A spiral fracture is a particularly nasty injury for human bones. For crustaceans exoskeletons, however, spiral fractures are something like a design feature. That is, they actually make their shells tougher.
To explain, lobster, crab and shrimp exoskeletons are made up of chitin protein fibers. The fibers are layered in a spiral pattern known as a Bouligand structure:
When the exoskeleton receives an impact and cracks, the crack doesn't travel straight down through the layers, like it would with a piece of wood. Instead the crack spirals through the structure in accordance with the layers. "Instead of a single crack continuing to propagate," write researchers at Purdue University, "numerous smaller cracks form – dissipating the energy absorbed by the material upon impact."
"New findings show that the composite material of the [exoskeleton] actually becomes tougher as a crack tries to twist, in effect halting its progress."
The researchers, led by engineering professor Pablo Zavattieri, have been 3D printing composites with layers in a Bouligand structure and stress-testing them to better understand the ideal patterns.
Here's Zavattieri demonstrating:
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Up next, we'll see how researchers on the other side of the world have taken this a step further and revealed the patterns they're working with.