Microstructure and 3-d Effects in Fretting Fatigue of Ti Alloys and Ni-base Superalloys

Damage and plastic deformation accumulation in fretting fatigue occurs within a depth of a few crystallographic grains. Therefore, more accurate assumptions concerning length scale, damage volume, and the material model are needed to establish a more solid physical foundation necessary for next generation fretting fatigue damage prediction. Major thrusts include: (1) development and implementation of a 3-D crystal viscoplasticity model for Ti-6A1-4V, (2) realistic 3-D fretting simulations that capture influence of key microstructure features, including distinct phase properties and crystallographic texture, and (3) experimental characterization of fretting experiments to both validate fretting simulation results and identify additional features to incorporate in the crystal viscoplasticity model and fretting simulations. Development of a Crystal Viscoplasticity Model for Ti-6A1-4V Improvements and extensions to our crystal viscoplasticity model for Ti-6A1-4V continue to be made each year. This year, a major extension to 3-D was undertaken. Our initial twodimensional crystal plasticity model of duplex Ti-6A1-4V employed a planar triple slip idealization (Goh et al., 2006a and references therein). This was a pioneering effort to show effect of grain orientation distribution, grain size and geometry, as well as the phase distribution and their arrangement in fretting contact problems, suggesting that the role of micro-textures (and indeed primary a size and orientation) might be significant in resisting fretting fatigue (Goh et al., 2006b). The 2-D crystal plasticity model was extended to a full 3-D version by Mayeur and McDowell (2006) as reported in last year's annual report. The 3-D model is capable of representing arbitrary crystallographic textures, the unique crystallography of the constituent phases, anisotropy of slip system strengths, and non-planar dislocation core structures. These features are essential to capturing the deformation behavior of these materials due to the low symmetry of the hep crystal structure and the resulting anisotropic properties. These material models are coded in FORTRAN as ABAQUS User MATerial subroutines. Consideration of more realistic microstructure morphologies using Voronoi tessellation coupled with simulated annealing techniques have also been considered (Zhang et al., 2006a; 2006b; 2006c). It is well known that fatigue crack formation within Ti-6A1-4V is associated with the impingement of slip on boundaries and decohesion of shear bands. To account for these mechanisms, our 3-D crystal viscoplasticity model will be further enhanced to address the shear