Research On Time-Dependent Mechanical Behaviour Of CFRP-Steel Composite Structure Interface

The interface between CFRP and steel play an important role in transferring the load and keeping compatible deformation as well as making CFRP and steel together working.Bond behavior of CFRP-to-steel interface is critical for the success of CFRP applications.As a high molecular polymer,the adhesive often undergoes time-variant deformation,due to the time-dependent viscoelastic behavior.The viscoelastic behavior of adhesive can cause stress redistribution at the CFRP-to-steel interface,which make the mechanical behavior of CFRP-to-steel interface even more complicated.Therefore,it is necessary to understand both the force transfer mechanism and failure mechanism at the CFRP-to-steel interface.In the present study,the interfacial mechanical behavior of the CFRP-to-steel composite structure has been investigated and the outline of present study is as follows.First,the double shear test method is first used to study the shear viscoelastic behavior of adhesive at CFRP-to-steel interface.A total of 27 test specimens are designed and manufactured,which are then divided into 5 groups for different propose:1 group of test specimens are used as benchmark for comparison,and the other 4 groups of test specimens are used to undergo various loading level and different loading time.In order to conduct the creep experiments,a loading equipment is first designed based on level principle.The creep test is carried out in the laboratory,and the test specimens have been applied with various loading level,i.e.,20%,40%,60%and 75%of the average ultimate load of the corresponding benchmark specimens in group 1.The total duration of the creep experiments for all the test specimen is 3 month.The creep test results show that the longer the loading level is,the more obvious the stress redistribution will be.This is also true to the loading time.Second,base on the creep test results and theory analysis,the modified Burgers model and the Findley model are used as constitutive model to predict the mechanical behavior of CFRP-to-steel interface subject to shear loading.The calculation formula of parameters used in the Modified Burgers model are given.Then,the parameters expression of two models are obtained from the creep experiments through the nonlinear regression analysis.In addition,sensitivity study has also been conducted to investigate the influence of interfacial shear stress and loading time on the various parameters in two models.The comparative analysis of prediction creep compliance curves and practical creep compliance curves show that the prediction curves by the modified Burgers model have good agreement with the test results and the prediction curves by the Findley model are bigger than test.Meanwhile,the analytical solution of the interfacial shear stress of the double shear specimen under pure shear is derived,based on viscoelastic theory.Third,the double shear test method is first used to study the influences of the viscoelastic behavior of adhesive on bonding behavior of CFRP-to-steel interface.Static tensile test has also been conducted on the double-shear specimens from 1 group of comparative specimens and 4 groups of different loading level specimens with 6 different loading time of each group.The mechanical behavior of the CFRP-to-steel interface,i.e.,failure process,failure characteristics,load-displacement curve,strain distributions of CFRP,and the bond-slip curves,have been studied in details.The experimental results indicate that the creep of adhesive is the major cause for the creep damage occurred near the loading end at the bonding interface.The higher loading level with the longer duration can lead to more creep damage.Fourthly,base on the static tensile test results and theory analysis,a new bilinear bond-slip model considering the degradation due to adhesive creep damage,has been proposed and used for the analysis of full-range behavior of CFRP-to-steel bonded joints.Based on the comprehensive review of current-state-of-art of the bilinear bond-slip constitutive model,the creep reduction factor ? of the peak shear stress,and the creep reduction factor ? of the peak slip,and the creep reduction factor y of the maximum slip,have been introduced.The calculation formula of the three creep reduction factor including the loading level and the loading time,have been proposed by the regressive analysis of test data.Fifthly,based on the deformation compatibility,the differential equation of the interfacial stress for the CFRP reinforced steel beams subject to external load and thermal load,has been derived.The analytical solution considering the uniformly distributed load,a single point load and two symmetrical point loads are derived,respectively.In addition,according to the fundamental principle of the sensitivity analysis,sensitivity analysis of the interfacial stress to various parameters is investigated quantitatively.The results show that the interfacial stress is inversely proportional to the thickness of the adhesive layer.The interfacial stress is proportional to the elastic modulus of the adhesive layer.The interfacial stress is proportional to the thickness of the CFRP laminate.The interfacial stress is proportional to the elastic modulus of the CFRP laminate.The interfacial stress is proportional to the distance from the end of the CFRP to the support.The experimental results also indicate that the interfacial shear stress at the CFRP plate end is most sensitive to the distance of the CFRP plate end to the bearing,while the interfacial normal stress is most sensitive to thickness of the CFRP plate.Finally,based on the modified Burgers model,the differential equations of the interfacial stress in Laplace image space,the axial force of the CFRP,the bending moment of the steel beam and the deflection of the reinforced beam are derived using the viscoelastic theory and the Laplace transformation.In addition,the finite element method(FEM)is also used to investigate the influence of the adhesive viscoelasticity on the mechanical behavior of the CFRP reinforced steel beam.The analysis results show that the adhesive viscoelasticity has some influence on the mechanical behavior of the reinforced steel beam.Both the peak shear stress and the peak normal stress at the CFRP plate end decrease with the increasing of loading time,as well as the axial force of the CFRP.Both the bending moment of the steel beam and the deflection of the reinforced beam increase with the increasing of loading time.