Photograph of Odontodactylus scyllarus, along with an image of a dactyl club that has been removed and sectioned (as indicated by the dashed line marks).The hammer-like club on the claw of a mantis or ‘smasher’ shrimp, which it uses to break open the shells of its prey, is the inspiration behind a new design for tougher carbon fiber-epoxy composites.
Natural composites such as mollusk shells are receiving considerable attention as a model for new designs. But David Kisailus of the University of California Riverside and colleagues from Brookhaven National Laboratory, Purdue University, and the University of Southern California looked instead at the mantis shrimp that is strong enough to smash through such shells [L.K. Grunenfelder, et al., Acta Biomaterialia (2014), DOI: 10.1016/j.actbio.2014.03.022].
The stomatopod’s heavily mineralized dactyl club, which contains aligned chitin fibers in a crystalline hydroxyapatite matrix, is able to withstand repeated impacts without failure. The strength of the club relies on the fact that each layer of chitin fibers is rotated by a small angle with respect to the layer below, forming what is known as a ‘helicoidal’ composite.
Kisailus and his team mimicked this structure with carbon fiber-epoxy composites, creating three helicoidal structures with different rotation angles (7.8°, 16.3°, and 25.7°) and compared them to conventional composites in which all the fibers are aligned in parallel (unidirectional) or have fiber layers oriented at 0°, ±45°, and 90° directions angles (quasi-isotropic structures). ‘Drop weight’ impact tests proved catastrophic for unidirectional and quasi-isotropic composites, which failed completely or were punctured upon impact. Helicoidal composites, by contrast, showed a much smaller dent – on average, 49% shallower than in conventional structures. In follow-up compression tests, the medium- and large-angle helicoidal composites also showed a 15-20% increase in residual strength compared with quasi-isotropic structures.
The key to the remarkable mechanical properties of the mantis shrimp’s club lies in the ability of the structure to propagate cracks between the mineralized fibers rather than breaking them, say the researchers. The helicoidal structural dissipates the energy from high-energy impacts by redirecting cracks and preventing their propagation to the surface, where they would lead to catastrophic failure. The bio-inspired structure also avoids a large-angle mismatch between the fiber layers, which reduces interlaminar shear stresses when impacted.
Although all the helicoidal structures showed less external damage on impact, the smallest angle samples exhibited the lowest residual strength. The researchers believe that this can be put down to experimental limitations and, in fact, small-angle helicoidal architectures could offer the best energy absorption performance.
“Biology has an incredible diversity of species, which can provide us new design cues and synthetic routes to the next generation of advanced materials for light-weight automobiles, aircraft and other structural applications,” says Kisailus.
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