We present a combined shape and mechanical anisotropy evolution model
for a two-phase inclusion-bearing rock subject to large deformation. A
single elliptical inclusion embedded in a homogeneous but anisotropic
matrix is used to represent a simplified shape evolution enforced on all
inclusions. The mechanical anisotropy develops due to the alignment of
elongated inclusions. The effective anisotropy is quantified using the
differential effective medium (DEM) approach. The model can be run for
any deformation path and an arbitrary viscosity ratio between the
inclusion and host phase. We focus on the case of simple shear and weak
inclusions. The shape evolution of the representative inclusion is
largely insensitive to the anisotropy development and to parameter
variations in the studied range. An initial hardening stage is observed
up to a shear strain of gamma = 1 irrespective of the inclusion
fraction. The hardening is followed by a softening stage related to the
developing anisotropy and its progressive rotation toward the shear
direction. The traction needed to maintain a constant shear rate
exhibits a fivefold drop at gamma = 5 in the limiting case of an
inviscid inclusion. Numerical simulations show that our analytical model
provides a good approximation to the actual evolution of a two-phase
inclusion-host composite. However, the inclusions develop complex
sigmoidal shapes resulting in the formation of an S-C fabric. We
attribute the observed drop in the effective normal viscosity to this
structural development. We study the localization potential in a rock
column bearing varying fraction of inclusions. In the inviscid inclusion
case, a strain jump from gamma = 3 to gamma = 100 is observed for a
change of the inclusion fraction from 20% to 33%.