Layers of weak and stretchy organic material between brittle mineral layers in nacre make the whole composite tough by managing cracks.

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Nacre (also known as mother of pearl) is the shiny biological material that lines the inner surface of many mollusc shells. It consists of approximately 95% inorganic minerals (calcium carbonate) and 5% organic material (a mix of proteins and polysaccharides, including chitin). Inorganic minerals make materials hard and stiff, which is important for supportive or protective structures like shells; however, they also typically make materials brittle and relatively easy to fracture (man-made glass is an example of a brittle material).

Nacre’s specific composition and construction make it tough and resistant to catastrophic failure that can result from spreading cracks. Here, higher toughness means that a greater amount of energy is needed to fracture or break the material. Hard microscale mineral layers in nacre are “glued” together by relatively soft nanoscale organic layers. The arrangement is much like staggered layers of bricks that are held together by mortar in a brick wall. When a crack starts in the nacre (say from a predatory attack), it quickly encounters the organic layers that are easy to stretch compared to the mineral layers. The cause behind the organic material’s stretchiness can vary among nacres from different species; one mechanism involves wavy or folded fibers that straighten out before experiencing any significant tension.

The overall effect is that the stretchy organic layers provide avenues for deflecting cracks and absorbing and dissipating energy. Cracks can be controlled and stopped before spreading through the whole shell and causing serious damage. Counterintuitively, built-in areas of weakness on the microscale make the whole material tougher on the macroscale.

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References

“High mineral content generally leads to brittleness, yet natural materials such as bone, mollusk shells or glass sponge achieve relatively high toughness [work of fracture] considering the weakness of their constituents through intricate microstructures. In particular, nanometers thick organic interfaces organized in micro-architectures play a key role in providing toughness by various processes including crack deflection, crack bridging or energy dissipation…We found that top shell nacre displayed the highest interfacial toughness, because of higher surface roughness and a more resilient organic material, and also through extrinsic toughening mechanisms including crack deflection, crack bridging and process zone…[T]he extensibility or ductility of the interfaces may be more important than their strength and toughness to produce toughness at the macroscale.” (Khayer Dastjerdi et al. 2013:50)

Journal article
The weak interfaces within tough natural composites: Experiments on three types of nacreJournal of the Mechanical Behavior of Biomedical MaterialsJanuary 1, 2013
Khayer Dastjerdi A; Rabiei R; Barthelat F

Journal article
Overcoming the brittleness of glass through bio-inspiration and micro-architectureNature CommunicationsJanuary 1, 2014
Mirkhalaf M; Khayer Dastjerdi A; Barthelat F.

Journal article
Nacre from mollusk shells: a model for high-performance structural materialsBioinspir. Biomim.January 1, 2010
Barthelat F.

“In a material with a high work of fracture, cracks do not easily propagate…[a]s a general rule, the more jagged the break, the more work it took to make the break, and the higher the work of fracture of the material.” (Vogel 2003:318)

Book
Comparative Biomechanics: Life's Physical WorldJanuary 1, 2003
Vogel S

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Organism
Black-lip Pearl ShellPinctadaSpecies


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