On the Fracture Toughness and R-Curve Behavior of Human Enamel

摘要

For decades the fracture toughness of human enamel has been quantified using indentation techniques. But due to limitations of this approach the R-curve behavior and nature of changes in toughness with crack extension have never been examined. In this study the fracture resistance of enamel was characterized as a function of distance from the DEJ using both the indentation approach and standard incremental crack growth resistance measurements. Sections of enamel were obtained from the molars of "young" (17<=age<=25) patients and prepared for indentation measurements or used to prepare a specially designed compact tension (CT) specimen. Results from the indentation measurements showed no significant difference between the apparent toughness of the outer and the inner enamel with the average being 0.83+-0.09 MPa(centre dot)m~(0.5). In contrast, results from the crack growth resistance experiments showed that enamel exhibits a marked rise in crack growth resistance with crack extension. Based on the distinct toughening behavior the responses were quantified in terms of the "initiation" (K_(o)), "growth" (K_(g)) and "plateau" (i.e. fracture) toughness (K_(c)). The Ko for enamel was 0.68 MPa(centre dot)m~(0.5) with values of K_(g) and K_(c) being 2.62 MPa(centre dot)m~(0.5)/mm and 2.07 MPa(centre dot)m~(0.5), respectively. The K_(o) and K_(c) were obtained at the outer and inner enamel, respectively, and were significantly different. It was found that indentation measurements are unable to quantify the important contributions of the microstructure to the toughening behavior of enamel.