There is a growing demand from the industry for nonlinear 3-dimensional material models and simulation tools. Nonlinearity in material response can be attributed to different phenomena - viscoelasticity, viscoplasticity and/or damage. In previous studies, authors have looked at viscoplastic behavior in 3D and developed material models with different complexity for the 3D cases. The first model assumes that all the nonlinearity functions are direction independent, and the second model assumes direction dependency. In this study, the viscoelasticity is analyzed in the axial and lateral directions in order to investigate the applicability of those developed material models. The results show high direction independence for viscoelastic nonlinearity functions for studied high-density polyethylene with and without different amounts of graphene nanoplatelets. The obtained information will be used in ongoing work to further adjust the nonlinear material model and implement the obtained expressions in a finite element code.