  In an attempt to describe cork-pulling, we model a cork as an incompressible
rubber-like material and consider that it is subject to a helical shear
deformation superimposed onto a shrink fit and a simple torsion. It turns out
that this deformation field provides an insight into the possible appearance of
secondary deformation fields for special classes of materials. We also find
that these latent deformation fields are woken up by normal stress differences.
We present some explicit examples based on the neo-Hookean, the generalized
neo-Hookean and the Mooney-Rivlin forms of the strain-energy density. Using the
simple exact solution found in the neo-Hookean case, we conjecture that it is
advantageous to accompany the usual vertical axial force by a twisting moment,
in order to extrude a cork from the neck of a bottle efficiently. Then we
analyse departures from the neo-Hookean behaviour by exact and asymptotic
analyses. In that process, we are able to give an elegant and analytic example
of secondary (or latent) deformations in the framework of nonlinear elasticity.
