Shell Provides Resistance Against Cracking

a pile of broken macadamia nut shells in varying shades of brown and cream, the shells are spherical and quite thick

Biological Strategy

Macadamia

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Image: Forest and Kim Starr / Wikimedia Commons / CC BY - Creative Commons Attribution alone

Manage Impact

An impact is a high force or mechanical shock that happens over a short period of time, such as a hammer hitting a nail rather than a hand pushing slowly against a wall. Because of their speed and force, impacts don’t allow materials to slowly adjust to the force, which can lead to cracks, ruptures, and complete breakage. Therefore, living systems have strategies that can absorb, dissipate, or otherwise survive that force without the need to add large amounts of material. For example, the Toco toucan’s large beak is very lightweight, yet can withstand impacts because it’s made of a composite material with rigid foam inside and layers of a hard, fibrous material outside.

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Manage Compression

When a living system is under compression, there is a force pushing on it, like a chair with a person sitting on it. When evenly applied to all sides of a living system, compression results in decreased volume. When applied on two sides, it results in deformation, such as when pushing on two sides of a balloon. This deformation can be temporary or permanent. Because living systems must retain their most efficient form, they must ensure that any deformation is temporary. Managing compression also provides an opportunity to lessen the effects of other forces. Living systems have strategies to help prevent compression or recover from it, while maintaining function. For example, African elephant adults weigh from 4,700 to 6,048 kilograms. Because they must hold all of that weight on their four feet, the tissues of their feet have features that enable compression to absorb and distribute forces.

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Prevent Fracture/Rupture

High force impact or stress can cause materials that comprise living systems to separate into two or more pieces (called fracturing) or to break or burst suddenly (called rupturing). For example, a scallop prevents structural failure from fracture because its shell is comprised of two materials of varying stiffness. When a crack moves from the scallop’s stiff material to the less stiff one, the latter reduces the force at the tip of the crack, thereby stopping it from spreading farther.

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Plants

Phylum Plantae (“plants”): Angiosperms, gymnosperms, green algae, and more

Plants have evolved by using special structures within their cells to harness energy directly from sunlight. There are currently over 350,000 known species of plants which include angiosperms (flowering trees and plants), gymnosperms (conifers, Gingkos, and others), ferns, hornworts, liverworts, mosses, and green algae. While most get energy through the process of photosynthesis, some are partially carnivores, feeding on the bodies of insects, and others are plant parasites, feeding entirely off of other plants. Plants reproduce through fruits, seeds, spores, and even asexually. They evolved around 500 million years ago and can now be found on every continent worldwide.

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The outer shells of the nuts of macadamia trees are extremely resistance to cracking due to its fullerene-like surface structure

Image: Kahuroa / wikimedia commons /

Shells of Macadamia nuts at a Macadamia farm near Helensville, Auckland district, New Zealand

Image: Forest & Kim Starr / wikimedia commons /

Macadamia integrifolia (nuts on ground). Location: Maui, MISC HQ Piiholo

“The stones of the macadamia fruits are generally called ‘macadamia nuts’. They are very hard to crack with fracture toughness comparable with that of glass and many ceramics, and their work of fracture is about an order of magnitude higher than that of high quality structural ceramics (Jennings and MacMillan, 1986).”(Kaupp and Kaupp 2007:150)

“The macadamia hardshell owes its extreme resistance against external cracking to its advanced fullerene-like* surface construction with additional elastic stiffeners. The polygons are about 50 μm wide and appear to consist of flat lying fibres according to the structure of the fractured hardshell in Figure 8 with overwhelming ends of bundles there. The round stiffeners are likely candidates for the bundles of fibres that end normal to the spheroid surface of the hardshell. This architecture is the reason for the extreme cracking toughness of the 160 μm thick shell.” (Kaupp and Kaupp 2007:158)

*”Fullerene-like” refers to being like buckyballs (as in Buckminster Fuller) and refers to polygons and stiffeners that make up the structure of the shell.

Last Updated March 25, 2020

References

Journal article

Advanced fullerene-type texture and further features of the macadamia nutshell as revealed by optical 3D microscopy

Kaupp G; Kaupp M

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