Research On Mechanical Behavior And Calculation Method Of Composite Joint Connected By FRP Bolts

Fiber Reinforced Polymer(FRP)connection plays a major role affecting the structural performance of FRP.At present,common connection modes are bonded connection,bolt connection and hybrid connection,among which bolt connection is the most widely used one.The bolts used include titanium bolts,stainless steel bolts and FRP bolts.FRP bolts are made of FRP materials,which possess advantages of insulation,voltage resistance and electromagnetic resistance.They are widely used in aerospace,electrical,transportation and other fields.At present,the research on bolt connection of FRP material is mainly focused on titanium bolts or stainless steel bolts,but the research on FRP bolts is relatively limited.Therefore,this thesis studies the load distribution,the ultimate strength and failure mode of composite joint connected by FRP bolts by means of experimental research,numerical simulation and theoretical analysis.The research findings are as follows:(1)FRP bolt tightening torque and bolt pre-tightening force demonstrate linear relationship and the torque coefficient is about 0.35.Compared with titanium bolts and stainless steel bolts,FRP bolts have higher bolt pre-tightening force under the same bolt tightening torque.The suitable tightening torque range of M10 FRP bolt is 5.0N.m-9.0N.m,which is significantly lower than that of traditional M10 bolt with 18.0N.m-25.0N.m range.(2)FRP bolt tightening torque has little effect on the ultimate bearing strength of composite joints,but too intensive FRP bolt tightening torque will lead to bolt shear failure.(3)The load distribution of bolts in composite joint is affected by bolt end spacing,bolt spacing,bolt edge spacing and number of bolts,among which the effect of bolt spacing is small.When the ratio of bolt end pitch to bolt diameter increases from 2.0 to 5.0,the shear ratio of bolts decreases by 19.4%;when the ratio of bolt edge pitch to bolt diameter increases from 2.0 to 4.0,the shear ratio of bolts increases by 8.6%;when the number of bolts increases from 2.0 to 5.0,the shear ratio of bolts decreases by 31.3%;compared with titanium bolts and stainless steel bolts,the use of FRP bolts can improve the shear ratio of bolt group of composite joint about 4.6%.(4)The ultimate strength and failure mode of composite joint are affected by bolt end spacing,bolt spacing,bolt edge spacing and number of bolts.When bolt end spacing is less than 3.0 times bolt diameter or bolt spacing is less than 4.0 times bolt diameter,the composite joint is prone to shear-out failure;when bolt edge spacing is less than 3.0 times bolt diameter,the composite joint is prone to net-tensile failure;with the increase of bolt number,the composite joint is prone to shear failure of FRP bolt;increasing bolt end spacing or reducing bolt spacing can improve the deformation ability of the composite joint.Compared with titanium bolts and stainless steel bolts,FRP bolts can improve the ultimate bearing strength and deformation capacity of composite joint.(5)Based on the node-spring method and the first strength theory,a method for calculating the ultimate strength of the composite joint under tension load is given,and the method is proved efficient.