Residual stress and localized lattice rotation under fatigue loading

Abstract

A significant change in residual stress around a fatigue crack has been observed in Al alloys. For Al 7075, the residual stress increases by 200% closer to the crack. However, in commercially pure Al (Al 1100), the residual stress decreases by 80% closer to the crack. The residual stresses are mostly the stored elastic energy as a result of deformation around the fatigue crack. To understand these results, we estimate the dislocation density as a function of distance from the crack using the full width at half maxima of 111 reflection for both alloys. The dislocation density increases by 25–30 % for Al 7075. However, it actually decreases by 10% for Al 1100. Such change in dislocation density cannot explain the observed change in residual stress closer to the crack. Additionally, the deformation leads to a significant lattice rotation, as observed by the x-ray diffraction and the electron backscatter diffraction measurements. We argue that dislocations are needed to produce the lattice rotation and the estimated lattice rotation is ≈ 11°, and conclude that the deformation associated with the lattice rotation would be a major contributing factor to the residual stress. This provides a new insight on the role of lattice rotation on the residual stress under fatigue loading.