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.
The stomatopod’s heavily mineralised dactyl club, which contains aligned chitin fibres 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 fibres is rotated by a small angle with respect to the layer below, forming what is known as a ‘helicoidal’ composite.
Helicoidal composites
Kisailus and his team mimicked this structure with carbon fibre 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 fibres are aligned in parallel (unidirectional) or have fibre 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.
Read more in this Materials Today article.
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