of the adhesive layer on the interface stresses is analyzed through the comparisons with the linearly elastic solusions.According to the maximum shear strain criterion of the adhesive layer,the ultimate bearing capacity of the FRP strengthened beam is deduced.