This program is focused on the definition, microstructural characterization and mechanism-based modeling of the limiting states of damage associated with the onset of high-cycle fatigue failure in titanium and nickel-base alloys for propulsion systems. Experimental and theoretical studies are aimed at three principal areas, namely high cycle/low cycle fatigue (HCF/LCF) interactions, the role of notches and foreign object damage and fretting fatigue.  The approach is to combine new experimental techniques for imaging microstructural damage with detailed micro-mechanical characterization and modeling of the salient micro-mechanisms to facilitate the prediction of the effects of such damage on HCF lifetimes.

The primary study is focused both at ambient temperatures on Ti-6Al-4V, with a bimodal processed blade microstructure, and at 700° and 1100°C on a single crystalline Ni-base blade alloy; additional studies, specifically to isolate the role of microstructure, are being performed on Ti-6Al-4V, with a lamellar structure, and on a fine-grained polycrystalline Ni-base disk alloy.   Specific objectives include:

(1) Systematic experimental studies to define crack formation and lower-bound fatigue thresholds for the growth of "small" and “large” cracks at high load ratios, high frequencies, and with superimposed low cycle loading, in the presence of primary tensile and mixed-mode loading.  Analysis of the applicability of the threshold stress-intensity factors to characterize crack initiation and growth in engine components subjected to high cycle fatigue.

(2) Similar definition of lower-bound fatigue thresholds for crack formation in the presence of notches, fretting, or projectile damage, on surfaces with and without surface treatment (e.g., shot or laser shock peened).

(3) Development of an understanding of the nature of projectile (foreign object) damage and its mechanistic and mechanical effect on initiating fatigue-crack growth under high-cycle fatigue conditions.

(4) Development of new three-dimensional computational and analytical modeling tools and detailed parametric analyses to identify the key variables responsible for fretting fatigue damage and failure in engine components.  Comparison of model predictions with systematic experiments.  Identification and optimization of microstructural parameters and geometrical factors and of surface modification conditions to promote enhanced resistance to fretting fatigue.

(5)  Development of a mechanistic understanding for the initiation and early growth of small cracks in order to characterize their role in HCF failure, with specific emphasis on initiation at microstructural damage sites and on subsequent interaction of the crack with characteristic microstructural barriers. Correlation of analytical models to experimental measurement.

(7) The ultimate aim of the work is to provide quantitative physical/mechanism based criteria for the evolution of critical states of HCF damage, enabling life-prediction schemes to be formulated for fatigue-critical components of the turbine engine.