  A model of a shear band as a zero-thickness nonlinear interface is proposed
and tested using finite element simulations. An imperfection approach is used
in this model where a shear band, that is assumed to lie in a ductile matrix
material (obeying von Mises plasticity with linear hardening), is present from
the beginning of loading and is considered to be a zone in which yielding
occurs before the rest of the matrix. This approach is contrasted with a
perturbative approach, developed for a J$_2$-deformation theory material, in
which the shear band is modelled to emerge at a certain stage of a uniform
deformation. Both approaches concur in showing that the shear bands
(differently from cracks) propagate rectilinearly under shear loading and that
a strong stress concentration should be expected to be present at the tip of
the shear band, two key features in the understanding of failure mechanisms of
ductile materials.
