Investigation of Fatigue Crack Bifurcation in AA7050 AL Alloys

The aims of this project are develop a methodology to investigate the fatigue crack deviation behaviors in high strength AA7000 series Al alloy plates. The microstructural and textural effects, as well as the stress triaxiality effect, in the vicinity of the crack tip will be studied on fatigue crack growth behaviors in these Al alloys. The crystallographic geometry and microstructure of the grain boundaries along which crack deviation takes place will be investigated using EBSD and TEM/STEM, in order to identify which types of grain boundaries are more prone to crack deviation. A focus ion beam will be used to fabricate micro-notches in the selected grains and grain boundaries to verify the effects of texture and stress traxiality on crack deviation behaviors by studying the crack growth from the notches under different stress level. The growth behaviors of the cracks initiated from the notches will be measured in details. From the measured crack growth rates before and after a grain boundary, and along a grain boundary, the resistance to crack growth across and along a grain boundary will be quantified and compared with the stress intensity thresholds for crack growth across and along the grain boundary, respectively. Stress analysis will be carried out to quantify the triaxial stress field at the crack tip as a function of the crack size and the nominal applied stress first in a simple mode- I loading, with a view to establishing a relationship between the stress field and crack deviation. Based on the relations of crack deviation with microstructure, texture and stress field, a criterion for fatigue crack bifurcation, as well a quantitative model that takes account both driving forces and resistance to crack growth along the nominal mode-I crack path and the potentially deviated path, will be developed to simulate the fatigue crack deviation behaviors.