Dental ceramics have become increasingly prolific as restorative materials because of their esthetic appearance, wear resistance, thermal insulation, and biocompatibility. Unfortunately, dental ceramics are brittle in comparison to dental alloys. This lack of fracture resistance compromises their strength and reliability, resulting in decreased lifetime expectancy. The magnitude of occlusal load rarely exceeds 324 N in humans, and finite element models have predicted these loads to induce stresses of 22 to 87 MPa in molar crowns. Traditional dental ceramics can withstand 11.6 MPa (aluminous porcelains) to 240 MPa (Dicor) of stress before single-cycle failure. In spite of this margin of safety, failure rates as high at 30% (Dicor) after seven years in vivo have been reported. Cyclic fatigue loading clearly has an impact on the maximum stress that can be withstood, and fatigue failure is rightfully a concern of dental ceramic researchers. Fatigue failure of ceramics occurs when initial flaws reach a critical stress intensity following subcritical crack growth (SCG). The overall goals of this project are to develop easier methods of measuring SCG for dental ceramics and to develop more accurate methods for predicting all-ceramic prothesis lifetimes through the following specific aims: (1) To develop a method for constructing accurate S-N-P (stress-lifetime-failure probability) models for dental ceramics from cyclic fatigue measurements on a minimum number of standard geometry specimens. (2) To develop a method for determining the individual contributions of stress corrosion and cyclic degradation to SCG for a given ceramic and stress function. (3) To verify the accuracy of prosthesis lifetimes predicted using finite element software in conjunction with time-dependent reliability software. (4) To determine how the distribution of fatigue strength relates to the distribution of single-cycle strength. (5) To determine whether crack healing through surface remodeling affects fatigue lifetime.
Specific Aims 1 -4 will investigate two dental ceramics: one that exhibits constant fracture toughness and one that exhibits dependence of fracture toughness on flaw size (R-curve behavior). A porcelain that exhibits crack healing behavior will be analyzed in Specific Aim 5.
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