  The strength of neutron star crust is crucial for modelling magnetar flares,
pulsar glitches and gravitational wave emission. We aim to shed some light on
this problem by analysing uniaxial stretch deformation (elongation and
contraction) of perfect body-centered cubic Coulomb crystals, paying special
attention to the inherent anisotropy of this process. Our analysis is based on
the semi-analytical approach of Baiko and Kozhberov (2017), which, for any
uniform deformation, allows one to calculate, in fully non-linear regime,
critical deformation parameters beyond which the lattice loses its dynamic
stability. We determine critical strain, pressure anisotropy and deformation
energy for any stretch direction with respect to the crystallographic axes.
These quantities are shown to be strongly anisotropic: they vary by a factor of
almost 10 depending on the orientation of the deformation axis. For
polycrystalline crust, we argue that the maximum strain for the stretch
deformation sustainable elastically is 0.04. It is lower than the breaking
strain of 0.1 obtained in molecular dynamic simulations of a shear deformation
by Horowitz and Kadau (2009). The maximum pressure anisotropy of
polycrystalline matter is estimated to be in the range from 0.005 to 0.014
$nZ^2e^2/a$, where $n$ is the ion number density, $Ze$ is the ion charge, and
$a$ is the ion-sphere radius. We discuss possible mechanisms of plastic motion
and formation of large crystallites in neutron star crust as well as analyse
energy release associated with breaking of such crystallites in the context of
magnetic field evolution and magnetar flaring activity.
