SUMMARY

Turbine engine components such as fan and compressor blades experience complex combinations of steady and vibratory loads that lead to in-service cracking in directions that cannot be predicted by current fracture criteria. Accurate crack path predictions are required in order to characterize the risk and extent of damage resulting from liberation of a fractured ligament from rotating components. Under proportional in-phase mixed Mode I / Mode II loading conditions, crack growth direction has been observed in some materials to shift from tensile-dominated Mode I to shear-dominated Mode II or mixed-mode crack growth at higher proportions of initial Mode II loading, but non-proportional loads are not well-characterized. An extensive database of crack growth direction under non-proprotional 2-D mixed-mode loading conditions is required to expand crack path prediction models, which are likely to vary between alloys. Polycrystalline nickel-base superalloy Inconel 718 was tested using standard and novel crack growth specimen configurations under proportional in-phase and 3 kinds of non-proportional fatigue loading, and crack tip conditions have been analyzed with boundary element fracture modeling software FRANC3D. The new specimen design has been validated for generating crack path data over a broad matrix of mixed-mode loading conditions. Qualitative microscopy of fracture surfaces was used to characterize the crack growth behavior. Impact of these results on current crack path models, and the general feasibility of a simple, linear elastic fracture mechanics-based approach to crack path prediction, is discussed.