COLLABORATIVE RESEARCH AND DEVELOPMENT (CR&D) Delivery Order 0037: Mechanisms Causing Fatigue Variability in Turbine Engine Materials

The research develop a comprehensive understanding of the causes of fatigue variability, especially the minimum life, in a +b titanium alloys and nickel base superalloys. A physics-based life-prediction model has been developed that provides accurate probabilistic fatigue lifetime limits in aerospace materials with respect to microstructural and extrinsic variables, and promises significant reduction in the uncertainty associated with life prediction. The modeling is based on some key findings, common to a range of aerospace material systems, in terms of the fatigue variability behavior. First, it is found that effect of microstructural and loading variables on fatigue variability is manifested as separate degree of influences on the mean fatigue behavior and the life-limiting, lower-tail behavior. Therefore, the lifetime distribution can be represented as a divergence or convergence of these two behaviors as a function of the imposed variables. Second, the life-limiting behavior is shown to be controlled by the crack growth regime, i.e., the crack initiation lifetime is negligible. This separation of mean and the crack-growth-controlled behavior is found to be promoted by the development of an array of heterogeneity scales in the material under any given nominal microstructure and loading condition. These heterogeneity scales, which are related to the underlying microstructural configurations, are suggested to produce sequential failure mechanisms in the order of decreasing heterogeneity level. These vital concepts have been developed with the help detailed investigation of the relationship of the mean and the life-limiting mechanisms to the underlying microstructural drivers. Quantitative analysis of crack initiation sites in terms of size, and spatial and crystallographic orientations of the crack initiating feature and its neighborhood were employed to support this purpose.
The original document contains color images. All DTIC reproductions will be in black and white.