A two-yield surfaces, viscoplastic constitutive model for ceramics and geomaterials

This work describes an innovative multi-axial, elasto-visco-plastic constitutive model based on the assumption that plastic deformations are induced by two mechanisms: a short term, instantaneous mechanism, and a long term, viscous mechanism. The need for this approximation has been recently deduced from experimental compaction of fine grained powders (consisting of clays). This constitutive approach implies the need to consider the existence of two yield surfaces: the first one is a conventional, rate-independent, elasto-plastic yield surface, whereas the second one is a Perzyna-type, visco-plastic yield surface. The method for combining the hardening laws of the two, very different yield surfaces within a consistent framework is discussed in detail and the numerical algorithms for the implicit, backward Euler integration of the proposed model are fully explained. In the final part of the work, model simulations are compared with experimental results, showing that model simulations are reliable for both ceramics and geomaterials.