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Abstract
Thin elastic sheets buckle to form a wide variety of morphologies that can be broadly categorised intotwo groups: smoothly wrinkled shapes, and sharply faceted shapes. According to common knowledge, the
former is energetically dominated by external sources of work (like a substrate or boundary tension), while
the latter is dominated by sources of work arising from the sheet’s intrinsic elasticity and geometry. In this
thesis, we analyse a buckled morphology that shows characteristics of both these aforementioned categories.
We call this intermediate category “faceted wrinkling”. Using numerical finite-element simulations, we study
a minimal two-dimensional system: a circular annulus contracted at the inner boundary by fraction ∆, so
that it buckles into a radial wrinkling pattern that shows sharp zig-zag faceting at the inner boundary. In
our first result, we argue that this morphology results from the fact that the wrinkling is asymptotically
isometric, i.e. its stretching energy approaches zero relative to its bending energy. To this end, we compare
our numerically generated solutions to an Ansatz zero-thickness solution made up of alternating triangles
and cones that is developable, and hence isometric, by design. We find this isometric cone-triangle Ansatz to
agree with simulations over a wide range of values of system size, thickness, and wrinkle wavenumber and
amplitude. In our second result, we address the mechanism that selects the wrinkle wavelength λ in such a
pure-bending configuration. Usually, wavelength selection in elastic wrinkling occurs through macroscopic
competition between the sheet’s bending energy and some external source of deformation work, like a (real
or effective) substrate. What could select λ in the absence of any competition to the bending energy? Using
our numerical simulations, we argue that competition between stretching and bending energies at mesoscopic
scales leads to the selection of a wavelength scale sensitive to both the width w and thickness t of the sheet:
λ∗ ∼ w2/3t1/3∆−1/6. This scale λ∗ corresponds to an arrest criterion for wrinkle coarsening in the sheet
starting from any wavelength finer than λ∗: λ ≲ λ∗. However, the sheet can support coarser wavelengths,
λ ≳ λ∗, since there is no penalty to their existence. Since this wavelength selection mechanism depends on
the initial λ, it is path-dependent (or hysteretic).