The fracture of restored teeth is a significant obstacle to lifelong oral health. Recent studies suggest that fatigue cracks originate at flaws introduced during cavity preparation and that fatigue crack growth is a principle cause of restored tooth fractures. In this study, the rate of fatigue crack growth in bovine dentin was estimated under mode I cyclic loading. Double cantilever beam (DCB) specimens were obtained from bovine molars and subjected to high cycle fatigue loading (10(5) < N < 10(6)). The fatigue crack growth rates were measured and used to estimate the crack growth exponent and coefficient according to the Paris Law. The average fatigue crack growth exponent was 4.7 +/- 0.6 for crack growth parallel to the dentin tubules, which was significantly larger than 4.3 +/- 0.5 for crack growth perpendicular to the tubules (t-test, CI > 80%). Although the crack growth rates varied considerably, there was no significant dependence on tubule orientation or tubule density. However, specific features of the fracture surfaces and tendencies for crack curving away from the tubules suggested preferential fatigue crack growth perpendicular to the dentin tubules. Results from this study are being used to guide an experimental investigation of fatigue crack growth in human dentin.

An experimental study of fatigue crack growth in dentin was conducted, and the influence of stress ratio (R) on the crack growth rate and mechanisms of cyclic extension were examined. Double Cantilever Beam (DCB) fatigue specimens were sectioned from bovine molars and then subjected to high cycle fatigue loading (10 5 Ͻ N Ͻ 10 6 ) under hydrated conditions. The evaluation consisted of Mode I loads with stress ratios that ranged from Ϫ0.5 to 0.5. The fatigue crack growth rates were measured and used to estimate the crack growth exponent (m) and coefficient (C) according to the Paris Law model. The fatigue crack growth rates for steady-state extension (Region II) ranged from 1E-7 to 1E-4 mm/cycle. It was found that the rate of cyclic extension increased significantly with increasing R, and that the highest average crack growth rate occurred at a stress ratio of 0.5. However, the crack growth exponent decreased with increasing R from an average of 4.6 (R ϭ 0.10) to 2.7 (R ϭ 0.50). The stress intensity threshold for crack growth decreased with increasing R as well. Results from this study suggest that an increase in the cyclic stress ratio facilitates fatigue crack growth in dentin and increases the rate of cyclic extension, both of which are critical concerns in minimizing tooth fractures and maintaining lifelong oral health.

ABSTRACT-In this paper, the fatigue and fracture properties of bovine dentin are evaluated using in vitro experimental analyses. Double cantilever beam (DCB) specimens were prepared from bovine maxillary molars and subjected to zeroto-tension cyclic loads. The fatigue crack growth rate was evaluated as a function of the dentin tubule orientation using the Paris law. Wedge-loaded DCB specimens were also prepared and subjected to monotonic opening loads. Moiré interferometry was used to acquire the in-plane displacement field during stable crack growth, and the instantaneous wedge load and crack length were acquired to evaluate the crack growth resistance and crack tip opening displacement (CTOD) with crack extension. The rate of fatigue crack growth was generally larger for crack propagation occurring perpendicular to the dentin tubules. The Moiré fringe fields documented during monotonic crack growth exhibited non-linear deformation occurring within a confined region adjacent to the crack tip. Both the wedge load and CTOD response provided evidence that a fracture process zone contributes to energy dissipation during crack extension and that dentin exhibits a rising R-curve behavior. Results from this preliminary investigation are being used as a guide for an evaluation of the fatigue and fracture properties of human dentin.

ABSTRACT-In this paper, the fatigue and fracture properties of bovine dentin are evaluated using in vitro experimental analyses. Double cantilever beam (DCB) specimens were prepared from bovine maxillary molars and subjected to zeroto-tension cyclic loads. The fatigue crack growth rate was evaluated as a function of the dentin tubule orientation using the Paris law. Wedge-loaded DCB specimens were also prepared and subjected to monotonic opening loads. Moiré interferometry was used to acquire the in-plane displacement field during stable crack growth, and the instantaneous wedge load and crack length were acquired to evaluate the crack growth resistance and crack tip opening displacement (CTOD) with crack extension. The rate of fatigue crack growth was generally larger for crack propagation occurring perpendicular to the dentin tubules. The Moiré fringe fields documented during monotonic crack growth exhibited non-linear deformation occurring within a confined region adjacent to the crack tip. Both the wedge load and CTOD response provided evidence that a fracture process zone contributes to energy dissipation during crack extension and that dentin exhibits a rising R-curve behavior. Results from this preliminary investigation are being used as a guide for an evaluation of the fatigue and fracture properties of human dentin.