  We present a general framework for the approximation of systems of hyperbolic
balance laws. The novelty of the analysis lies in the construction of suitable
relaxation systems and the derivation of a delicate estimate on the relative
entropy. We provide a direct proof of convergence in the smooth regime for a
wide class of physical systems. We present results for systems arising in
materials science, where the presence of source terms presents a number of
additional challenges and requires delicate treatment. Our analysis is in the
spirit of the framework introduced by Tzavaras [A. Tzavaras, Commun. Math.
Sci., 3-2, 2005] for systems of hyperbolic conservation laws.
