Progressive Damage Analysis Of Bisphthalonirtile Based Three-dimensional Six-directional Braided Composites

Three-dimensional braided composite materials are widely used in the aerospace field due to their unique advantages of high specific strength,high specific stiffness,resistance to multi-directional complex stress and thermal stress,and integral molding.Three-dimensional braided composite materials have high damage tolerance,good impact resistance and structural design features,and gradually change from secondary secondary bearing members to primary bearing members,making it a third-generation fiber-reinforced composite material with greater potential.The prerequisite for the widespread application of three-dimensional braided composite materials is to make a reasonable analysis and evaluation of its strength,damage tolerance and reliability.The uncertainty brought about by the three-dimensional braided composite material braiding method and the molding process makes its mesostructure very complicated.It is difficult for a single research method to make a correct analysis of the performance and failure mode of braided composite materials.In this paper,the ultrasonic C scan of the three-dimensional six-directional braided composite material was carried out to observe the pore defects on the surface of the sample,quantitative measurement of the porosity and fiber volume content,and the unidirectional stretching and three points of the braided composite material sample Bending and interlayer shear experiments obtained the mechanical properties such as the failure mode and strength and modulus of the material.Combined with macro experiments,from the perspective of mesomechanics,based on the movement law of yarn in the three-dimensional six-way weaving process and the assumptions made on the yarn,a Python program was written to establish a representative volume unit cell,according to the unit cell meso structure period The characteristic characteristics impose periodic boundary conditions,and the representative volume unit cell model with periodic boundary conditions applied to simulate the destruction of macroscopic materials.The three-dimensional Hashin criterion is used as the initial damage criterion of the fiber bundle,and the maximum stress criterion is used as the damage criterion of the resin matrix.In order to reduce the dependence of the grid size and improve the convergence rate,a damage evolution model related to the element characteristic length and Sidoroff energy equivalence principle was established.The second-order damage variables in the Murakami-Ohno model are used to reduce the stiffness of the component materials.A UMAT subprogram for the damage of 3D braided composites has been compiled,and a Jacobian matrix containing damage variables was defined in the program to perform progressive damage analysis on the 3D braided composites.The effects of different braid angles and different periodic boundary conditions on the properties of three-dimensional sixdirectional braided composites are discussed and analyzed.