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.