On the magnitude of error in determination of rotation axes

Abstract

Rotation axes (together with rotation angles) are often used to describe crystal orientations and misorientations, and they are also needed to characterize some properties of crystalline materials. Since experimental orientation data are subject to errors, directions of axes obtained from such data are also inaccurate. A natural question arises: given a resolution of input rotations, what is the average error of the rotation axes? Assuming that rotation data characterized by rotation angle $\omega$ deviate from actual data by error rotations with fixed angle $\delta$ but otherwise random, the average error of rotation axes of the data is expressed analytically as a function of $\omega$ and $\delta$. Moreover, a scheme for using this formula in practical cases when rotation errors $\delta$ follow the von Mises-Fisher distribution is described. Finally, the impact of crystal symmetry on determination of average errors of axis directions is discussed. The presented results are important for assessing the reliability of rotation axes in studies where directions of crystal rotations play a role, e.g. in identifying deformation slip mechanisms.