Meso Architecture And Mechanical Models Of Three-dimensional Braided Composites

In this thesis, the meso architecture and meso numerical mechanical models of 3D braided composites were studied. The work can be concluded as three main aspects as follow:(1) The movement of yarns of 3D braided composites in four-step braiding procedure was analyzed. The spatial motion tracks of yarns were simulated. Then a new method to divide the fabric into unit cells was proposed. Cross section shape and direction of yarns in different regions were obtained by microscopic section photographs of real 3D braided composites which were taken from continuous cross sections and improved by specific graph software. They provided proof for mesomechanical modeling.(2) Based on the analysis of meso architecture, 3D microscopic finite cell model was created .The selection of yarn packing factor for different regions and the reasonable cell boundary conditions was also discussed in detail. The average elastic constants of inner and surface cells were calculated. In the mean time, the curves of elastic constants of both interior unit cell and surface unit cell versus fiber volume fraction were obtained.(3) Based on the study of (2), the tensile modulus and Poisson's ratio of real 3D braided composites were predicted, which were compared with the calculation results of inclined laminar combination model. At the same time, the experimental results demonstrated the reliability of the meso mechanical models. The law of the surface yarn influence and the braiding dimension effect on its whole elastic properties were discussed.