  Ductile fracture through void growth and coalescence depends significantly on
the plastic anisotropy of the material and on void size, as shown by
experiments and/or numerical simulations through several studies. Macroscopic
(homogenized) yield criteria aiming at modeling nanoporous materials have been
proposed only for the growth regime, i.e. non-interacting voids. The aim of
this study is thus to provide a yield criterion for nanoporous materials
relevant for the coalescence regime, i.e. when plastic flow is localized
between voids. Through homogenization and limit analysis, and accounting for
interface stresses at the void-matrix interface, analytical coalescence
criterion is derived under the following conditions: axisymmetric loading,
orthotropic material obeying Hill's plasticity, cylindrical voids in
cylindrical unit-cell. Incidentally, an orthotropic extension of the existing
isotropic modeling of interface stresses through limit analysis is described
and used. The proposed coalescence criterion is then extended to account for
combined tension and shear loading conditions. Numerical limit analyses have
been performed under specific conditions / materials parameters to get
supposedly exact (up to numerical errors) results of coalescence stress. A good
agreement between the analytical coalescence criteria derived in this study and
numerical results is found for elongated spheroidal voids, making them usable
to predict the onset of void coalescence in ductile fracture modeling of
nanoporous materials.
