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Creature feature: Twisting cracks impart superhero toughness to animals

June 25, 2018

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 mainly performed at Purdue University with collaborators at 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.

"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. A YouTube video explaining this concept is available at

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."

Two papers were published in the Journal of the Mechanical Behavior of Biomedical Materials and the International Journal of Solids and Structures. The papers were co-authored by Purdue doctoral student Nobphadon Suksangpanya; UC Riverside doctoral student Nicholas A. Yaraghi; David Kisailus, a UC Riverside professor of chemical and environmental engineering and materials science and engineering; and Zavattieri.

"The novelty of this work is that, on the theory side, we developed a new model, and on the experimental side we used established materials to create composites that validate this theory," Zavattieri said.

Previous research has shown this helicoidal architecture is naturally designed to survive the repeated high-velocity blows, revealing that the fibers also are arranged in a herringbone pattern in the appendage's outer layer.

In the new research, the team has learned specifically why this pattern imparts such toughness: as cracks form, they follow the twisting pattern rather than spreading straight across the structure, causing it to fail. Images taken with an electron microscope show that instead of a single crack continuing to propagate, numerous smaller cracks form - dissipating the energy absorbed by the material upon impact.

The researchers created and tested 3D-printed composites modeled after the phenomenon, capturing the crack behavior with cameras and digital image correlation techniques to study the deformation of the material.

Bryon Pipes, Purdue's John L. Bray Distinguished Professor of Engineering, helped Suksangpanya to fabricate glass fiber-reinforced composites incorporating this phenomenon.

"We are establishing new mechanisms that were not available to us before for composites," Zavattieri said. "Traditionally, when we produce composites we put fibers together in ways that are not optimal, and nature is teaching us how we should do it."
The research has been funded by the National Science Foundation through a CAREER award, the U.S. Air Force Office of Scientific Research (AFOSR- FA9550-12-1-0245) and Multi-University Research Initiative (AFOSR-FA9550-15-1-0009).


Crack twisting and toughening strategies in Bouligand architectures

Nobphadon Suksangpanya, Nicholas A. Yaraghi b, R. Byron Pipesc, David Kisailus b,c , Pablo Zavattieria aLyles School of Civil Engineering, Purdue University, bMaterials Science and Engineering Program, University of California, Riverside, cSchool of Aeronautics and Astronautics, Purdue University, dDepartment of Chemical and Environmental Engineering, University of California, Riverside

The Bouligand structure in some arthropods is a hierarchical composite comprised of a helicoidal arrangement of strong fibers in a weak matrix. In this study, we focus on the Bouligand structure present in the dactyl club of the smashing mantis shrimp due to its exceptional capability to withstand repetitive high-energy impact without catastrophic failure. We carry out a combined computational and experimental approach to investigate the high damage resistance of the Bouligand structure through a biomimetic composite material. This is studied by performing specific fracture experiments on the helicoidal composites specimens, where it was found that crack twisting, driven by the fiber architecture, is the main fracture mechanisms. This crack twisting mechanism competes with other alternative mechanisms such as crack branching and delamination, delaying catastrophic failure. The main mechanism of crack twisting is studied through specifically designed specimens in which the crack propagation path is controlled. Further quantification of the toughening mechanisms and crack growth rate is analyzed with analytical and finite element models. The biomimetic helicoidal composites are shown to have improved fracture resistance as the crack twists mainly driven by the increase in crack surface area and fracture mode mixity. Our analysis allowed us to study the effect of crack front shape, stress distribution and energy dissipation mechanisms.

Twisting Cracks in Bouligand Structures

Nobphadon Suksangpanya, Nicholas A. Yaraghi b, David Kisailus b,c , Pablo Zavattieria

aLyles School of Civil Engineering, Purdue University, bMaterials Science and Engineering Program, University of California, Riverside, cDepartment of Chemical and Environmental Engineering, University of California, Riverside

The Bouligand structure, which is found in many biological materials, is a hierarchical architecture that features uniaxial fiber layers assembled periodically into a helicoidal pattern.

Many studies have highlighted the high damage-resistant performance of natural and biomimetic Bouligand structures. One particular species that utilizes the Bouligand structure to achieve outstanding mechanical performance is the smashing Mantis Shrimp, Odontodactylus Scyllarus (or stomatopod). The mantis shrimp generates high speed, high acceleration blows using its raptorial appendage to defeat highly armored preys. The load-bearing part of this appendage, the dactyl club, contains an interior region [16] that consists of a Bouligand structure. This region is capable of developing a significant amount of nested twisting microcracks without exhibiting catastrophic failure. The development and propagation of these microcracks are a source of energy dissipation and stress relaxation that ultimately contributes to the remarkable damage tolerance properties of the dactyl club. We develop a theoretical model to provide additional insights into the local stress intensity factors at the crack front of twisting cracks formed within the Bouligand structure. Our results reveal that changes in the local fracture mode at the crack front leads to a reduction of the local strain energy release rate, hence, increasing the necessary applied energy release rate to propagate the crack, which is quantified by the local toughening factor. Ancillary 3D simulations of the asymptotic crack front field were carried out using a J-integral to validate the theoretical values of the energy release rate and the local stress intensity factors.

Purdue University

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