Crack growth resistance of human tooth enamel: Mechanisms of toughening and energy dissipation

Enamel, located on the surface of teeth, is the hardest and the most highly calcified tissue of the human body. Cracks and craze lines are often observed in the enamel, but they rarely cause tooth fracture. The primary objective of this dissertation is to characterize the crack growth resistance of human enamel and develop a mechanistic understanding of crack extension and the fracture toughness. Controlled crack growth under Mode I cyclic and monotonic loads was achieved in unique inset Compact Tension (CT) specimens embodying a section of cuspal enamel. Cracks were grown in the forward (from outer enamel inwards) and reverse (from inner enamel outwards) directions and the responses were compared quantitatively and with the responses of sintered hydroxyapatite (HAp). In addition, a hybrid approach was adopted where experimental measures of crack extension and the near tip displacement field were used as solutions for a finite element model that quantify the contributions from toughening mechanisms to the critical stress intensity for fracture.;Results from the fatigue crack growth evaluation showed that crack growth was more stable in the forward direction and occurred over twice the spatial distance achieved in the reverse direction. The fatigue crack growth exponent (m) for enamel (m = 7.7+/-1.0) was similar to that for the HAp (m = 7.9+/-1.4), whereas the crack growth coefficient (C) for enamel (C=8.7E-04 (mm/cycle)•(MPa•m 0.5)-m) was significantly lower (p<0.0001) than that for HAp (C = 2.0E+00 (mm/cycle)•(MPa•m0.5) -m). Fatigue crack growth in enamel was accompanied by toughening mechanisms such as microcracking, crack bridging, crack deflection and crack bifurcation. These mechanisms of toughening were not observed in the crack growth response of the sintered HAp. Results from the fatigue crack growth evaluation showed the importance of microstructure towards achieving the crack growth resistance exhibited by enamel.;Results from monotonic crack growth experiments showed that enamel undergoes an increase in crack growth resistance (i.e. rising R-curve) with crack extension from the outer to the inner enamel, and that the rise in toughness is function of distance from the Dentin Enamel Junction (DEJ). The outer enamel exhibited the lowest apparent toughness (0.67 +/- 0.12 MPa•m0.5), and the inner enamel promoted a growth toughness with extension from 1.13 MPa•m0.5/mm to 3.93 MPa•m0.5/mm. The maximum crack growth resistance at fracture (i.e. fracture toughness (Kc)) ranged from 1.79 to 2.37 MPa•m0.5. Crack growth in the inner enamel was accompanied by host of mechanisms operating from the micro- to the nano-scale. Acting in concert, these mechanisms promoted more than 300% increase in toughness from initiation to fracture. In comparison, monotonic crack extension in the reverse direction was highly unstable and consumed only a third of the total energy required for extension in the forward direction.;Based on the inverse approach the maximum crack closure stress (CCS) for growth in the forward direction reached approximately 20 MPa and extended over a cohesive zone length ranging between 0.4 to 0.8 mm. When assessed using elastic-plastic fracture mechanics (EPFM), the fracture toughness of enamel was found to be 2.89+/-0.50 MPa•m0.5. The intrinsic mechanisms contributed to approximately 23% of total toughness, whereas the extrinsic mechanisms contributed to more than 50% of the total toughness. Results from the present investigation showed that enamel is primarily an extrinsically toughened tissue and the microstructure of enamel is designed to be most effective at resisting crack extension initiating from damage at the tooth's surface.