Research Of Mechanical Properties Under Axial Load Of The Novel Improved CFRP-Steel Tube Composite Member

Circular steel tube members are one of the most important forms of force-bearing members of large-span space steel structures.With the increasing development and application of large-span space grid,the self-weight of the structure becomes a key factor limiting its force performance.Moreover,due to the poor corrosion resistance of steel and the great influence of temperature change,these also further limit the further development of large-span space steel structure.Among them,axially loaded steel tube members are one of the most typical member forms of large-span spatial steel structures.Therefore,in order to reduce the self-weight of structural members and improve the load-bearing capacity and durability of members,the use of fiber-reinforced composite materials（FRP）to strengthen and reinforce steel pipe members is one of the promising research topics.The FRP material connection method has been widely used owing to the advantages of easy operation,low damage to the original structure and low stress concentration.However,the FRP end is easily damaged by the adhesive connection method.This limits the strength of the FRP material and the performance of the combined section to a large extent.Therefore,based on the above background,this study proposes a new type of CFRP-steel tube composite member（CFRP-STCM）with thick-walled steel tube at the end based on the demand for light weight,high strength and corrosion resistance of structural members for large-span steel structures.Experimental studies,numerical simulations and theoretical analyses were carried out for the bonding and anchorage characteristics,damage modes,load carrying capacity and ductility characteristics of the new composite member under axial tension and compression and when applied to the truss structure.The main contents of the study and related conclusions are as follows:（1）The mechanical performance of the novel CFRP-STCM and CFRP reinforced circular steel tube members with equal wall thickness and steel tube members under axial tensile loading was investigated based on experimental and finite element model comparisons.The effects of parameters such as the number of CFRP layers and the length of thick-walled steel tubes were mainly considered.The typical failure mode,load carrying capacity characteristics,bonding and anchoring performance and damage evolution of the adhesive layer of the specimens are discussed.The results show that the load carrying capacity,the stiffness of the steel tube after yielding and the ductility performance of the novel CFRP-STCM are better than those of the CFRP reinforced equal-wall-thickness steel tube members and steel tube members.Among them,the CFRP reinforced equal-wall-thick steel tube specimens have obvious peeling damage under tension,while the novel CFRP-STCM can effectively suppress the peeling damage of CFRP（L₀≥100mm）due to the adoption of thick-walled steel tube.Based on the refined finite element model,the stress distribution pattern and force transmission mechanism of the bonded anchorage area of the novel composite member were also revealed.（2）Based on the axial compression test combined with finite element simulation,the vertical bearing capacity and stiffness characteristics of the novel CFRP-STCM as well as the typical damage phenomena were investigated.The effect of winding direction and number of layers of CFRP fibers are considered to derive the influence on the load bearing capacity and damage phenomenon of the members.The results show that:fiber winding along the longitudinal direction has the most significant effect（12.9%）on the load bearing capacity of the member,while fiber winding along the circumferential direction and symmetrical winding along±45°has limited effect（≤5%）on the load bearing capacity of the member.The effect of parameters such as the number of CFRP layers,initial defect value,steel tube diameter-thickness ratio and adhesive layer thickness on the bearing capacity of the novel CFRP-STCM was investigated through the supplementary parametric finite element analysis.Finally,based on the theory of stable equilibrium of the compression rod,a modified formula for calculating the stable bearing capacity that can take into account the parameters of the number of CFRP layers,initial geometric defects and material properties is obtained.The comparison between the theoretical calculation results and the finite element analysis results shows that the mean value of the error is 1.0145%and the coefficient of variation is 0.0144,indicating that the overall accuracy of the calculation formula is high enough to accurately predict the axial compressive load capacity of the novel composite member.（3）Based on the axial tensile test study,it is known that the butt weld on both sides of the steel pipe due to the different wall thickness,so there will be a significant mutation effect of stress.Therefore,based on the refined finite element model,three common types of steel pipe butt joint configurations,i.e.,insertion type,thick-walled steel pipe with open bevel and direct butt weld type（Type-A,Type-B and Type-C,respectively）,are studied.The overall stress characteristics（load bearing capacity,ductility）of the novel CFRP-STCM,the influence law of the adhesive layer,the steel pipe and the CFRP stress are studied for the characteristics of the steel pipe butt joint construction.The results show that the three types of construction methods have limited effects on the bearing capacity and ductility of the novel CFRP-STCM and on the distribution of CFRP tensile stresses,but have a greater effect on the distribution of local shear and peel stresses in the adhesive layer,in which the shear stress amplitude in the adhesive layer of the insertion type joints（Type-A）decreases by 25.2%and 36.7%compared with Type-B and Type-C,respectively.The improvement effect is significant.Therefore,Type-A joints are more reasonable than the other two types of Joints.（4）According to the experimental study of axially-tensioned CFRP-STCM in Chapter 2,it is known that the bond stress distribution at the end of the member is the key to calculate the bond strength of the member,so the influence of the geometric parameters of the member,material properties and other factors on the bond stress distribution pattern of the adhesive layer of the axially-tensioned member is studied based on the parametric finite element model.The results show that the distribution range and magnitude of shear and peel stresses in the adhesive layer increase as the number of CFRP layers increases.Compared with the normal stress of the adhesive layer,the interface shear stress is the controlling factor for the damage of the adhesive layer at the end of the member.After that,based on the static equilibrium and deformation coordination conditions of each component of the bonded section,the state of CFRP,steel pipe and adhesive layer is analyzed,and the theoretical formula for the distribution of bond stress at the end of the member is obtained and the value of the effective bond length at the interface of the adhesive layer is given.The results of the finite element model analysis are used to verify the theoretical formula,and the results show that the formula has high accuracy and can meet the needs of the calculation.Finally,based on the results of the theoretical analysis,a reasonable value of the length of the new CFRP-STCM end thick-walled steel pipe is given.（5）In order to investigate the stress characteristics of the novel CFRP-STCM composite truss structure,the indexes of vertical stiffness,ultimate load capacity and damage mode of the novel CFRP-STCM composite truss,steel tube truss and CFRP reinforced equal thickness steel tube truss were studied comparatively based on the refined finite element model.And the whole process load-displacement curves,cohesive joint stress development law,damage state of adhesive layer and CFRP stress distribution law of the truss structure were obtained.The results show that the vertical load capacity,stiffness and nodal domain stress of the novel CFRP-STCM composite truss structure are better than those of steel tube truss and CFRP reinforced equal wall thickness steel tube truss.The new composite truss structure has the highest load capacity due to the overall destabilization of the pressurized composite members;the steel pipe truss has the lowest load capacity due to the overall unstabilization of the pressurized webs;the CFRP-reinforced thick steel pipe truss has the lowest load capacity due to the failure of the exposed steel pipe at the end of the member,and the strength of the composite section formed by CFRP and steel pipe cannot be fully developed.Finally,a comparative study on the effect of different arrangements of the novel composite members on the structural forces of the truss is carried out,which provides useful reference for related research and application.Finally,based on the main contents of this paper and the development research needs of the topic,the research focus and the results obtained are summarized,and the directions for further research on the topic are proposed to provide reference for further research on the topic.