Analysis On Mesoscopic Mechanics Of 2D Biaxial Braided Composites

2D biaxial braided composites are new fiber composites which were developed at the end of 20th century. Compared with traditional laminated composites, 2D biaxial braided composites have better interlaminar stiffness, interlaminar strength and toughness, thus enhance the ability of protesting delamination. In addition, they also have higher specific strength, higher specific modulus, good anti-fatigue properties, good damping properties and possibility of designing material properties. Therefore they are widely used in aeronautics and astronautics structure and other fields. For the complication of fiber braided structure, the geometry of yarns used to be simplified and the research object was only the overall performance of composites, the 3D stress, strain distribution and damnification of 2D braided composites in microscale were not discussed. It is obvious that, developing the analysis of their mechanics properties, which was based on the refined braided structure model, and developing the relativity of braided structure, composites mechanics and deformation have important scientific significance and engineering value.For two common modes of 2D biaxial braided composites-1×1 braided mode and 2×2 braided mode, this article numerically simulates the mechanical behavior of composites with different braided angles under different loads, calculates the effective elastic modulus in the braided direction and analyzes the influencing factors and regulation, through CAD software UG NX4 and finite element software ANSYS combined with composites mechanics.Specifically, this article sets up the solid model of yarns, describes the orientation and the bend of yarns, adopts a parallelogram cell in which every yarn involved has the cyclical structure of the smallest, sets six boundary conditions and numerically simulates the stress, strain distribution of the 2D biaxial braided composites in three-dimensional space. The conclusion are as follows:1. When the composites are under unidirectional tensile load along +θdirection (-θdirection), for 1×1 braided composites, stress concentrations occur at one side of overlapping face of +θyarns (-θyarns) which bear the main load. While for 2×2 braided composites, in the middle and at both sides of overlapping face there are tress concentrations. Under the tensile load, the effect of braid angle on the stress distribution of +θyarn (-θyarns) is not very obvious, but the effect of braid angle on the stress distribution of -θyarn (+θyarns) is notable.2. When the side surface is under simple shear load and has a tangential deformation, stress concentrations occur at the fixed end of yarns which are the main load-bearing bodies. When the shear load is along the thickness direction and the model is bent, with the increasing braid angle, the stress in the 2×2 braided composites increases, but in the 1×1 braided composites it doesn't diversify. When the shear load is along the +θor -θdirection, with an increase in braid angle, the stress both in 1×1 and 2×2 gradually decreases.3. When the composites are under unidirectional tensile load along thickness direction, stress concentrations occur at the overlapping parts of±θyarns. For 1×1 braided composites, the stress concentration on the overlapping face of overlapping parts is more serious than the back face of overlapping parts. While for 2×2 braided composites, it is more obvious that the stress concentration of the overlapping parts at both ends of yams is more serious than in the middle of yarns, and the tress on the overlapping face is not different with the tress on the back face. With an increase in braided angle, the stress rises. And it is more obvious in 1×1 braided composites than in 2×2 braided composites. When the upper and bottom face are under shear load, the stress concentrations also occur at overlapping parts. With the increasing braided angle, tress in the composites declines and the deformation gets easier.4. This article compares the effective modulus in the braided direction with different braided angle through the finite element method and analytical method respectively. With the increasing braided angle, the modulus reduces. With a same braided angle, the modulus is higher in 2×2 braided composites than in 1×1 braided composites, and when the braided angle increases, the diversity of them gradually reduces.