A New Method for Studying Fractures of Porcelain Enameled Specimens*

Abstract

Journal of The American Ceramic Society-Petersen, Jones, and Allen relaxation develops through the viscous flow of the low-temperature component. However, in the beginning of such a heat-treatment, the strains increase temporarily because of the comparatively rapid shrinkage of the undercooled low-temperature component as it approaches equilibrium a t the treating temperature. This effect merely delays the detection of the relaxation which ultimately allows the high temperature component and the whole mass of the glass to expand accordingly. This expansion together with the shrinkage of the low-temperature component appreciably increases the intermolecular space. These effects together with the changes which heat-treatment causes in the expansivity are responsible for the extraordinary density variations that are peculiar to some Pyrex-brand glasses when they are heat-treated.Previously, an empirical equation was developed to express the density-time relation as the density of ordinary glasses is altered by annealing treatments a t constant temperatures. Because of the rather unusual expansion effects observed in two-component glasses, this equation is obviously inadequate in the case of these glasses. However, it is shown that the equation has a limited applicability even under such circumstances.The equation was designed to take into account the change in viscosity as a glass advances toward equilibrium while at a constant annealing temperature. For this reason, presumably, it generally yields results that are much more satisfactory than those obtained when the logarithmic equation is applied to the same experimental data. The first equation has been found to be superior not only when applied to density data, as in this investigation, but also when applied to the changes that occur in the refractivity when a glass is annealed a t a constant temperature. Moreover, when both equations are modified so that they apply to the relaxation of strains in an annealing glass, the same superiority for the first equation has been found. As the logarithmic equation is derived from the assumption that the viscosity of a glass remains constant as long as the temperature is constant, experience in applying these equations t o various data suggests that the change in viscosity as glass advances toward equilibrium at any annealing temperature is an important factor in determining the rate of that advance and also in determining the rate a t which strains relax. ABSTRACT This method shows great promise in the study of different types of fractures of porcelain enameled specimens. An analysis of the stress conditions in the enamel-iron-enamel system before, during, and after thermal shock is also included.A process is described which locates microscopic cracks in an enamel layer.

Microstructure of ceramic materials

“…Important results with decoration by sodium vapor to reveal cracks in glasses [112,113] , decoration of sodium chloride with gold particles [llh] resulting from the decomposition of gold chloride, decoration of magnetic materials with Fe 2 02and the use of small charged particles in crack location and polarity determinations [115,116] all suggest that decoration is well worth exploring further…”