Mechanistic aspects of in vitro fatigue-crack growth in dentin

Although the propagation of fatigue cracks has been recognized as a problem of clinical significance in dentin, there have been few fracture mechanics-based studies that have investigated this issue. In the present study, in vitro cyclic fatigue experiments were conducted over a range of cyclic frequencies (1-50 Hz) on elephant dentin in order to quantify fatigue-crack growth behavior from the perspective of understanding the mechanism of fatigue in dentin. Specifically, results obtained for crack extension rates along a direction parallel to the dentinal tubules were found to be well described by the stress-intensity range, DeltaK, using a simple Paris power-law approach with exponents ranging from 12 to 32. Furthermore, a frequency dependence was observed for the crack-growth rates, with higher growth rates associated with lower frequencies. By using crack-growth experiments involving alternate cyclic and static loading, such fatigue-crack propagation was mechanistically determined to be the result of a "true" cyclic fatigue mechanism, and not simply a succession of static fracture events. Furthermore, based on the observed frequency dependence of fatigue-crack growth in dentin and observations of time-dependent crack blunting, a cyclic fatigue mechanism involving crack-tip blunting and re-sharpening is proposed. These results are deemed to be of importance for an improved understanding of fatigue-related failures in teeth.