A model is constructed for prediction of fatigue crack initiation based on the material’s microstructure. This approach allows us to take a quantifiable description of the microstructure and predict fatigue life and scatter properties, which can be used to design materials with optimum life. In the material of interest, a nickel-based superalloy, the crack initiation mechanism is strain localization via formation of persistent slip bands (PSBs). In the model, we calculate the energy of each PSB, which evolves with increasing number of loading cycles and search for a minimum energy configuration as our failure criterion for fatigue crack initiation. The strength of the individual barriers to slip is calculated from atomistic simulations of dislocations transmitting through and nucleating from distinct types of grain boundaries (GBs) or shearing precipitates. These energy barriers along with dislocations slipping through the stress field within a PSB (modeled on the continuum scale) are incorporated in the calculation of the total energy of the PSB, which inherently accounts for the local microstructure of the material. Through this methodology, the fatigue life is predicted based on EBSD scans of the material, and the fatigue scatter is predicted by using various realizations of the materials microstructure (created by redistributing the spatial location of the grains). Excellent agreement is shown between the model predictions and experimental data. Of equal importance, the model predicts the microstructural features that are most likely to initiate cracks, i.e. a combination of preferential orientation, single large grains, clusters of grains connected by low-angle GBs, and PSBs impinging upon twins.
Project Sponsors: Rolls-Royce Corporation
Collaborators: Huseyin Sehitoglu (University of Illinois, Urbana-Champaign); Hans J. Maier (University of Paderborn, Germany); David U. Furrer, Michael G. Glavicic, Jeff D. Stillinger (Rolls-Royce Corporation)
Sangid MD, Sehitoglu H, Maier HJ, Furrer DU, Glavicic MG, Stillinger JD, “Role of microstructure in prediction fatigue performance,” AIAA Journal, American Institute of Aeronautics and Astronautics, Honolulu, Hawaii, April 23-26, 2012.