Research On Mechanical Properties Of Two-Dimensional Triaxially Braided Composites Based On Multiscale Analysis Method

Composite materials refer to materials with new properties optimized from two or more different materials at the macro(micro)level,generally including reinforcement phase and matrix phase,which have been widely used in many industries such as aviation,aerospace,and so on.Nowadays,the most widely used structural form is laminated composite material,whose manufacturing process is relatively mature,but there are shortcomings such as easy delamination and low toughness,which often limit its application in main loadbearing situations.Compared to laminated composite materials,braided composite materials have advantages such as high specific strength,high specific stiffness,and delamination resistance.The research object of this thesis is two-dimensional triaxially braided composite materials,whose internal structure is relatively complex,with axial and biaxial fiber bundles(warp and weft)interwoven with each other.Its mechanical properties and damage and destruction are not only affected by macro factors such as loading and constraints but also related to micro and meso properties such as component properties.To utilize two-dimensional triaxially braided composite materials more efficiently,it is necessary to grasp the influence of their micro and meso structures on their macro properties,so it is necessary to carry out multiscale research on braided composites.In this thesis,based on continuous damage mechanics,a multiscale analysis method is used to research the mechanical properties and damage of two-dimensional triaxially braided composite materials.Key research contents and relevant conclusions include:(1)At the micro scale,a finite element analysis model of microfiber bundles is established based on DIGIMAT to predict their equivalent engineering constants;Based on the HASHIN 3D failure criterion,the strength analysis of representative volume elements of fiber bundles under different loads is performed.The results indicate that the predicted results of the finite element model are in good agreement with the results calculated by the Chamis formula,and the predictive errors are consistently less than 12%.The longitudinal mechanical properties of the fiber bundles are strong,whereas the transverse and shear mechanical properties mainly depend primarily on the matrix properties.The damage is concentrated mainly in the contact area between the fiber and the matrix,and propagates along the edge of the fiber.The failure strength of the fiber bundle is obtained through the stress-strain curve.(2)At the meso scale,a finite element model is established to consider the kink defects generated by fiber bundles along the fiber path.The equivalent engineering constants of different braiding structures are predicted,and the influences of braiding process parameters on them are analyzed.The stress distribution of the material under uniaxial tensile and compressive loading is predicted.The results indicate that the rules of influence are consistent with those obtained from reference experiments.The overall stress distribution of RVE is not uniform,and there is significant stress concentration in the areas where the yarns are twisted together.The performance difference between transverse compression and longitudinal compression is large,and the overall stress level under transverse compression load is low.(3)At the macro scale,a homogenization method is used to equivalent the meso model to homogenization elements and array them into a macro model.Based on the ABAQUS/Explicit solver and the VUMAT subroutine,the tensile properties and damage of the macro specimens(excluding defects,open hole defects,and notch defects)are studied.The results indicate that the braiding angle is inversely proportional to the tensile strength.The overall trend of the stress distribution around the opening in Lekhnitskii’s theory is consistent with that calculated by the proposed finite element method.The relative error between the predicted strength values of different openings predicted by the average stress failure criterion and the finite element calculation results is less than 5%.When the width of the specimen is consistent,the strength value and fracture strain are inversely proportional to the curvature radius of the notch,and are not sensitive to the position of the notch/opening.