Multiscale viscoelastic behavior of 3D braided composites with pore defects

The multiscale modeling is adopted to investigate the viscoelastic behavior of 3D braided composites considering pore defects. The homogenization method deals with determining the equivalent viscoelastic properties of matrix with pore defects. A semi-theoretical model aiming to compute the viscoelastic properties of yarns, is achieved by the generalized method of cells (GMC) with the homogenization theory, and then verified by the representative unit cell (RUC) using commercial finite element package. The viscoelastic behavior of braided composites is characterized by regarding the yarns as transversely isotropic material whose properties are explained by the user-defined material subroutine (UMAT). The proposed multiscale modeling is experimentally validated by stress relaxation tests of braided composites. All the multiscale results are in good agreements. The pore defects can significantly reduce the transverse viscoelastic behavior of braided composites, and show smaller effects on the longitudinal long-term properties. As can be seen, the longitudinal long-term properties are more sensitive to the braiding angles compared with the transverse direction. The present multiscale viscoelastic analysis scheme can well capture the long-term behavior of 3D braided composites with pore defects.