The onset of shear band formation in granular materials has been linked
to the governing partial differential equations becoming ill-posed which
has in turn been linked to nonassociativity of the flow rule. If
uniform material properties and uniform deformation are assumed,
ill-posedness occurs simultaneously at all points in the sample.
This work derives a one-dimensional model from a two-dimensional model
for granular flow with a nonassociative flow rule and shows that,
shortly before the onset of ill-posedness, deformation can become
highly non-uniform at a point where the material is slightly weakened.