The role of near-threshold small-crack behavior in life prediction of titanium alloys for use in advanced turbine engines

Increasingly accurate life prediction models are required to utilize safely the full capability of current and future structural materials in advanced gas turbine engines. As an aid to understanding the role of crack size and threshold crack growth in these life predictions, two classes of material are examined: a conventional high strength titanium alloy and a gamma TiAl intermetallic alloy. Residual life calculations and fatigue life maps are used to illustrate the significance of crack size under realistic loading. Using weight function analyses, it is shown that surface residual stresses can greatly increase the threshold crack size for propagation of surface cracks. In the conventional titanium alloy, large-crack fracture mechanics data appear to be applicable to small fatigue cracks of practical, or inspectable, sizes. In a coarse microstructure of the gamma alloy, however, the scale of small cracks and initial damage can be of the order of microstructural dimensions, leading to rapid crack propagation. Local anisotropy in this material is shown to explain this behavior, leading to guidance for optimizing microstructure to improve durability and useful lifetime under realistic loading conditions.