On the Origins of Toughness in <i>Corymbia calophylla</i> (Marri Tree) Nuts

Fruits and nuts develop resilient outer shells to safeguard their embryos against environmental hazards and predators. The nuts of the marri tree (Corymbia calophylla) possess unusual strength and toughness. In contrast to other tough nuts, seasoned parrots avoid cracking them and instead resort to drawn-out attempts to snatch their seeds from a dispersal opening at their base. Here, we reveal the subtle strengthening mechanisms of the marri nut and compare its mechanical response and properties to those of other nuts and commercial materials. The analysis involves an extensive examination of the various material phases of the nutshell, the chemical composition of its fibers, and an analysis of its microscopic and macroscopic mechanical properties. The study reveals that, akin to self-standing structures, the interplay of the densely packed fibers of the shell held together by a highly ductile soft matrix, while arranged in a spherical configuration, gives the nut its notable toughness, ductility, strength, and remarkable fracture resistance. Additionally, we design and test the mechanical behavior of a bioinspired interpenetrating phase composite material mimicking the nut's microstructure, demonstrating comparable deformability and toughness. These findings lay a new foundation for designing highly efficient bioinspired structures for energy absorption, protection, and armor.