Study On The Fbaric Structure Design And Composite Compressive Properties Of Three Dimensional Rotary Braided Preforms

Three-dimensional(3D)braided composites have been successfully applied to aerospace structural components subjected to complex and extreme environments because of their mechanical properties of high impact resistance,non-delamination property,high damage tolerance and high energy absorption rate.At the same time,they attracted great interests from various fields because they have the ability of near-net-shape forming of complex structures and the design flexibility of fabric structure,which can meet the requirements of different loading conditions through fabric structure design.However,at present,the preparation of 3D braided composite preforms mainly relies on the four-step method,and the braiding process is less automated,which requires a lot of manual assistance,resulting in low efficiency and expensive products,which limits their application scope.The bisectional 3D rotary braiding machine inherits the advantages of the traditional rotary bariding machine with a high degree of automation,and at the same time,its bisectional switch structure increases the number of yarn carriers and improves the defects of the rotary braiding machine with small yarn capacity,which is expected to be a promising solution to realize the automated and low-cost manufacturing of 3D braided preforms.However,there is a lack of theoretical research on the process flexibility and fabric structure characteristics brought by the improved bisectional switch structure,which means it cannot provide guidance on the preform manufacturing.Therefore,this dissertation takes the bisectional 3D rotary braided preform structure as the key point,and focuses on its braiding trajectory,topology,geometry,and the influence law of the preform structure on composite mechanical properties.In this dissertation,the working principle of bisectional 3D braiding machine is used to analyze the horn-gear arrangement law and its structural forms to satisfy the principle,and two basic structural forms named the four-wing horn-gear and six-wing horn-gear are obtained.Aimed at the two basic forms of bisectional 3D rotary braiding machines,the rotary braiding machine coordinate system and a matrix representation of braiding motion parameters are proposed,based on which the mathematical expression of the carrier motion is given.Based on the mapping relationship between the carrier path and yarn trajectory,a fitting algorithm is proposed to obtain the yarn trajectory from the carrier path under arbitrary braiding motion,thus the braiding yarn trajectory model is established.Using this model,the influence law of horn-gear and switch motions on yarn interlacing topology are obtained,which provides the basis to choose braiding motion parameters.The process simulation interface of two kinds of braiding machines is developed and used to explore potential braiding trajectories,then the influence of braiding motion on the fabric reinforcement format was analyzed.Based on the yarn trajectory model,a writing method of the braiding motion parameter matrix of the fabric with a combined cross-section of four-wing horn-gear rotary braiding method is given,the scheme of changing partial movements to obtain novel fabric structure is proposed and the mathematical expression of horn-gear arrangements belongs to hexagonal rotary braided fabrics of four basic cross-sections are summarized.The braiding motion model is experimentally verified.In order to investigate the constituents of the bisectional 3D rotary fabric topology and their influence law,the spatial lattice node displacement method is proposed to study the fabric topology according to the similarity between braided fabrics and the spatial lattice.Aimed at the 45° enhanced braiding trajectory,the method to establish the braiding spatial lattice based on braiding motion parameter matrix is proposed,and the braiding spatial lattice consisting of nodes and yarn segments intersecting at the nodes is obtained.Combining the influence of the switch rotation direction on the yarn interlacing topology,the method of obtaining the fabric topology by moving the yarn segments at the nodes according to the switch motion is proposed,and the necessary switch rotation combination condition for yarns to remain straight in the representative volume unit is derived,according to which the design method of the fabric topology under straight yarn condition is given.The reason of the existence of two different surface braiding angles on the bisectional rotary braided fabrics is deduced,and the method of adding edge yarns for the rotary braiding process is proposed.One kind of definition of interlacing frequency is proposed,which gives interlacing frequency a quantitative expression.The influence law of interlacing frequency on the fabric topology is obtained,and the minimum interlacing frequency to maintain the uniformity of fabric structures is deduced and experimentally verified.A method to calculate interlacing frequency from the braiding trajectory and carrier arrangement is proposed,and the interlacing frequencies of common-used braiding process are summarized.Aimed at the characteristic of the fabrics with high interlacing frequencies which have severve yarn deformation,a fiber bundle modelling method is proposed to characterize the bending and distortion deformation by cross-sectional changes.The restrictive geometric condition of mutual squeezing of fiber bundles inside the braided preforms is established,and the deformation law of fiber bundles is derived,then the fiber bundle cross-section parameters are obtained.Based on the periodicity of the braided preform structure,a reasonable scheme is selected to divide the preform into unit cells,and the geometric models of unit-cells in the 3D four-dimensional bisectional rotary braided preform are established.The relationship among the geometric parameters of the preform is deduced,and a general braiding angle measurement method is proposed for preforms of different interlacing frequency.The restrictive geometric condition of the squeezing between axial yarns and braiding yarn is established,and the relationships between the geometric parameters of the 3D full-five directional braided preform are obtained.The influence of the braiding angle and axial yarn size on the interior unit-cell geometric parameters is analyzed.The correctness of the above geometric model is verified experimentally.In order to verify the influence of fabric topology and axial yarn size on the fabric structure and explore their influence on the mechanical properties of the composites,braided preforms with different fabric topologies,axial yarn sizes,and braiding angles were fabricated and their surface morphology was observed and analyzed.The composite was prepared by curing of the preforms,and the structure was observed according to the distribution pattern of yarn morphology.Longitudinal compression and transverse compression tests were carried out,and the damage modes of the specimens were analyzed with the stress-strain curves and damage morphology.The influence of above factors on the compression properties of the braided composites was demonstrated.