Multiaxial fatigue of forged Inconel 718 at room and high temperature

Abstract

This work investigates the effects of high temperature on the fatigue behavior of forged Inconel 718 under axial/torsional loading conditions. To this end, strain-controlled tension, torsion, and proportional fatigue tests were performed at 20 °C and 450 °C on thin-walled tubular specimens extracted from an as-forged turbine disk. All tests were conducted with axial and shear strain ratios equal to zero, and the applied equivalent von Mises strain amplitude was such to result in fatigue lives within the range of 104 to 106 cycles, which is the typical fatigue life of the component of interest in this study. It was observed that for a given loading condition and prescribed equivalent strain amplitude, the fatigue lives of tests at 450 °C were longer than the room temperature tests. Moreover, the longest lives were observed in the torsion tests, followed by the proportional and tension tests in all temperature and prescribed equivalent strain amplitude tested. The capability of the cyclic plasticity model proposed by Chaboche in describing the cyclic response of Inconel 718 was also addressed. This model successfully simulated the stress–strain behavior of this material under all loading conditions, and for both temperatures investigated. In addition, the cracking behavior of the material was examined. For both temperatures, the tension tests failed by Mode I tensile cracks, while Mode II shear cracks were observed in the torsion and proportional tests. After a scanning electron microscope analysis, it was concluded that the transgranular propagation mode was the predominant failure mechanism of all tested conditions. The fatigue life of Inconel 718 was predicted by the Fatemi–Socie (FS) and the multiaxial version of the Smith–Watson–Topper (SWT) fatigue models, resulting in estimations within a factor-of-two and a factor-of-four, respectively. Moreover, crack directions were also determined by the critical plane models and compared with the orientation of the observed cracks. The SWT model correctly predicted the crack direction of the tension tests, while the FS model was more accurate in determining the orientation of the cracks seen in the torsion tests. None of the two models was able to correctly predict the angles of the critical plane in all tested loading conditions.