Research On Interface Problems Of 3D Braided Composite Based On XFEM

Because of light weight,high strength,impact resistance and strong designability,braided composite materials have been widely used in aerospace,automobile,ship,construction engineerings.Most of the existing researches on 3D braided composites assume that the displacement and stress at the interface between fiber and matrix are continuous,that is,the influence of interface on the mechanical properties of 3D braided composites is not considered.However,the previous studies have shown that the mechanical properties and damage failure of the braided composite are obviously related to the interface property between the fiber and the matrix.Therefore,some errors in the prediction results will occur for 3D braided composites by neglecting the interface effect.The extended finite element method(XFEM)can be used to establish the theoretical model for predicting the mechanical properties of 3D braided composites,which considers the influence of the interface by adding an additional function to reflect the discontinuous characteristic of the interface.However,the current theory of XFEM is not perfect,and the previous XFEM are mostly used in the simulation of two-dimensional problems.In this paper,a three-dimensional modified XFEM is established to study the mechanical behavior of 3D braided composites.The mechanical behaviors of 3D braided composites were studied considering the interface effect.The main contents of this paper include the following aspects:(1)Based on Partition of Unity Method and Level Set Method,the level set function of the interface of the three-dimensional fiber bundle reinforced composite was derived.The basic method and steps of XFEM were used to describe the selection of strengthening function,the derivation of control equation and the integration of element stiffness matrix.(2)The non-thickness interface model of 3D braided composite was established.The expression of the interface stiffness matrix of 3D four-directional braided composite with the interface was derived by increasing the additional freedom of the joints.The numerical integration method of the element and the interface stiffness matrix were proposed.(3)The stress and displacement distributions of a single fiber bundle composite and 3D four-directional braided composite with interface were investigated.Displacement jump and stress concentration were found in the interface between the fiber bundles and matix.