The Effect of a Starter Notch on the Isothermal and Thermomechanical Fatigue Life of a Nickel-Based Superalloy

The effect that a starter notch has on the life of a Nickelbased superalloy which is used in the high temperature regions of aeroengines has been considered for this study under both isothermal fatigue (IF) and thermomechanical fatigue (TMF) conditions. The components made from this material in these regions are usually subjected to cyclic mechanical and thermal loading and hence it is necessary that this form of loading does not lead to cracking and hence crack growth leading to failure. It is therefore important to build up an understanding of the effect that these loading conditions have on the crack growth and therefore failure life of such materials. Such TMF conditions usually incorporate high temperatures which permit creep to play a significant role in the evolution of deformation and damage of the material with time. An experimental programme of TMF crack growth testing for a Nickel-based superalloy, has been carried out using an Instron TMF test machine with induction coil heating using uniaxial as well as initially corner notched (CN) specimens. A variation of conditions in terms of strain and temperature range(s) have been studied in order to build up life curves as a function of these conditions. TMF test results have also been compared to corresponding isothermal data, in order to investigate the effect that temperature cycling has on the material response. Details of the experimental set-up, as well as the experimental results are presented. Future application of the test data for component life prediction has been addressed.