Fracture toughness of sea otter enamel

Objective: To compare the fracture behaviour of the enamel of carnivore teeth to the human by chipping tests. Methods: Sea otter (Enhydra lutris, Mustelidae) teeth were obtained, still within jaws (from Melissa Miller, California Fish and Game, with full permission). Teeth were excised and their roots embedded in epoxy resin blocks. Teeth were kept hydrated prior to experiment and cut so that a flat enamel (transverse) surface was obtained. A Vickers pyramid, ground onto the end of a 6-mm diameter tungsten carbide rod, was fixed in a mechanical tester with its tip directed vertically to contact the enamel at a distance h from the tooth edge. The load was gradually increased until, at a peak force P_F, a scallop-shaped chip broke away from the enamel. Previous work on chipping fine-grained ceramic materials and human enamel suggests that P_F =9.3Th^1.5, where T is the fracture toughness. Thus, log-log plots of P_F-h for varying h were constructed such that the gradient could reveal T for sea otter enamel. Enamel structure was examined by light (LM) and environmental scanning electron (ESEM) microscopy. Results: Data for 12 chips were obtained. Plots of log P_F vs. log h were linear, but with substantial scatter. The apparent average toughness of sea otter enamel was 1.8 MPa m^1/2, close to double that in the human. However, there was considerable plasticity under the indenter prior to fracture. Structural study by LM confirmed that decussation of the inner enamel of sea otters was greater than that in humans. Crack paths observed using ESEM showed that fractures often crossed enamel rods. Conclusion: Sea otters appear to have tougher enamel than humans. As carnivores suffer frequent fractures to their teeth, it is logical to suppose that their teeth have been under strong selective pressure to combat fracture damage.