Study On The Shear Deformation Mechanism And Creep Law Of CFRP-steel Reinforced Interface

Compared with traditional structural materials,New lightweight Carbon Fibre-reinforced Polymer（CFRP）has many advantages,such as high strength,lightweight,corrosion resistance,strong designability,and convenient construction.The widespread application of CFRP brings new opportunities and challenges to the research of steel structure reinforcement technology.The use of CFRP sheets（plates）for reinforcement,reinforcement,and repair of steel structures,especially in large-span bridges and high-rise buildings,has received increasing attention and achieved fruitful results,providing an essential foundation for the rapid and widespread application of CFRP-steel reinforcement technology.However,the existing research primarily focuses on the technology and methods of specific structural engineering,and the study results on CFRP-steel strengthening mechanisms are relatively few.This article takes the interface reinforced with CFRP-steel as the research object and comprehensively adopts experimental research,theoretical analysis,and other methods and means to systematically study the shear deformation mechanism,creep law,and interface failure micro mechanism of CFRP-steel reinforced interface.The following innovative achievements have been achieved:（1）Based on the stress and shear deformation characteristics of the CFRP-steel reinforced interface,a double-shear composite structural specimen based on traditional electrical measurement methods was designed and developed.The expression of the relationship between the axial measured strain of the steel and CFRP plate in the specimen and the shear strain and stress at the reinforced interface was derived.Indirect shear strain and stress measurements at the supported interface were achieved.（2）A systematic static test and analysis were conducted on the developed sample to obtain the ultimate bearing capacity P_kavg and its variation with the surface roughness R_s of the sample steel plate and the thickness t_a of the reinforcement interface adhesive layer.As t_a increases,P_kavg first increases and then decreases;As R_s increase,P_kavgshows an overall increasing trend.The test analysis shows that the shear strain and shear stress at the interface of CFRP-steel reinforcement are approximately linearly distributed along the axial direction of the specimen,and the slope of the linear distribution varies significantly with the changes of R_s and t_a.A characteristic point on the reinforced interface was selected,and the shear stress-strain relationship curve of the interface was given.The variation patterns of its maximum shear stress,maximum shear strain,and shear deformation modulus with R_s and t_a were analyzed.It was shown that when R_s=0.767 and t_a=1.0mm,the reinforced interface can exhibit good shear strength and shear deformation resistance.（3）The shear creep characteristics of the interface reinforced with CFRP-steel were tested using a graded loading method,including the creep time history,creep rate,and long-term strength of each characteristic point on the interface.The research results showed that as the thickness of the adhesive layer increased,both instantaneous and creep strains showed a nonlinear decrease,while with the increase of roughness R_s,they showed an approximately linear increase;The long-term strength of the interface first increased and then decreases with the increase of adhesive layer thickness t_a,and offers an approximately linear increasing trend with the increase of roughness R_s;The steady-state shear creep rate shows a decreasing trend with the increase of adhesive layer thickness t_a and roughness R_s.（4）The effect of cyclic loading on the shear creep law of the CFRP-steel reinforced interface was tested.The study found that with the increase of the number of cycles n or the cyclic load level S_max,the instantaneous strain and creep strain of the interface showed an approximately linear increase;As the number of cycles n and the level of cyclic load S_max increase,the steady-state shear creep rate of the interface increases,while the long-term strength of the interface decreases.The results indicate that the accumulation of interface fatigue damage caused by the increase of cycle number n and cycle load level S_max is the main reason for the long-term strength reduction of the CFRP-steel-reinforced interface.（5）By using microscopic testing and analysis methods,the microscopic morphology characteristics of the shear creep failure section at the interface of CFRP-steel reinforcement were analyzed and classified.In particular,statistical analysis was conducted on the area proportion and fractal characteristics of the dimple morphology characterized by tensile failure.The study showed that as the thickness of the adhesive layer increased,the area proportion of the dimple morphology characterized by tensile failure at the interface showed an increasing trend,while as the surface roughness of the steel plate increased,then it showed a decreasing pattern;The fractal dimension of dimple morphology has a good correlation with the proportion of tensile failure area.The fractal dimension of dimple morphology is more sensitive to the thickness of the adhesive layer than the surface roughness of the steel plate;The creep failure of CFRP-steel reinforced interface is mainly caused by shear failure.The microscopic mechanism of shear creep failure at the interface strengthened with CFRP-steel has been revealed.（6）The NRC model was introduced to characterize the nonlinear accelerated creep characteristics of the CFRP-steel reinforcement interface.A nonlinear viscoelastic plastic shear creep constitutive model（MSSB-NRC creep model）was established for the CFRP-steel reinforcement interface,considering factors such as adhesive layer thickness,steel plate surface roughness,number of load cycles,and cyclic load level.The creep model parameters were effectively identified based on the creep test results;the comparison between the model results and the shear creep characteristics test results of the CFRP-steel reinforced interface shows good confirmatory properties.The research results can provide an essential basis for the CFRP-steel structure reinforcement technology and process research.This thesis has 114 figures,32 tables and 159 references.