  We give an a posteriori analysis of a semi-discrete discontinuous Galerkin
scheme approximating solutions to a model of multiphase elastodynamics, which
involves an energy density depending not only on the strain but also the strain
gradient. A key component in the analysis is the reduced relative entropy
stability framework developed in [Giesselmann 2014]. This framework allows
energy type arguments to be applied to continuous functions. Since we advocate
the use of discontinuous Galerkin methods we make use of two families of
reconstructions, one set of discrete reconstructions [Makridakis and Nochetto
2006] and a set of elliptic reconstructions [Makridakis and Nochetto 2003] to
apply the reduced relative entropy framework in this setting.
