Back to the thermodynamic admissibility of creep-relaxation functions in linear viscoelasticity

The recent discussion on admissibility criteria for the kernels of linear viscoelastic liquids is completed by the results for viscoelastic solids. The necessary and sufficient conditions of thermodynamic admissibility are established. The class of thermodynamically admissible creep-relaxation functions is proved to be wider than it is usually supposed, including as well numerous nonmonotone and even locally negative functions. These nontraditional viscoelasticity kernels find applications in physics of heterogeneous materials, like for instance in mechanics and theology of composites.