Temperature evolution and life prediction in fatigue of superalloys
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Temperature Evolution and Life Prediction in Fatigue of Superalloys L. JIANG, H. WANG, P.K. LIAW, C.R. BROOKS, L. CHEN, and D.L. KLARSTROM Low-cycle fatigue behavior of two superalloys, ULTIMET® alloy, Co-26 pct Cr-9 pct Ni (wt pct), and HAYNES® HR-120® alloy, Ni-33 pct Fe-25 pct Cr, was studied at room temperature. An infrared thermography system was employed to monitor the temperature evolution of fatigue processes for both superalloys. Temperature changes during fatigue were related to the hysteresis effect, and were successfully predicted, based on the consideration of the hysteresis effect and heat conduction. The temperature increase of a specimen from the initial to the equilibrium stages was used as an index to predict the fatigue life of the two superalloys. It was found that the fatigue-life predictions using the present model were in good agreement with the experimental results.
I. INTRODUCTION
FATIGUE has long been recognized as one of the major causes for the rupture of materials and catastrophic damage in components or even entire systems. Investigations of the fatigue process in its intricacy and complexity have been carried out for more than 160 years. A diversity of theories, ranging from constitutive models, micromechanics, damage mechanics, to empirical solutions, were developed to predict constitutive behavior and fatigue life of materials. However, many issues regarding fatigue mechanisms and phenomena remain unresolved. The difficulties in studying fatigue result from (a) many internal and external factors, such as material properties, microstructures, loading conditions, geometry, etc., which affect the fatigue behavior; (b) interdependency of these factors; and (c) simplifications and assumptions that have to be made in order to understand the fatigue process.[1,2] The strain-energy-based approach for fatigue-life prediction seems promising, since the low-cycle fatigue of materials is a complicated process involving the gradual accumulation of damage, which is mainly controlled by the amplitude of the plastic-strain component.[2] It is well known that stored energy is generally only a small portion of the energy dissipated during fatigue.[3–5] Even during the fatigue-propagation process, the stored energy, which is the integration of the increase of the internal energy over all the elements in the plastic zone, is very small, compared with the dissipation of mechanical energy, which can be measured from the stress-strain hysteresis loops.[4,5] At high cyclic stresses, the plastic strain is the pre-
dominant cause of energy dissipation. Because the dissipated strain energy is intimately related to the fatigue process, various theoretical approaches have been attempted to predict the fatigue life of materials, based on the plastic-strain energy as a criterion of fatigue damage. In materials undergoing cyclic loading, most of the dissipated energy due to the hysteresis effects manifests itself as heat, and the heat is removed from the material by conductive, convec
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