18 MOMENTUM • VIRGINIA TECH MECHANICAL ENGINEERING
Researchers find cuttlebone ’ s microstructure sits at a ‘ sweet spot ’ for lightweight , stiff , and damage-tolerant design
by Suzanne Irby , Virginia Tech College of Engineering
Ling Li has a lesson in one of his mechanical engineering courses on how brittle materials like calcium carbonate behave under stress . In it , he takes a piece of chalk composed of the compound and snaps it in half to show his students the edge of one of the broken pieces . The break is blunt and straight .
Then , he twists a second piece , which results in sharper shards broken at a 45-degree angle , indicating the more dangerous direction of tensile stress on the chalk . The broken chalk helps Li demonstrate what brittle calcium carbonate will do under normal forces : it tends to fracture .
“ If you bend it , it will break ,” Li said .
In Li ’ s Laboratory for Biological and Bio-Inspired Materials , many of the ocean animals he studies for their biological structural materials have parts made of calcium carbonate . Some mollusks use it in photonic crystals that create a vivid color display , “ like a butterfly ’ s wings ,” Li said . Others have mineral eyes built with it , into their shells . The more Li studies these animals , the more he ’ s amazed by the uses their bodies find for intrinsically brittle and fragile material . Especially when the use defies that fragility .
In a study published by Proceedings of the National Academy of Sciences of the United States of America , Li ’ s research team focused on the cuttlefish , another one of those inventive , chalk-built animals and a traveler of the ocean ’ s depths . The researchers investigated the internal microstructure of cuttlebone , the mollusk ’ s highly porous internal shell , and found that the microstructure ’ s unique , chambered “ wall-septa ” design optimizes cuttlebone to be extremely lightweight , stiff , and damage-tolerant . Their study goes into the underlying material design strategies that give cuttlebone these high-performance mechanical properties , despite the shell ’ s composition mostly of brittle aragonite , a crystal form of calcium carbonate .
In the ocean , the cuttlefish uses cuttlebone as a hard buoyancy tank to control its movement up and down the water column ,