Towards measuring absolute residual stress by HR-EBSD with simulated reference patterns

The High-Resolution Electron Backscatter Diffraction (HR-EBSD) technique has drawn attention in recent years, mainly thanks to its ability to assess the elastic strain/stress state with a sub-micron spatial resolution. Each diffraction pattern is correlated with a reference one, and the strain level relative to the reference is retrieved in the procedure. Several studies have attempted to use dynamically simulated diffraction patterns as references to access absolute strain/stress states to circumvent the difficulty of finding a strain-free reference pattern, which is often inaccessible. Numerous factors need to be considered and corrected to register better experimental and simulated diffraction patterns, such as the accurate positioning of the projection center, Kikuchi band (K-band) brightness asymmetry, K-band gray level reversal, optical distortion, or non-uniform electron energy. Here, an integrated digital image correlation method is proposed to register experimental patterns to a master pattern defined on a sphere, where the difficulties with experimental patterns are, for the most part, corrected efficiently. Due to the shallow inspection depth of EBSD, the free-surface property (vanishing stress vector at the observation surface) is also adopted to reduce the degrees of freedom and enhance precision. Through several tests on high-quality experimental patterns, the proposed method proves robust and precise. Both the systematic and random errors of the residual strain results are several ${10}^{-4}$. The method also allows the simultaneous determination of residual stress and the projection center coordinates.