Analysis of tempering stresses in bilayered porcelain discs.

Previous studies of opaque-porcelain/body-porcelain discs have shown that compressive stresses which develop in the porcelain surface by being tempered in air can inhibit the sizes of cracks induced within the surface. The objective of this study was to develop a theoretical model for analysis of transient and residual stresses in opaque-porcelain/body-porcelain discs which were produced under variable cooling conditions. The model incorporates the effects of stress and structural relaxation. Transient and residual stresses were calculated for bilayered porcelain discs 16 mm in diameter and 2 mm in thickness for three opaque-porcelain/body-porcelain combinations. Transient temperature distributions in the discs for simulated convective cooling were calculated by finite-element analysis. Data from microhardness indentations reported by Anusavice et al. (1989) indicate that crack lengths measured for bilayered porcelain discs subjected to slow cooling conditions, for which the model predicted residual tensile stresses, were greater than those combinations for which residual compressive stresses were calculated. Calculated values of residual compressive stress for tempered specimens were considerably higher than those for specimens that were slowly cooled and those that were cooled by free convection. In general, residual stress levels calculated by use of the analytical model were in fairly good agreement with the trends observed for crack lengths and bi-axial flexural strengths reported by Anusavice and Hojjatie (1991). The results of the present study indicate that a visco-elastic model is a viable approach for determination of transient and residual stresses in opaque-porcelain/body-porcelain discs.