A Novel Braiding Method And Impact Behaviors Of 3-D Braided Composite Three-way Integrated Circular Tubes

The 3-D integrated braided composite multi-way tubes are connecting joints with the characteristics of lightweight,high impedance,and high damage tolerance,which have great application potential in satellite truss structures,airship frames,bicycle parts,and other engineering fields.The net-shape seamless forming strength is beneficial to avoid the defects caused by cutting,splicing,and stitching compared with traditional joints manufacture,and reduce the number of parts.It is inevitable for the composite tubes to encounter low-velocity impact events during production and service,which will result in visible or invisible damage to materials.The purpose of this research is to develop a novel braiding method for Y-shaped tubular preform and explore the effects of impact energy,branch tube length,and braiding layers on the axial impact compression performance of three-way composite tubes.The damage evolution mechanism was revealed.The dual effects of boundary condition and braiding structure were also discussed.The research contents are as follows:(1)The integrated three-way tubular preforms based on 1×1 four-step braiding method were developed.The composite tubes were prepared via vacuum-assisted resin transfer molding process.The tracer yarn was used to characterize the spatial movement path of fiber tows.The fiber volume fraction of each part was obtained by combustion and drainage method.(2)Low-velocity axial impact test was carried out on drop weight tester,and the real-time mechanical response curves were obtained.The damage propagation process during impact was recorded using high-speed camera.The surface strain distribution was calculated using digital image correlation technology to reveal the failure mechanism.The internal damage information was acquired through micro-computed tomography results.(3)The effects of impact energy level,branch tube length and braiding layer on the mechanical properties of the three-way tube were explored based on the experimental characterization results.The difference in mechanical properties was compared with single-way tube under the same test conditions.The braided structure effect of surface strain distribution was discussed.The main conclusions are as follows:(1)The novel braiding method of ‘yarn-moved,yarn-added,yarn-realigned’ can ensure the continuity of carbon fiber spatial distribution,the structural uniformity,and consistency of mechanical properties in any axial direction of branch tube,which are beneficial to the stress transfer at the junction.The fiber volume fraction at the triangle area of composite tube increases with the number of layer augments.And the value at the joint region is smaller than the branch part due to the widened area covered by the same number of yarns.(2)The boundary condition of the three-way tube was set to be fixed support at both ends.When the tube was subjected to axial impact compression,the branches only played the role of load transmission,and the damage was mainly concentrated at the junction.The failure modes included:matrix cracking and peeling off,fiber buckling and breakage,interface debonding,and collapse of the junction.Because the triangle area was squeezed simultaneously by the side branches,the surface strain distribution presented the opposite variation trend compared with the single tube.It was also the reason for the local tensile stress concentration and the formation of compressive shear band,which was formed in less than 60μs.(3)The peak load,maximum displacement,specific energy absorption,and damage degree of the tube increased with higher impact energy,while the initial compression stiffness was insensitive to the energy level.The ultimate strength and compression stiffness were negatively correlated to the branch length but positively correlated to the number of braiding layers.The branch length affected the stress concentration position.The shorter the branch length,the higher the damage initiation point and the closer the shear band was to the upper part of the triangle area.(4)Compared with the shear damage mode in the middle of the single tube with free end,the special structure of the three-way tube can avoid the failure of the load-bearing component connected with it.There also existed structural effect on the strain distribution of the single tube and topside branch,where the strain at the interweaving point was larger than that at the float because of the weak strength of the matrix.And the surface strain value of the branch was insensitive to the tube length.In this project,the integrated forming technique and low-velocity axial impact compression behaviors of 3-D braided composite three-way tubes were explored through experimental method.This study provides theoretical references for the optimal design and application of special-shaped braided joints.