  In a recent feature article in this journal, co-authored by Gert van der
Heijden, I described the static-dynamic analogy and its role in understanding
the localized post-buckling of shell-like structures, looking exclusively at
integrable systems. We showed the true significance of the Maxwell energy
criterion load in predicting the sudden onset of 'shock sensitivity' to lateral
disturbances. The present paper extends the survey to cover non-integrable
systems, such as thin compressed shells. These exhibit spatial chaos,
generating a multiplicity of localized paths (and escape routes) with complex
snaking and laddering phenomena. The final theoretical contribution shows how
these concepts relate to the response and energy barriers of an axially
compressed cylindrical shell.
  After surveying NASA's current shell-testing programme, a new non-destructive
technique is proposed to estimate the 'shock sensitivity' of a laboratory
specimen that is in a compressed meta-stable state before buckling. A probe is
used to measure the nonlinear load-deflection characteristic under a rigidly
applied lateral displacement. Sensing the passive resisting force, it can be
plotted in real time against the displacement, displaying an equilibrium path
along which the force rises to a maximum and then decreases to zero: having
reached the free state of the shell that forms a mountain-pass in the potential
energy. The area under this graph gives the energy barrier against lateral
shocks. The test is repeated at different levels of the overall compression. If
a symmetry-breaking bifurcation is encountered on the path, computer
simulations show how this can be supressed by a controlled secondary probe
tuned to deliver zero force on the shell.
