  We study mechanical behavior of soft rubber-like digital materials used in
Polyjet multi-material 3D-printing to create deformable composite materials and
flexible structures. These soft digital materials are frequently treated as
linear elastic materials in the literature. However, our experiments clearly
show that these materials exhibit significant non-linearities under large
strain regime. Moreover, the materials demonstrate pronounced rate-dependent
behavior. In particular, their instantaneous moduli as well as ultimate strain
and stress significantly depend on the strain rate. To take into account both
hyper- and viscoelasticity phenomena, we employ the Quasi-Linear Viscoelastic
(QLV) model with instantaneous Yeoh strain-energy density function. We show
that the QLV-Yeoh model accurately describes the mechanical behavior of the
majority of the soft digital materials under uniaxial tension.
