Meso-structure And Tensile Failure Mechanism Of 3D Non-periodic Braided Composites

With the increasing maturity of 3D preforming technology and the increasing degree of mechanical automation,braided composite materials with heterogeneous 3D preforming bodies as the reinforcement skeleton as the main bearing members and functional members are widely used in aerospace,military protection,transportation and ocean and other fields.There are a variety of molding methods for heteromorphic 3D braided composites.No matter how the process changes,its meso-structure will have certain non-periodicity,and the stress concentration will be formed in the non-periodic region when subjected to external forces.At the same time,in the composite process,the fiber bundles are subjected to external compression deformation,which change the microstructure and affect the mechanical properties of the material.Therefore,it is very important to study the microstructure of 3D non-periodic braided composites and predict their mechanical properties.Based on Micro-CT tomography and image processing technology,this paper constructed a microscopic model close to the real internal structure of the material,and analyzed the tensile mechanical behavior and the failure mechanism of 3D braided composites by combining finite element analysis and mechanical test.The main research contents include:(1)Based on the four-step braiding principle,select the method of yarn reduction in the whole row,design process and structural parameters,and braid the non-periodic braided preform body.Three-dimensional five-directional non-periodic braided composites were prepared by resin transfer molding(RTM).At the same time,three dimensional five-directional periodic braided composites were designed and prepared for comparison.(2)Micro-CT tomography was selected and reasonable scanning parameters were set to obtain multi-slice images.The image processing software is used to filter,adjust contrast and gray to improve the sharpness of fiber bundle.The internal unit is divided into several parts in the direction of pitch height,and the cross-section profiles of braided yarn and axial yarn under different section positions are extracted,and the coordinate system is established for the statistical analysis of path and cross-section parameters.The results show that the cross section shape,area and circumference of the braiding yarn do not change obviously,the deflection degree and the compaction degree of the cross section will change,the cross-section shape of axial yarn alternates with triangle and sector shape.At the same time,the observation of nonperiodic region shows that in a braiding cycle after yarn reduction,holes will be formed in the yarn reduction place,and the cross-section shapes of fiber bundles around the holes are varied.The braiding angle,the degree of squeezing between fiber bundles and the gap between fiber bundles all changed,and the density of fiber bundle braiding is obviously reduced.After a braiding cycle,the arrangement law of all fiber bundles was restored,and with the arrangement of pitch,the density of braiding yarn increased gradually.(3)Using the characterization technology of universal material testing machine and highspeed camera,the tensile mechanical behavior and progressive failure process of threedimensional five-directional non-periodic braided composites were obtained;On this basis,the ultimate failure morphology of non-periodic structure samples was observed and analyzed by Micro-CT and scanning electron microscope(SEM),and the failure mechanism was revealed.The results show that compared with the three dimensional periodic braided composites,the tensile strength of non-periodic braided materials decreases obviously after the yarn reduction process.The main damaged area of the non-periodic braided composite material is the fiber bundle on the left and right sides of the yarn reducing position,where only resin cracking and single filament tiny slip occur in the fiber bundle,and the final failure mode is dominated by resin cracking and fiber bundle extraction.(4)Based on Micro-CT scans and statistical analysis,more real was realized using the Python language mesoscopic structure model of parameterized,use for discrete Voxel grid,impose periodic boundary conditions,simulation,the failure mode of tensile load conditions,analysis of the damage evolution of materials,and the test results and existing ideal mesoscopic model predicted results were compared.The results show that the statistical model based on Micro-CT can accurately predict its mechanical behavior and failure process.