Material and Failure Models for Textile Composites

Abstract

The complex three-dimensional structure of textile composites makes the experimental determination of the material parameters very difficult. Not only the number of constants increases, but especially through-thickness parameters are hardly quantifiable. Therefore an information-passing multiscale approach for computation of textile composites is presented as an enhancement of tests, but also as an alternative to tests. The multiscale approach consists of three scales and includes unit cells on micro- and mesoscale. With the micromechanical unit cell stiffnesses and strengths of unidirectional fiber bundle material can be determined. The mesomechanical unit cell describes the fiber architecture of the textile composite and provides stiffnesses and strengths for computations on macroscale. By comparison of test data and results of numerical analysis the numerical models are validated.

To consider the special characteristics of epoxy resin and fiber bundles two material models are developed. Both materials exhibit load dependent yield behavior, especially under shear considerable plastic deformations occur. This non-linear hardening is considered via tabulated input, i.e. experimental test data is used directly without time consuming parameter identification. A quadratic criterion is used to detect damage initiation based on stresses. Thereafter softening is computed with a strain energy release rate formulation. To alleviate mesh-dependency this formulation is combined with the voxel-meshing approach.

Epoxy resin is modeled with the first, isotropic elastoplastic material model regarding a pressure dependency in the yield locus. As the assumption of constant volume under plastic flow does not hold for epoxy resin, a special plastic potential is chosen to account for volumetric plastic straining.

To describe the material behavior of the fiber bundles, the second, transversely isotropic, elastoplastic material model is developed. The constitutive equations for the description of anisotropy are formulated in the format of isotropic tensor functions by means of structural tensors. Opposed to the isotropic case the hardening curves are not obtained by experiment but by simulations performed done with the micromechanical model. So the hardening and softening curves from the micro model simulation, reflecting the homogenized material parameters from the micro model, are submitted to the next scale, the mesomechanical model.