A group of Canadian researchers have investigated the optimal design for lightweight armour, using 3D printing and mechanical testing
If asked to suggest animals that use armour to protect their soft, squishy innards, you might think of the quills of a porcupine, the thick hide of a rhino, or the hard shell of a tortoise. But there is another class of natural armours that are incredible effective – flexible ones, based on carefully arranged scales and plates. Used by alligators, armadillo and fish, they offer a unique combination of hardness, flexibility and low weight.
The contrast between the stiffness of the scales and the soft tissue that they protect means that such armours offer protection without overly impeding movement. For this reason, they’ve caught the attention of researchers looking to design smart, lightweight armour materials. But despite the growing interest, the current literature contains very few systematic studies into how the performance of such materials can be influenced by the design of the individual components.
So that is exactly what McGill University mechanical engineers have tackled in their latest paper, soon to be published in Acta Biomaterialia [DOI: 10.1016/j.actbio.2017.03.025]. They’ve explored how the geometry and arrangement of ‘hard’ scales could be tailored to offer a combination of puncture resistance and flexural compliance.
To do this, they 3D printed scales made from acrylonitrile butadiene styrene (ABS). The complexity and placement of these scales were varied from simple squares with no overlap, to scales that overlap and interlock, mimicking those found on real freshwater fish. They submitted each of the eight arrays to the same puncture and flexural tests, in order to determine which design offered the optimal combination of puncture resistance and flexibility.
They found that promoting scale-scale interactions was the key to designing an armour that offered maximum protection from puncture. The highest puncture resistance was measured in their ‘simplified elasmoid design’, in which each scale overlapped with surrounding scales at multiple points. The force needed to puncture these scales was a staggering 12.75 N, 792% higher than the force that caused the simple array of square scales to fail.
However, armour isn’t all about strength. The team found that the best compromise between protection and manoeuvrability was offered by the scale geometries that mimicked those of fish like bass or sturgeon. The authors say that “This striking observation suggests that natural evolution has shaped the geometry and arrangement of natural scales to maximize protective efficiency.” This study certainly yields new insights into the mechanisms of natural dermal armour, and it may also inspire new designs for personal protective systems.
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R. Martini, Y. Balit and F. Barthelat, “A comparative study of bio-inspired protective scales using 3D printing and mechanical testing.” Acta Biomaterialia, In Press, Accepted Manuscript. DOI: 10.1016/j.actbio.2017.03.025