Lifetime predictions for resin-based composites using cyclic and dynamic fatigue. 
Because dental restorative materials undergo fatigue in use, testing is often performed in the laboratory to evaluate material responses to cyclic loading.  The purpose of this study was to compare the lifetime predictions resulting from two methods of fatigue testing: dynamic and cyclic fatigue.  Model composites were made in which one variable was the presence of a silanizing agent, and specimens tested in 4-point flexure.  Cyclic fatigue was carried out at a frequency of 5 Hz, while dynamic fatigue testing spanned seven decades of stress rate application.  Data were reduced and the crack propagation parameters for each material were calculated from both sets of fatigue data.  These parameters were then used to calculate an equivalent static tensile stress for a 5-year survival time.  The 5-year survival stresses predicted by dynamic fatigue data were approximately twice those predicted by cyclic fatigue data.  In the absence of filler particle silanization, the survival stress was reduced by half.  Aging in a water-ethanol solution reduced the survival stresses by a factor of four to five.  Cyclic fatigue is a more conservative means of predicting lifetimes of resin-based composites.