Evaluation of sub-critical fatigue crack propagation in a restorative composite

Significance: A method is presented to determine slow crack propagation using a four-point bending test. Although fatigue lifetime varied widely, stable crack growth prior to fracture was consistent. This consistency allowed formulation of stress-based crack propagation relationships that can be used in concert with numerical simulations to predict composite restoration performance. The large variation found for specimen lifetime was attributed to the initiation process that precedes sub-critical crack propagation.