  Soft constituent materials endow biological composites, such as bone, dentin
and nacre, with viscoelastic properties that may play an important role in
their remarkable fracture resistance. In this paper we calculate the scaling
properties of the quasi-static energy release rate and the viscoelastic
contribution to the fracture energy of various biological composites, using
both perturbative and non-perturbative approaches. We consider coarse-grained
descriptions of three types of anisotropic structures: (i) Liquid-crystal-like
composites (ii) Stratified composites (iii) Staggered composites, for different
crack orientations. In addition, we briefly discuss the implications of
anisotropy for fracture criteria. Our analysis highlights the dominant
lengthscales and scaling properties of viscoelastic fracture of biological
composites. It may be useful for evaluating crack velocity toughening effects
and structure-dissipation relations in these materials.
