Perspectives on strength

Strength values are often relied upon as indicators of structural performance for brittle dental materials. Strength, however, is more of a “conditional” than an inherent material property, and strength data alone cannot be directly extrapolated to predict structural performance. Strength data are meaningful when placed into context via knowledge of material microstructure, processing history, testing methodology, testing environment and failure mechanism(s). Structural failure is determined by additional failure probability variables (in concert with strength) that describe stress distributions, flaw size distributions, and that can account for either single or multiple failure modes. Lifetime predictions require additional information about the time dependence of slow crack growth. Basic fracture mechanics principles and Weibull failure modeling will be reviewed for the perspective they provide in understanding strength and the data obtained using various laboratory tests. Examples will be given to demonstrate how failed specimens can provide crucial information to either validate or question the failure mechanisms invoked during laboratory testing. The role of interfacial stresses is discussed as applied to dental structures of current interest. Overall, it is emphasized that an understanding of actual clinical failure modes is absolutely necessary before results of in vitro strength testing can be considered to have clinical validity.