Tempering Stresses in Feldspathic Porcelain

Abstract

The objective of this study was to develop an analytical model to calculate transient and residual (tempering) stresses in dental porcelain plates subjected to cooling rates used by commercial laboratories. The model incorporates linear viscoelasticity and structural relaxation effects.

The viscosities of three experimental body porcelains and three experimental opaque porcelains as a function of temperature were calculated from creep rates measured in a bending beam viscometer. Measurements were made under thermal equilibrium conditions for temperatures ranging from 550°C to 625°C. Thermal expansion data measured in a differential dilatometer at slow heating rates were supplied by the manufacturer. Temperature distribution in the plates as a function of convective heat transfer coefficient, initial plate temperature, and plate thickness was calculated by use of standard numerical techniques.

Calculations of transient and residual stress were performed for one body porcelain, for two plate thicknesses, and for three variable cooling rates. Calculated surface residual stresses were strongly dependent on plate thickness, cooling rate, and initial soak temperature. For the cases studied, the maximum residual surface compressive stress was 26.4 MPa.

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References

Anusavice, K.J. and Lee, R.B. (1985): Effect of Firing Temperature and Water Exposure on Crack Propagation in Unglazed Porcelain, J Dent Res 64:296, Abst. No. 1092.

Bertolotti, R.L. and Shelby, J.E. (1979): Viscosity of Dental Porcelain as a Function of Temperature, J Dent Res 58: 2001-2004.

Gardon, R. (1980): Thermal Tempering of Glass. In: Class: Science and Technology, D.R. Uhlmann and N.J. Kreidl , Eds., New York, NY: Academic Press, pp. 145-216.

Hagy, G.E. (1963): Experimental Evaluation of Beam Bending Method of Determining Glass Viscosities in the Range 108 to 1015 Poises, J Am Ceram Soc 46: 93-97.

Kreith, F. (1973): Principles of Heat Transfer, 3rd ed.. New York, NY: Intext Educational Publishers, pp. 161-165.

Lee, E.H. ; Rogers, T.G. ; and Woo, T.C. (1965): Residual Stresses in a Glass Plate Cooled Symmetrically from Both Surfaces, J Am Ceram Soc 48: 480-487.

Muki, R. and Sternberg, E. (1961): Transient Thermal Stresses in Viscoelastic Materials with Temperature-dependent Properties, J Appl Mech 28: 193-207.

Narayanaswamy, O.S. (1971): A Model of Structural Relaxation in Class, J Am Ceram Soc 54: 491-498.

Narayanaswamy, O.S. (1978): Stress and Structural Relaxation in Tempering Glass, J Am Ceram Soc 61: 146-152.

10.

Narayanaswamy, O.S. and Gardon, R. (1969): Calculations of Residual Stress in Glass, J Am Ceram Soc 52: 554-558.

11.

Rekhson, S.M. and Mazurin, O.V. (1974): Stress and Structural Relaxation in Na2O-CaO-SiO2 Glass, J Am Ceram Soc 57: 327-328.

12.

Tool, A.Q. (1946): Relation Between Inelastic Deformation and Thermal Expansion of Glass in its Annealing Range, J Am Ceram Soc 29: 240-253.

13.

Twiggs, S.W. ; Anusavice, K.J. ; and Fairhurst, C.W. (1980): Effect of Cooling Rate on Porcelain-Alloy Compatibility , AADR Progr & Abst 59: No. 329.