  From soda cans to space rockets, thin-walled cylindrical shells are abundant,
offering exceptional load carrying capacity at relatively low weight. However,
the actual load at which any shell buckles and collapses is very sensitive to
imperceptible defects and can not be predicted, which challenges the reliable
design of such structures. Consequently, probabilistic descriptions in terms of
empirical design rules are used and reliable design requires to be very
conservative. We introduce a nonlinear description where finite-amplitude
perturbations trigger buckling. Drawing from the analogy between imperfect
shells which buckle and imperfect pipe flow which becomes turbulent, we
experimentally show that lateral probing of cylindrical shells reveals their
strength non-destructively. A new ridge-tracking method is applied to
commercial cylinders with a hole showing that when the location where buckling
is nucleated is known we can accurately predict the buckling load of each
individual shell, within $\pm 5\%$. Our study provides a new promising
framework to understand shell buckling, and more generally,
imperfection-sensitive instabilities.
