Journal of Dental Research, Vol 68, 134-138, Copyright © 1989 by International & American Associations for Dental Research Online Journals
Tempering stresses in feldspathic porcelain
P. H. Dehoff and K. J. Anusavice
Department of Mechanical Engineering and Engineering Science, University of North Carolina, Charlotte 28223.
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 degrees C
to 625 degrees 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.
