title: Piston-on-three-ball versus piston-on-ring in evaluating the biaxial strength of dental ceramics.
abstract: OBJECTIVE
  Piston-on-three-ball tests have been selected by the International Organization for Standardization to establish ISO 6872 for the evaluation of the biaxial strength of dentistry-ceramic materials. However, the formula adopted in ISO 6872 for the fracture load-biaxial strength relationship was an approximate equation originally derived for piston-on-ring tests of monolayered discs. This formula was modified and extended to the case of multilayered discs subjected to piston-on-ring loadings recently. The purpose of the present study is to evaluate the adequacy of applying the formula for piston-on-ring to piston-on-three-ball tests for both monolayered and multilayered discs.
  
  
  METHODS
  Finite element analyses were performed to simulate both piston-on-three-ball and piston-on-ring tests. Different degrees of friction between the specimen supporting surface and the loading fixture were considered in the simulations. The simulated biaxial stress distributions through the disc thickness for both tests were then compared to the formula to examine how the predictions agree.
  
  
  RESULTS
  Examples of monolayered, bilayered and trilayered discs subjected to piston-on-three-ball and piston-on-ring loadings were simulated for comparison with the formulae. The results depended on friction when the disc was supported by a ring, however the results became insensitive to friction when the disc was supported by three balls. For the frictionless contact, both loading tests yield almost the identical biaxial stress distribution through the disc thickness and agree well with the formula. On the other hand, the maximum tensile stress on the surface of the disc decreased when the friction increased. As a result the biaxial tensile strength was overestimated for piston-on-ring tests if friction was neglected.
  
  
  CONCLUSIONS
  The results of our finite element analyses demonstrate how the friction between the specimen supporting surface and the loading fixture affects the biaxial strength evaluation in piston-on-ring and piston-on-three-ball tests. It is critical to have frictionless contact between the disc and the supporting ring when evaluating the biaxial strength by piston-on-ring tests. Otherwise, the application of the approximate formula, which was derived for piston-on-ring, to piston-on-three-ball is even better than piston-on-ring. This is true for not only monolayered discs but also multilayered discs.
authors: Department of Civil Engineering, Chung Yuan Christian University, Chung Li, Taiwan. cwhuang@cycu.edu.tw
authors: C W
authors: CW
authors: Huang
authors: C H
authors: CH
authors: Hsueh
mesh_terms: D000465:Algorithms; D000538:Aluminum Silicates / Q000737:chemistry; D002516:Ceramics / Q000737:chemistry; D003198:Computer Simulation; D003776:Dental Porcelain / Q000737:chemistry*; D055119:Elastic Modulus; D020342:Finite Element Analysis; D005544:Forecasting; D017276:Friction; D006801:Humans; D018020:Lithium Compounds / Q000737:chemistry; D008422:Materials Testing / Q000295:instrumentation* / Q000379:methods / Q000592:standards; D018583:Pliability; D013314:Stress, Mechanical; D013499:Surface Properties; D013718:Tensile Strength; D015040:Zirconium / Q000737:chemistry
chemical_list: D000538:Aluminum Silicates; C085095:Cerinate; C118557:Empress 2; C075815:IPS-Empress ceramic; D018020:Lithium Compounds; D003776:Dental Porcelain; C023506:Glass ceramics; D015040:Zirconium; C028541:zirconium oxide