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.
 
 

The objective of the AFOSR-MURI High-Cycle Fatigue program is to characterize and model the limiting damage states at the onset of high-cycle fatigue to facilitate a mechanistic understanding and to develop a basis for life prediction. Efforts have been focused on the influence of HCF/LCF interactions, foreign object damage (FOD) and fretting, initially on a Ti-6Al-4V blade alloy and on a polycrystalline Ni-base disk alloy.

Notable highlights during the third year include the characterization and quantitative modeling of fretting and FOD and the definition of the role of mixed-mode loading on HCF thresholds in Ti-6Al-4V.  Accomplishments of the program are outlined below:

• Worst-case fatigue threshold stress intensities have been measured in STOA Ti-6Al-4V using large (> 5 mm) cracks under representative HCF conditions (R > 0.95, 1000 Hz).  Values provide a practical, frequency-independent (20 – 20,000 Hz) lower-bound for the growth of naturally-initiated, physically-small (> 40 ?m) cracks.

• Mixed-mode thresholds, at mixities of KII/KI ~ 0.5 to 8, have been measured in Ti-6Al-4V, with both STOA and lamellar microstructures. Using a G-based approach, Mode I is found to be the worst-case threshold condition in the STOA alloy.

• Stress-intensity solutions have been developed for small, semi-elliptical, surface cracks under mixed-mode loading.  Such solutions are being used to experimentally measure (for the first time) small-crack, mixed-mode thresholds in Ti-6Al-4V.

• FOD, simulated with high velocity 200-300 m/s steel-shot impacts, has been found to severely reduce the smooth-bar fatigue life in Ti-6Al-4V microstructures. However, worst-case thresholds are again seen to provide a lower-bound for the onset from small fatigue-crack growth from damaged regions.

• The local residual stress gradients surrounding FOD regions have been analyzed using a quasi-static analytical model; predictions are being verified using synchronous X-ray micro-diffraction techniques.

• Large-crack threshold behavior in a polycrystalline Ni-base disk alloy has been characterized at 1000 Hz at 22º and 650-900ºC, with respect to the role of microstructure, frequency and load ratio.

• Theoretical solutions for the crack-tip opening and crack-shear displacements controlling the growth of small fatigue cracks have been developed.

• New computational (finite-element) methods for 3-D simulations of fretting fatigue (Fretting Fatigue Simulator) have been developed using a ring-element approach.

• Through an analogy between the asymptotic fields at contact edges and ahead of a crack, a crack-analogue approach to contact fatigue (Crack Analogue) has been developed, and validated by experiment in Al and Ti alloys.

• A continuum level mechanics model (Adhesion Model), incorporating interfacial adhesion, material properties and contact loads, for predicting contact fatigue crack initiation for a variety of loading states and contact geometry, has been developed.

• The influence of contact and bulk stresses, contact geometry, material microstructure and surface finish on the fretting fatigue behavior of Ti-6Al-4V has been investigated through controlled experiments, using the MURI-developed fretting fatigue device.

• A new theoretical model for the fretting of coated metal surfaces has been developed which specifically addresses the role of plastic deformation of the metal substrate.

• Quantitative analytical and experimental tools for evaluating the effectiveness of different palliatives, e.g., shot-peening, laser shock-peening, coatings, for fretting fatigue has been investigated.