Braiding Techniques And Quasi-static Compressive Behavior Of 3-D Braided Composite Three-way Circular Tubes

Three-dimensional(3D)braided carbon fiber reinforced polymer composites have been applied in the aerospace,marine,and automotive industries for their integrated structures,excellent mechanical performances and high design flexibility.The joints are mainly divided into metal and composite joints according to the material,where the composite joints are generally prepared by moulding or lamination.The lack of reinforcing fibre bundle at the connection region generates stress concentrations due to the presence of defects.Here we purpose a new method to prepare three-way integrated braided perform,and its braided structure at the connection region is revealed.The quasi-static compression properties of the three-way braided composite joints are investigated,and the effect of braiding structure on the compression properties is studied to pave a way develop the multi-way composite joints.The research contents are as follows:(1)The braiding method of four-step three-way integrated braided tube has been studied,and the braided structure and yarn distribution at the connection region have been investigated.(2)The longitudinal compressive behavior of 3D braided three-way composite tube has been analyzed.Load-displacement curves,initial stiffness,compressive strength and damage morphologies were obtained from experiments.The effects of braiding layer(2layers,3layers and4layers)and branch length(50mm,60 mm and 70mm)on the longitudinal compressive performance of three-way composite tube were studied.Micro-CT was employed to analyze the damage mechanisms of three-way braided composite tube.The effect of braiding layer and branch length on damage mechanisms were also studied.(3)3D digital image correlation method(3D-DIC)was used to analyze the deformation mechanisms of the branch tube and the connection region,and relationship between shear strain localization and failure mechanisms of three-way braided composite tube with various braiding layer and branch length was also found.The main conclusions are:(1)With glass fiber as tracer yarns,yarn paths at the connection region were constructed by Micro-CT.The images show that yarns spiral continuously and interlace orderly throughout the three-way tube,which indicates the integral braiding structure on the connection region.Yarn crosssection on the connection region appears ellipse,while on the branch tube shows ellipse with one flat side.(2)The effects of braiding layer and branch length on compressive behavior of 3D braided three-way tube are significant.Compressive strength,initial stiffness and energy absorption all increased with the increase in braiding layer or the decrease in branch length.Specimens with shorter branch length failed more quickly and seriously.More braiding layer enhanced the integrity of three-way braided composite tube,which improved the ability to resist deformation and damage.Shear fracture on the intersection and matrix cracking on the connection region are the main damage modes of the three-way braided composite tube.(3)With three-dimensional digital image correlation technique,we found that the fracture of the three-way composite tube was related to shear strain distribution.The local shear strain appeared mainly on the intersection,and cracks were inclined to occur in the region of strain concentration,which led to the shear fracture of the three-way composite tube.Strain field on branch tube was strongly regulated by braided structure.Out-of-plane displacement field showed that the connection region bulged under compression of three branch tubes,and the cross section of upper branch tube was ovally deformed due to the squeeze of two side branch tubes.(4)The three-way braided composite tube with different braiding layer and branch length showed different damage modes.Yarns intertwined tightly in the two-layer specimen led to poor performance in fiber/epoxy interface,and the main damage mode was fiber breakage.The main damage modes in three-layer and four-layer specimens were matrix cracking and kink band.With the decrease in branch length,fiber breakage was the main damage mode,while matrix cracking and kink band increased with increasing branch length.The damage occurred around the intersection instead of on the intersection that resulted from yarn packed tightly here causing the high fiber volume fraction.