The effect of the level of maximum temperature (Tmax), the temperature range (ΔT) and the mode of convective heat transfer on the thermal fatigue resistance of brittle structural materials is analysed. Expressions are derived for the number of thermal cycles to failure in terms of the appropriate mechanical and thermal properties, crack growth parameter, ΔT andTmax. For simultaneous changes inTmax and ΔT commonly used in practice, the change in thermal fatigue life is governed by both the thermal stress intensity exponent (n) and the activation energy (Q) for subcritical crack growth, in contrast to the results of other studies. For constantTmax but variable ΔT, thermal fatigue life is affected byn only, whereas, for constant ΔT but variableTmax, the value ofQ alone governs changes in fatigue-life. Heat transfer by natural or forced convection will result in differences in thermal fatigue resistance. Recommendations are made for the design and analysis of thermal fatigue experiments. Figures-of-merit for the selection of materials with high thermal fatigue resistance are presented.

Analysis of thermal fatigue behaviour of brittle structural materials

The heat transfer characteristics of a fluidized bed used as a quenching medium for the thermal stress testing of brittle ceramics, were determined by measurements of the thermal shock behaviour of rods of a soda-lime—silica glass. The heat transfer coefficient was found to be strongly dependent on the mean particle size of the powder and air flow rate, and was relatively independent of the position within the bed. The results indicate that the heat transfer coefficient during thermal shock fracture may have a value lower than that obtained under heat transfer conditions which more closely resemble steady state. The heat transfer data inferred from the quenching experiments with the glass gave excellent agreement between calculated and measured values for the thermal shock behaviour for rods of a polycrystalline aluminium oxide. It is concluded that fluidized beds are excellent inert quenching media with variable heat transfer coefficient controlled by particle size and flow rate.

Observations on the characteristics of a fluidized bed for the thermal shock testing of brittle ceramics

The aim of this study was to evaluate the healing of self-contained coronal defects on a sand-blasted, large-grit, acid-etched (SLA) surface implant, which had a calcium phosphate (CaP) coating applied by ion-beam-assisted deposition (IBAD). We also evaluated the effect of heating the coating to different temperatures. The CaP-coated SLA implants exhibited a slightly larger bone healing capacity in the self-contained coronal defect than SLA implants, indicating that combining SLA surface implants and a CaP coating by the IBAD method had synergistic effects on bone healing. There was no difference in the healing capacity between 350 °C and 450 °C heat treatment of the coating layer.

https://iopscience.iop.org/article/10.1088/1748-6041/4/4/044107/pdf

Effects of calcium phosphate coating to SLA surface implants by the ion-beam-assisted deposition method on self-contained coronal defect healing in dogs

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 ΔTi1/4, where ΔTi is the instantaneous temperature difference between the specimen and quenching medium, the stresses are appreciably less than those for ΔTi-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 ΔTi was established experimentally. The relevance of this result to the interpretation of thermal quenching studies is discussed.

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

An original method of characterizing thermal fatigue of ceramic materials has been proposed. This method is based on after-shock measurements of the degree of damage through a compliance calibration using compact tension (CT) test pieces. This method has been applied to a fibre-reinforced refractory material subjected to repeated thermal shock between 20 and 800° C. It has been demonstrated from both experiments and finite element analysis that the CT specimen is a convenient shape for the evaluation of thermal fatigue behaviour. In these specimens it has been established that the damage primarily affects the notch tip. The thermal fatigue behaviour of the CT specimens depends on notch length: when the notch length is greater than 30 mm, catastrophic failure occurs after a few cycles. When the notch length is less than 30 mm, the crack formed at the notch tip during the first cycle grows slowly during subsequent cycles. This behaviour has been explained by the variation of the stress intensity factor KI.

A method for evaluating the thermal fatigue resistance of ceramics and its application to a fibre reinforced refractory material

The thermal fatigue behavior of hot pressed silicon nitride

The spring constant of a single crystal spring

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The thermal fatigue behavior of hot pressed silicon nitride

The spring constant of a single crystal spring