Thermal gradients in round TMF specimens

The effect of radial thermal gradients in thermomechanical fatigue (TMF) specimens on the number of cycles to failure has been studied. Different radial gradients were obtained by asymmetric placement of the specimen in the coil. And by using different induction heating frequencies. The different induction frequencies give different depths of penetration for the heat. A special thermal gradient specimen was used to assess the gradients through the thickness of the specimen. Measurements performed with different induction heating coil set-ups and different heating/cooling rates show that the internal temperature varies from the external temperature. The minimum difference measured was to about ±10 °C for a specimen cycled using a 400 kHz coil and a heating rate of 2 °C/s and the maximum ±40 °C using a 10 °C/s heating rate. TMF out of phase tests were performed with three different thermal gradients. Specimens tested with high heating rates exhibit more significant compressive creep compared to lower heating rates. The specimens exhibit a stress range that is 10% higher when using a coil with heating frequency of 66 kHz compared to 400 kHz.