title: Step-stress analysis for predicting dental ceramic reliability.
abstract: OBJECTIVE
  To test the hypothesis that step-stress analysis is effective to predict the reliability of an alumina-based dental ceramic (VITA In-Ceram AL blocks) subjected to a mechanical aging test.
  
  
  METHODS
  Bar-shaped ceramic specimens were fabricated, polished to 1μm finish and divided into 3 groups (n=10): (1) step-stress accelerating test; (2) flexural strength-control; (3) flexural strength-mechanical aging. Specimens from group 1 were tested in an electromagnetic actuator (MTS Evolution) using a three-point flexure fixture (frequency: 2Hz; R=0.1) in 37°C water bath. Each specimen was subjected to an individual stress profile, and the number of cycles to failure was recorded. A cumulative damage model with an inverse power law lifetime-stress relation and Weibull lifetime distribution were used to fit the fatigue data. The data were used to predict the stress level and number of cycles for mechanical aging (group 3). Groups 2 and 3 were tested for three-point flexural strength (σ) in a universal testing machine with 1.0MPa/s stress rate, in 37°C water. Data were statistically analyzed using Mann-Whitney Rank Sum test.
  
  
  RESULTS
  Step-stress data analysis showed that the profile most likely to weaken the specimens without causing fracture during aging (95% CI: 0-14% failures) was: 80MPa stress amplitude and 10(5) cycles. The median σ values (MPa) for groups 2 (493±54) and 3 (423±103) were statistically different (p=0.009).
  
  
  CONCLUSIONS
  The aging profile determined by step-stress analysis was effective to reduce alumina ceramic strength as predicted by the reliability estimate, confirming the study hypothesis.
authors.affiliation: Post-graduate Program in Dentistry, Dental School, University of Passo Fundo, Passo Fundo, RS, Brazil. marciaborb@bol.com.br
authors.forename: Márcia
authors.initials: M
authors.lastname: Borba
authors.forename: Paulo F
authors.initials: PF
authors.lastname: Cesar
authors.forename: Jason A
authors.initials: JA
authors.lastname: Griggs
authors.forename: Alvaro
authors.initials: A
authors.lastname: Della Bona
mesh_terms: D000465:Algorithms; D001732:Bite Force; D003775:Dental Polishing; D003776:Dental Porcelain / Q000737:chemistry*; D003799:Dental Stress Analysis / Q000295:instrumentation / Q000706:statistics & numerical data; D005544:Forecasting; D006801:Humans; D008422:Materials Testing / Q000706:statistics & numerical data; D008855:Microscopy, Electron, Scanning; D018583:Pliability; D015203:Reproducibility of Results; D013314:Stress, Mechanical; D013499:Surface Properties; D013696:Temperature; D013997:Time Factors; D014867:Water / Q000737:chemistry
chemical_list: C483026:In-Ceram Alumina; D014867:Water; D003776:Dental Porcelain
keywords: Aging; Ceramics; Fatigue