James J-W. Lee


Teeth are highly damage tolerant, and yet tooth enamel is brittle with low fracture toughness.  This work reports the results of fracture experiments on extracted human molars and other mammalian teeth.  We describe deformation and fracture evolution in simple contact tests and during larger scale physiologically relevant testing protocols designed to simulate dental occlusion.  Damage accumulation in the enamel is mitigated by the special tooth microstructure and geometry. Such mechanical property testing reveals differences in properties at different locations within the enamel layer, as well as between species. Detailed sectioning of failed teeth elucidates the role of enamel microstructure on the crack evolution.  Critical loads for the onset and propagation of damage are controlled primarily by enamel thickness and tooth size, rather than by variations in enamel material properties.  The relative importance of competing deformation and fracture modes in dental enamel of different animal species, specifically great apes and sea otters, is considered.  It is shown that incipient defects within the enamel provide weak pathways for fracture, yet act to inhibit prolonged crack propagation by deflecting and bridging the cracks or by self-healing. Implications relating to diet of these species are also discussed.