December 24, 2009 — Vol. 2, Issue 12
Cracked on Impact
The Secrets of Tooth Enamel
The next wave of aircraft structural composites may be inspired by the resilient structure of mammalian tooth enamel.
Tooth enamel is naturally weak. Its fracture toughness is on the order of glass, but despite this weakness, enamel demonstrates a surprising resilience to repeated impacts. This curiosity led scientists from the National Institute of Standards and Technology (NIST), Tel Aviv University, and George Washington University to investigate the mechanism behind enamel's extended lifetime. What they found was a resilient structure worthy of attention in the aerospace community.
When researchers performed contact experiments on extracted human and sea otter teeth and investigated the changes beneath the surface, they concluded that teeth don't avoid damage — rather they contain it.
Enamel has a structure similar to that of a woven fiber design, explained James Lee of George Washington University. When a crack forms, there is no obvious path along which the crack can propagate. Enamel also has intrinsic flaws, which manifest as a multitude of micro-cracks called "tufts." These tufts, the researchers found, diffuse the stress of future impacts and applied loads. In combination with enamel's self-healing properties, these tufts decrease the overall damage and slow the propagation of cracks.
Typical aerospace composites used today are composed of grid-like layers of glass or carbon fibers, versus the woven, three-dimensional structure of enamel. As a result, it is easier for cracks to propagate through the material than through enamel. "The structure [of enamel] is much more sophisticated or smarter than synthetic composites," said Herzl Chai, a mechanical engineer at Tel Aviv University.
The self-healing capabilities of enamel also hold great interest. The micro-cracks that form seem to be repaired to a certain extent by a protein-rich fluid present in the enamel. This aspect is what Wayne Gregg, a damage tolerance engineer at NASA's Marshall Space Flight Center, found most interesting. "We expect to see micro-cracking in structural composites," Gregg said. "A composite's ability to heal would be a very significant leap of technology.
The characteristics of enamel offer a spark of inspiration for the creation of future structural composites. "[It's] great for technology development studies," said Gregg, "but not likely to be implemented on real hardware in the near future unless we have some great gains." Chai echoed this sentiment. "We may learn a few tricks from Mother Nature, although the implementation will not be so easy and immediate."
These findings were published in "The Remarkable Resilience of Teeth" in the May 2009 issue of Proceedings of the National Academy of Sciences
Article by HS