  Flutter instability in an infinite medium is a form of material instability
corresponding to the occurrence of complex conjugate squares of the
acceleration wave velocities. Although its occurrence is known to be possible
in elastoplastic materials with nonassociative flow law and to correspond to
some dynamically growing disturbance, its mechanical meaning has to date still
eluded a precise interpretation. This is provided here by constructing the
infinite-body, time-harmonic Green's function for the loading branch of an
elastoplastic material in flutter conditions. Used as a perturbation, it
reveals that flutter corresponds to a spatially blowing-up disturbance,
exhibiting well-defined directional properties, determined by the wave
directions for which the eigenvalues become complex conjugate. Flutter is shown
to be connected to the formation of localized deformations, a dynamical
phenomenon sharing geometrical similarities with the well-known mechanism of
shear banding occurring under quasi-static loading. Flutter may occur much
earlier than shear banding in a process of continued plastic deformation.
