  Voids can be observed at various scales in ductile materials, frequently of
sizes lower than the grain size, leading to porous single crystals materials.
Two local deformation modes of porous ductile (single crystals) materials have
been identified and referred to as void growth and void coalescence, the latter
being characterized by strong interactions between neighboring voids. A simple
semi-analytical coalescence criterion for porous single crystals with periodic
arrangement of voids is proposed using effective isotropic yield stresses
associated with a criterion derived for isotropic materials. An extension of
the coalescence criterion is also proposed to account for shear with respect to
the coalescence plane. Effective yield stresses are defined using Taylor theory
of single crystal deformation, and rely ultimately on the computation of
average Taylor factors. Arbitrary sets of slip systems can be considered. The
coalescence criterion is assessed through comparisons to numerical limit
analysis results performed using a Fast-Fourier-Transform based solver. A good
agreement is observed between the proposed criterion and numerical results for
various configurations including different sets of slip systems
(Face-Centered-Cubic, Hexagonal-Close-Packed), crystal orientations, void
shapes and loading conditions. The competition between void growth and void
coalescence is described for specific conditions, emphasizing the strong
influence of both crystal orientation and void lattice, as well as their
interactions.
