title: Evaluation of sub-critical fatigue crack propagation in a restorative composite.
abstract: OBJECTIVE
  Fracture is a major reason for clinical failure of dental restorations. Many clinical fractures are likely to be preceded by a slow sub-critical crack propagation. The purpose of this study was to determine slow crack propagation in a restorative composite.
  
  
  METHODS
  Notched composite (Z100, 3M ESPE) specimens were fatigued in a four-point bending test. The notch (1 mm) was created by embedding a sharpened razor blade in the specimen mold. The specimens were load-cycled at 5 Hz between -5 and -30 N until failure. Displacement and load were recorded during the fatigue tests and used to derive crack propagation based on beam-compliance.
  
  
  RESULTS
  The number of cycles until failure ranged between 34 and 82,481. In the last 1500 cycles prior to final fracture, the beam compliance increased consistently, indicating sub-critical crack propagation. It was calculated from the compliance change that the crack length increased 8% (77 +/- 14 microm) before final failure. The crack growth rate during sub-critical crack propagation was determined as a function of the stress intensity for the last 1500 cycles before fracture.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Oral Science, Minnesota Dental Research Center for Biomaterials and Biomechanics, University of Minnesota, 16-212 Moos Tower, 515 Delaware Street SE, Minneapolis, MN 55455, USA.
authors.forename: Galyna M
authors.initials: GM
authors.lastname: Loughran
authors.forename: Antheunis
authors.initials: A
authors.lastname: Versluis
authors.forename: William H
authors.initials: WH
authors.lastname: Douglas
mesh_terms: D003187:Compliance; D003188:Composite Resins; D019232:Dental Restoration Failure; D003793:Dental Restoration, Permanent; D003799:Dental Stress Analysis / Q000379:methods*; D008422:Materials Testing; D018583:Pliability; D012822:Silicon Dioxide; D015040:Zirconium
chemical_list: D003188:Composite Resins; C083568:Z100 composite resin; D012822:Silicon Dioxide; D015040:Zirconium