Relating fatigue strain accumulation to microstructure using digital image correlation

While fatigue has been studied extensively, fatigue life predictions are still phenomenological and relatively simple {e.g. Miner's rule, Paris relationship, etc.). Many improvements have been suggested to such models to incorporate newly discovered phenomena, but fatigue life predictions still have limited accuracy and scope. A quantitative understanding of fatigue at the grain level would lead to models with better predictive capability and/or broader applicability. In this work, fatigue crack growth in Hastelloy X, a high-temperature nickel based alloy, was examined using an ex-situ digital image correlation technique. Electron backscatter diffraction (EBSD) was performed on a region of interest in front of the crack tip in a single edge notched tension specimen to obtain microstructural characteristics. The specimen was then fatigue loaded to advance the crack. At regular intervals of crack growth, the specimen was removed from the load frame and the region of interest was imaged with an optical microscope. By performing digital image correlation on these images, a full-field measure of the accumulated plastic strain was obtained as the crack approached and passed through the region of interest. Strain fields were compared to EBSD results to elucidate the relationship between microstructure and fatigue crack growth. The presence of strain concentrations at grain and (annealing) twin boundaries was seen to be instrumental in the evolution of plastic strain accumulation during fatigue.