FEM implementation of a three-dimensional viscoelastic constitutive model for particulate composites with damage growth

In order to analyze viscoelastic behavior of particulate composites with growing damage, an existing three dimensional viscoelastic continuum damage model developed originally for solid propellant is generalized for wider use in a Finite Element model (FEM). This equation allows for damage induced anisotropy (local transverse isotropy). The constitutive equation is modified here to account for the change of the material continuously from a compressible, undamaged isotropic state into the damaged anisotropic state. A fully viscoelastic time-dependent implementation of the constitutive equation in a FEM is achieved that allows for future extension of the FEM to simultaneously take viscoplasticity into account. The computational results are compared to experimental results for uniaxial and multiaxial stress states in displacement-driven experiments for solid propellant. The multiaxial stress experiments used wide strips with a center hole. The model predicts the experimental load and local strain response up to, and slightly beyond, the peak load, very well. The algorithm is shown by example to be stable far past the peak load.