Effect of ΔT‐ and Spatially Varying Heat Transfer Coefficient on Thermal Stress Resistance of Brittle Ceramics Measured by the Quenching Method

A study was conducted of the effect of a spatial variation in heat transfer coefficient during forced convection and the effect of temperature dependence of the heat transfer coefficient during free convection on the magnitude of thermal stress encountered during the thermal shock testing of brittle ceramics by the quenching method. For specimens with circular geometry, the results obtained by the finite element method indicate that, depending on the value of the Biot number, a factor of three spatial variation in heat transfer coefficient changes the tensile thermal stresses on cooling by a maximum of about 17% over the value for spatially uniform heat transfer. On heating, the corresponding tensile thermal stresses are lower by a maximum of about 6%. In free convection for a heat transfer coefficient, h, proportional to ΔTi^1/4, where ΔT_i is the instantaneous temperature difference between the specimen and quenching medium, the stresses are appreciably less than those for ΔT_i‐independent heat transfer for the same initial value of h at the onset of the thermal quench. The validity of the dependence of h on ΔT_i was established experimentally. The relevance of this result to the interpretation of thermal quenching studies is discussed.