Mechanics of Functionally Graded Panicum miliaceum Seedcoat with Sutural Tessellation

The seedcoat of common millet (Panicum miliaceum) has a unique jigsaw puzzle-like microstructure. The epidermis cells on the seedcoat articulate via wavy intercellular sutures, forming a compact layer to protect the kernel inside. The intercellular boundaries of the cells are joined by thin, wavy suture interfaces. Previous studies have shown that this tessellation design can have amplified strength, toughness, and can have auxetic properties under higher stiffness ratios.

The goal of this study is to quantify the functionally graded mechanical properties of the seedcoat under different levels of humidity. Nanoindentation mechanical experiments are performed at selected locations on the surface of the seedcoat in a humidity chamber. The effective stiffness at different locations under different levels humidity are determined. The relationship between the complexity of local suture tessellation and the corresponding mechanical properties are explored. With the experimental input, finite element models of suture tessellations at different locations are developed to reveal the mechanics of the functionally graded property of the seedcoat.