Abstract:

Objective Braided composites have many advantages such as high specific strength, high specific stiffness, high impact damage tolerance and designable mechanical properties, and have been widely used in aerospace, machinery and other fields, and it is particularly important to optimize or design their mechanical properties. In this paper, based on a new composite simulation software DIGIMAT, a representative volume element (RVE)considering the kink defects of fiber bundles with fiber path is established to accurately and quickly predict the equivalent elastic properties of the material at different braiding angles and to explore the influence of the weaving angle on them; and then the stress distribution of two-dimensional triaxially braided composite RVE is obtained to provide a basis for further study of its damage and failure.

Method Although the internal structure of two-dimensional triaxially braided composites is relatively complex, it has a certain periodic distribution on the mesoscale. Therefore, an RVE considering the kink defects caused by fiber bundles along the fiber path was established by using the nonlinear composite modeling platform DIGIMAT, and the equivalent elastic properties of materials were predicted based on the DIGIMAT-MF module mean field homogenization method. Based on the strength failure criterion, the DIGIMAT-FE module was adopted to predict the mechanical properties of braided composites with different braiding angles under uniaxial tensile loading (peak strain of 0.5%).

Results The equivalent engineering constants of two-dimensional triaxial braided composite RVE were predicted with the longitudinal tensile modulus E₁ of 48.33 GPa, the transverse tensile modulus E₂ of 6.70 GPa, and the longitudinal Poisson's ratio μ₁₂ of 0.71, the transverse Poisson's ratio μ₂₃ of 0.45, and the shear modulus G₁₂ of 6.77 GPa. In addition, nine braiding angles of 15°, 19°, 23°, 27°, 30°, 35°, 37°, 41° and 45° were selected to analyze their influence on the equivalent engineering constant. The longitudinal tensile modulus E₁ was inversely proportional to the braiding angle. With the increase of braiding angle, E₁ demonstrated a gradual decrease, whereas the transverse tensile modulus E₂ showed an opposite trend. With the increase of braiding angle, the longitudinal shear modulus G₁₂ firstly increased and then decreased, while the transverse shear modulus G₂₃ remained virtually unchanged (Fig. 3). With the gradual increase of braiding angle, the longitudinal Poisson's ratio μ₁₂ first increased and then decreased, while the transverse Poisson's ratio μ₂₃ shows a decreasing trend (Fig. 4). The longitudinal uniaxial tensile simulation of two-dimensional triaxially braided composite RVE with different braiding angles showed that the elastic modulus of the material decreases with the increase of braiding angle, while the fracture strain is the opposite (Fig. 5). From the contour, it can be observed that the overall stress distribution is not uniform, and the stress peak tends to appear at the yarn, while the stress valley appears at the matrix, and a large stress gradient exists in the contact area between the two (Fig. 6 and Fig. 7). This is mainly because under the longitudinal tensile load, the axial fiber bundle bears most of the load, and the warp and weft yarns also bear part of the load, while the matrix basically does not bear the load effect, the warp and weft yarns improve the longitudinal bearing capacity of the material, so that the structure bears the load more uniformly. There are obvious stress concentrations in the mutual twist zone and the contact zone between the yarns and the matrix, which may lead to local deformation and crack expansion, and then cause material failure.

Conclusion The effect law of braiding angle on the equivalent engineering constant derived in this study is consistent with the law derived by experimental methods in the documents, the accuracy of the finite element model developed in this paper is verified. Based on this finite element model, the stress distribution of the two-dimensional triaxially braided composite RVE was predicted. The overall stress distribution is not uniform, and the stress of the yarn is significantly higher than the stress of the matrix. There are obvious stress concentrations in the yarn mutual kink area and the contact area between the yarn and the matrix, which may lead to the occurrence of local deformation and crack expansion, and then cause the material failure.

Key words: braided composite, mechanical property, representative volume element, DIGIMAT, braiding angle