  Consideration is given to a non-convex variational model for a shear
experiment in the framework of single-crystal linearised plasticity with
infinite cross-hardening. The rectangular shear sample is clamped at each end,
and is subjected to a prescribed horizontal or diagonal shear, modelled by an
appropriate hard Dirichlet condition. We ask: how much energy is required to
impose such a shear, and how does it depend on the aspect ratio? Assuming that
just two slip systems are active, we show that there is a critical aspect
ratio, above which the energy is strictly positive, and below which it is zero.
Furthermore, in the respective regimes determined by the aspect ratio, we prove
energy scaling bounds, expressed in terms of the amount of prescribed shear.
