Interlocking Increases Material’s Toughness

close up photograph of purple, blue, pink and green shell

Biological Strategy

Mollusks (Snails, Octopuses)

Eleanor Banwell

Image: Spitsyna Anastasiia / Shutterstock / Some rights reserved

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 Tension

When a living system is under tension, it means there is a force pulling on it, like a person pulling on a rope tied to a horse. When applied to a living system, unless the system is completely rigid, the result is that it gets stretched. If stretching exceeds the strength of the living system’s material, it can damage it. Living systems manage tension using materials that are flexible and stretchable enough to survive most tension that occurs in their environment. The ocean’s intertidal zone offers a good example. The waves and incoming and outgoing tides put tension on soft-bodied organisms. Mussels resist tension with flexible threads that hold them onto rocks; in contrast, large algae have stretchy fronds.

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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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Physically Assemble Structure

Living systems use physical materials to create structures to serve as protection, insulation, and other purposes. These structures can be internal (within or attached to the system itself), such as cell membranes, shells, and fur. They can also be external (detached), such as nests, burrows, cocoons, or webs. Because physical materials are limited and the energy required to gather and create new structures is costly, living systems must use both conservatively. Therefore, they optimize the structures’ size, weight, and density. For example, weaver birds use two types of vegetation to create their nests: strong, a few stiff fibers and numerous thin fibers. Combined, they make a strong, yet flexible, nest. An example of an internal structure is a bird’s bone. The bone is comprised of a mineral matrix assembled to create strong cross-supports and a tubular outer surface filled with air to minimize weight.

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Mollusks (Snails, Octopuses)

Phylum Mollusca (“soft”): Snails, clams, octopus, oysters, slugs

There are 50,000 known species of mollusk, and Members of this phylum have a mantle, which is a thick layer of tissue that contains the internal organs and can secrete a calcium carbonate shell. Mollusks also have radula, which is a circular band of teeth made of chitin. The muscle on the underside of mollusks forms a foot that can be used to move or attach to surfaces. The animals across the seven classes vary greatly in body shape, size, and life history.

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Plates in nacre increase toughness by interlocking

Nacre, or mother of pearl, is the iridescent material that forms the inner layer of the shells of some molluscs. It is a natural composite of plates of aragonite (a form of calcium carbonate) and organic layers that go around and through the plates, giving them some flexibility and holding them together. Nacre is 3000 times tougher than aragonite alone.

In nacre, aragonite is formed into stacked hexagonal plates. The plates are twisted relative to each other by 5% and sunk. That is, each plate overlaps its top and bottom neighbors by 20% of its depth, leaving the twisted offset corners as overhangs. These overhangs create lips that prevent the aragonite plates sliding past each other.

When nacre is subjected to force, the overhanging lips interlock, making the material tough. Subjected to more force the overhangs begin to break off. Although the force required to fracture a single overhang is small, summed throughout the material, the energy required to create large fractures in nacre becomes relatively large.

Brittle materials like aragonite are brittle in part because, when they start to fail, cracks can propagate easily throughout the bulk material. Because the aragonite in nacre is arranged in a staggered brick-like arrangement, and because the interlocks are the sites of first fracture, when cracks start, there is nowhere for them to spread to. To see another way the organic material between aragonite plates makes nacre tougher by stopping cracks from propagating, check out the related strategy here.