Twisting cracks impart superhero toughness to animals

The helicoidal architecture of a mantis shrimp’s dactyl club is naturally designed to survive repeated high-velocity blows. (Purdue University image/Pablo Zavattieri)

WEST LAFAYETTE, Ind. – Super-resilient materials found in the animal kingdom owe their strength and toughness to a design strategy that causes cracks to follow the twisting pattern of fibers, preventing catastrophic failure.

Researchers in a recent series of papers have documented this behavior in precise detail and also are creating new composite materials modeled after the phenomenon. The work was performed by a team of researchers at Purdue University in collaboration with the University of California, Riverside.

The researchers studied the preternatural strength of a composite material in a sea creature called the mantis shrimp, which uses an impact-resistant appendage to pummel its prey into submission.

The mantis shrimp conquers its prey with a “dactyl club” appendage, which is made up of a composite material that grows tougher as cracks twist. 

“However, we are seeing this same sort of design strategy not just in the mantis shrimp, but also in many animals,” said Pablo Zavattieri, a professor in Purdue’s Lyles School of Civil Engineering. “Beetles use it in their shells, for example, and we also are seeing it in fish scales, lobsters, and crabs.”

What makes the mantis shrimp stand out is that it can actually smash and defeat its armored preys (mostly mollusks and other crabs), which are also known for their damage-tolerance and excellent mechanical properties. The mantis shrimp conquers them with its “dactyl club,” an appendage that unleashes a barrage of ferocious impacts with the speed of a .22 caliber bullet.

New findings show that the composite material of the club actually becomes tougher as a crack tries to twist, in effect halting its progress. This crack twisting is guided by the material’s fibers of chitin, the same substance found in many marine crustacean shells and insect exoskeletons, arranged in a helicoidal architecture that resembles a spiral staircase.

“This mechanism has never been studied in detail before,” Zavattieri said. “What we are finding is that as a crack twists the driving force to grow the crack progressively decreases, promoting the formation of other similar mechanisms, which prevent the material from falling apart catastrophically. I think we can finally explain why the material is so tough.”

Read more: Twisting cracks impart superhero toughness to animals

Images courtesy of