  Dynamic shearing banding and fracturing in unsaturated porous media is a
significant problem in engineering and science. This article proposes a
multiphase micro-periporomechanics (uPPM) paradigm for modeling dynamic shear
banding and fracturing in unsaturated porous media. Periporomechanics (PPM) is
a nonlocal reformulation of classical poromechanics to model continuous and
discontinuous deformation/fracture and fluid flow in porous media through a
single framework. In PPM, a multiphase porous material is postulated as a
collection of a finite number of mixed material points. The length scale in PPM
that dictates the nonlocal interaction between material points is a
mathematical object that lacks a direct physical meaning. As a novelty, in the
coupled uPPM, a microstructure-based material length scale is incorporated by
considering micro-rotations of the solid skeleton following the Cosserat
continuum theory for solids. As a new contribution, we reformulate the
second-order work for detecting material instability and the energy-based crack
criterion and J-integral for modeling fracturing in the uPPM paradigm. The
stabilized Cosserat PPM correspondence principle that mitigates the multiphase
zero-energy mode instability is augmented to include unsaturated fluid flow. We
have numerically implemented the novel uPPM paradigm through a dual-way
fractional-step algorithm in time and a hybrid Lagrangian-Eulerian meshfree
method in space. Numerical examples are presented to demonstrate the robustness
and efficacy of the proposed uPPM paradigm for modeling shear banding and
fracturing in unsaturated porous media.
