Almost 23 million crowns and 4 million bridges are delivered to American patients annually, most of which are partly or completely made of porcelain or other brittle ceramics. Unfortunately, failure due to porcelain fracture necessitates the replacement of many restorations after only short periods of service, and currently limits the application of all-ceramic restorations. Increased use of all-ceramic restorations is desirable due to superior biocompatibility and more pleasing esthetics, but high clinical failure rates due to catastrophic fracture limit their use. The goal of this proposal study is to define and characterize the failure mechanism of dental porcelain using a fracture mechanics model system. Laboratory testing of dental porcelains and ceramics has used static single-load test methods in the past. These static tests determine the maximum stresses that can be tolerated, but are not valid for predicting failure in clinical conditions where prostheses are usually subjected to many subcritical load cycles before failure occurs. Dental ceramics and glasses undergo chemical static fatigue in the presence of moisture at ambient temperatures, but it has previously been assumed that these materials do not undergo mechanical fatigue. However, White (1993) used a fracture mechanics approach with a sharp indentation technique to demonstrate the susceptibility of a dental porcelain to mechanical fatigue, but was unable to characterize the phenomenon using that sharp indentation technique. Therefore, the susceptibilities of dental ceramics of mechanical fatigue, and the possible interaction between cyclic mechanical and static chemical fatigue must be investigated to identify failure mechanisms responsible for clinical failure. A fracture mechanics approach using a new indentation technique is suited to the fatigue characterization of brittle coarse grained materials, like dental porcelains. Materials are indented multiple times and microscopically examined of tested further. Experiments will test the hypotheses that: (1) dental porcelain is susceptible to mechanical fatigue, (2) mechanical fatigue, chemical fatigue, and their interaction decrease the strength of dental porcelain, (3) all fatigue damage is cumulative, (4) cracks may be initiated by a critical pressure, much smaller than the pressure necessary for static failure, (5) the rate of crack propagation varies with respect to pressure changes during load- unloading cycles. The results will define and quantify failure mechanisms. This knowledge of failure mechanisms is essential to identify the best materials, processing, and strengthening techniques, and is critical to the development of new materials for in vivo applications. Better ceramic materials will substantially decrease patient suffering and health care costs by reducing the replacement of fractured prostheses, and will allow greater application of all-ceramic prostheses.