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Critical Thickness Ratio for Buckled and Wrinkled Fruits and Vegetables
Fruits and vegetables are usually composed of exocarp and sarcocarp and they
take a variety of shapes when they are ripe. Buckled and wrinkled fruits and
vegetables are often observed. This work aims at establishing the geometrical
constraint for buckled and wrinkled shapes based on a mechanical model. The
mismatch of expansion rate between the exocarp and sarcocarp can produce a
compressive stress on the exocarp. We model a fruit/vegetable with exocarp and
sarcocarp as a hyperelastic layer-substrate structure subjected to uniaxial
compression. The derived bifurcation condition contains both geometrical and
material constants. However, a careful analysis on this condition leads to the
finding of a critical thickness ratio which separates the buckling and
wrinkling modes, and remarkably, which is independent of the material
stiffnesses. More specifically, it is found that if the thickness ratio is
smaller than this critical value a fruit/vegetable should be in a buckling mode
(under a sufficient stress); if a fruit/vegetable in a wrinkled shape the
thickness ratio is always larger than this critical value. To verify the
theoretical prediction, we consider four types of buckled fruits/vegetables and
four types of wrinkled fruits/vegetables with three samples in each type. The
geometrical parameters for the 24 samples are measured and it is found that
indeed all the data fall into the theoretically predicted buckling or wrinkling
domains. Some practical applications based on this critical thickness ratio are
briefly discussed.Comment: 11 pages 9 figures 2 table
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