Mechanical Properties Of Local Reinforcement In Bolted Connections Of CFRP Laminates

Composite materials have the advantages of light weight,high specific strength,high specific stiffness and good fatigue resistance,and are widely used in fields such as aerospace,civil infrastructure,automobiles,yachts and sports goods.Due to requirements or limitations in design,manufacturing use,and maintenance,composite material structures must adopt certain connection technologies and connection joints are the weakest link of composite material components,which are prone to damage and failure,seriously restricting the load-bearing capacity of the structure.Therefore,improving the efficiency of bolted connections has become an urgent problem to be solved in composite material applications.This article proposes a reserved screw hole process for carbon fiber composite material (CFRP)bolted connections,which is based on traditional reinforcement design and does not cut off the fibers passing through the bolt holes.Based on this process,local reinforcement design is carried out by embedding metal reinforcement plates layer by layer and fixing metal reinforcement plates with surface bolts.Through experiments and numerical simulationsmthe influencing factors of the failure of these two metal local reinforcement CFRP joints are analyzed,Evaluated their enhancement effect.The main work content and conclusions are as follows:(1)Firstly,a CFRP laminated plate[0/90]4 with reserved screw holes was made.During the production process,metal sheets were embedded layer by layer In the screw hole area to enhance the CFRP joint.Among them,the thickness of the metal sheet is taken as 0.08 mm,0.10mm,and 0.12mm,respectively,to evaluate the impact of the thickness of the metal reinforcement plate on the ultimate bearing capacity of bolted connections.Through single bolt double lap static tensile test,it was found that when the metal reinforcement plate content was 26%,the single hole CFRP specimen has the highest bearing capacity,and its strength increased by 43.3%.Secondly,the static tension of the bolt connection in the enhanced design was treated as a quasi-static problem for numerical simulation,and the results were in good agreement with the experimental results,verifying the effectiveness of the numerical method in this paper.(2)Based on the effectiveness of numerical simulation methods and combined with the spring mass model method,the effects of metal reinforcement plate end distance(e),screw hole diameter(D),and screw pitch(P)on the failure load,failure mode,and stress distribution of series double bolt connections were simulated for local reinforcement design of embedded metal reinforcement plates.It was found that when the end distance of the m etal reinforcement plate e=6.25mm,the bolt diameter D=5mm,and the screw pitch P=10mm,the bearing capacity of the double bolt connection specimen is the highest,and its strength is increased by 58%compared to the single bolt connection.(3)In order to compare with the embedded reinforcement method,this article also studied the effects of metal reinforcement plates with different opening shapes on the failure load and load transfer coefficient of surface reinforced CFRP single bolt specimens through experiments and numerical simulations.The results show that the metal reinforcement plate with a long circular opening has the best surface reinforcement design effect:compared to the CFRP specimen with a single opening,its failure load has increased by 22.7%,and the load transfer coefficient is the highest.But this result also indicates that for single hole CFRP bolt connections,the surface metal reinforcement design is weaker than the embedded reinforcement design.