| As a new type of textile structural composite material, the3D braided compositesovercome shortcomings of weak performance between layers and easy impact injury oftraditional two-dimensional reinforced material. With excellent mechanical properties and gooddesignability, Development so far，3D four-directional braided composites, five-directionalbraided composites and other multi-direction composite materials were included. So itswidely used in the aerospace and other civilian areas. However, the mechanical properties of3D four-directional and five-directional braided composites still cannot meet the actualproduction use, especially as the need for the main load-bearing components. Therefore, the3D full five-directional braided composites which can greatly improves the mechanical propertiestheoretically are performed. But the research on microstructure and mechanical properties of3Dfull five-directional braided composites is still in a blank stage, which severely limits thepromotion and use of the materials. Therefore, this paper, according to the3D braiding processof the3D full five-directional braided materials of this article, the microstructure wasdetailedly analysis, and through a series of mechanical tests the mechanical properties andfailure mechanism were studied deeply. The main content of this article includes three parts:(1) The process analysis, design and preparation of3D full five-directional braidedcomposites. Design was achieved by the selection of raw materials, the structural design ofpreforms and the optimization of the RTM process. Respectively the movement of carriers,the distribution and the yarn paths in preform were obtained. And the sample with differentprocess parameters (such as the braiding angle and the fiber volume fraction, etc.) wereprepared for supporting the following analysis of microstructural and mechanical property.(2) Establishment of the microstructure model of3D full five-directional braidedcomposite. Firstly, the cross-section pictures of3D full five-directional braided compositewere obtained and analyzed by super depth of field microscopy; Then according to theprocess of3D full five-directional braided composite, the microstructure was established byadopting the volume control method, and the relationship between its microstructuralparameters and process parameters was derived, and the theoretical Calculating formula offiber volume fraction was also derived and confirmed by experiment to show the effectivenessof the model.(3) The researches of mechanical properties and failure mechanism of3D fullfive-directional braided composite. Basic mechanics experiments such as axial tensile,compressive and three point bending test were adopted to study the mechanical properties、stress-strain curves characteristics and the failure mechanisms.The results showed that the axial tensile, compressive and flexural properties decrease asthe braiding angle increases, and increase when the fiber volume fraction increase of3D fullfive-directional braided composite. The tensile strength, compressive and flexural strength issignificantly higher than the ordinary3D five-directional braided composite; The tensile andcompressive stress-strain curves show linear characteristics,and they are obvious brittlefracture. Tensile fracture is flush due to fiber fracture and accompanied by the cracking of thematrix. Compression fracture is flush along the specimen width direction and has a45°angle with the direction of compression force, mainly shows the separation of interface betweenfiber and matrix, and the fiber bundles’ shear failure due to the role of compression. Theflexural stress-strain curve also shows a significant linear feature before the flexural strength,and then plummet after the flexural strength. Fibers and matrix were stretched and bended tosnap with parts of fibers were pulled out from the matrix, while the fibers in the compressedpart of the bend specimens were broken by combined shear force and bending with largepieces of matrix debonding. |
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