  One possibility to adjust material properties to a specific need is to embed
units of one substance into a matrix of another substance. Even materials that
are readily tunable during operation can be generated in this way. In
(visco)elastic substances, both the matrix material as well as the inclusions
and/or their immediate environment can be dynamically deformed. If the typical
dynamic response time of the inclusions and their surroundings approach the
macroscopic response time, their deformation processes need to be included into
a dynamic macroscopic characterization. Along these lines, we present a
hydrodynamic description of (visco)elastic composite materials. For this
purpose, additional strain variables reflect the state of the inclusions and
their immediate environment. These additional strain variables in general are
not set by a coarse-grained macroscopic displacement field. Apart from that,
during our derivation, we also include the macroscopic variables of relative
translations and relative rotations that were previously introduced in
different contexts. As a central point, our approach reveals and classifies the
importance of a new macroscopic variable: relative strains. We analyze two
simplified minimal example geometries as an illustration.
