20 MOMENTUM • VIRGINIA TECH MECHANICAL ENGINEERING
“ The exact morphology we haven ’ t seen , at least in other models ,” said Li of the design .
This wall-septa design gives cuttlebone control of where and how damage occurs in the shell . It allows for graceful , rather than catastrophic , failure : when compressed , chambers fail one by one , progressively rather than instantaneously .
The researchers found that cuttlebone ’ s wavy walls induce or control fractures to form at the middle of walls , rather than at floors or ceilings , which would cause the entire structure to collapse . As one chamber undergoes wall fracture and subsequent densification — in which the fractured walls gradually compact in the damaged chamber — the adjacent chamber remains intact until fractured pieces penetrate its floors and ceilings . During this process , a significant amount of mechanical energy can be absorbed , Li explained , limiting external impact .
A scanning electron microscopic image of cuttlebone , showing its chambered microstructure .
Li ’ s team further explored the high-performance potential of cuttlebone ’ s microstructure with computational modeling . Using measurements of the microstructure made with the earlier 3D tomography , postdoctoral researcher Zian Jia built a parametric model , ran virtual tests that altered the waviness of the structure ’ s walls , and observed how the shell performed as a result .
“ We know that cuttlebone has these wavy walls with a gradient ,” Li said . “ Zian changed the gradient so we could learn how cuttlebone behaved if we went beyond this morphology . Is it better , or not ? We show that cuttlebone sits in an optimal spot . If the waviness becomes too big , the structure is less stiff . If the waves become smaller , the structure becomes more brittle . Cuttlebone seems to have found a sweet spot , to balance the stiffness and energy absorption .”
Li sees applications for cuttlebone ’ s microstructural design in ceramic foams . Among foams used for crush resistance or energy
Supporting the chambers of cuttlebone ’ s microstructure are uniquely wavy walls .
Left photo by James C . Weaver , RIGHT photo by Ting Yang , Zian Jia , and Liuni Chen