Effect of Dislocation Emissions on Localized Lattice Rotation and Residual Stress Under Fatigue Loading

The effect of tensile residual stress is quite significant in determining the lifetime of a material as it affects the crack propagation by influencing significantly the fatigue life.  Here we demonstrate that the emission of dislocations in fatigue results in significant lattice rotation and tensile residual stresses around the fatigue crack. The lattice rotation would be related to the size of the plastic zone and the redistribution of dislocations and slip processes in front of crack. We measure the elastic strain ex-situ, owing to the change in lattice spacing, with the high-resolution x-ray diffraction (HRXRD) around the crack, and obtain the residual stresses, and correlate with the microstructural change.  The residual stress increases gradually around the fatigue crack by ≈ 200% for Al 7075, and decreases by ≈ 80% for Al 1100, as we approach the crack.  Additionally, the deformation leads to a significant lattice rotation and change in dislocation density as observed by the XRD, the electron backscatter diffraction measurements (EBSD) and transmission electron microscopy (TEM). The lattice rotation that aligns {111} planes across grains, which is consistent with increase in shear-2 and near Cu texture, increases the stress. However, the decrease in residual stress in Al 1100 is associated with the lattice rotation that decreases the Cu texture. These findings provide  a new insight on the role of lattice rotation and dislocation configurations around the fatigue crack in determining the residual stress.