Piston-on-three-ball versus piston-on-ring in evaluating the biaxial strength of dental ceramics
Resource
Dental Materials, 27(6), e117-e123
Journal
Dental Materials
Pages
e117-e123
Date Issued
2011
Date
2011
Author(s)
Huang, C.W.
Abstract
Objectives: 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. Significance: 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. © 2011 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
Type
journal article
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