Digital Image Correlation under Scanning Electron Microscopy: Methodology and Validation

The recent combination of scanning electron microscopy and digital image correlation (SEM-DIC) enables the experimental investigation of full-field deformations at much smaller length scales than is possible using optical digital image correlation methods. However, the high spatial resolution of SEM-DIC comes at the cost of complex image distortions, long image scan times that can capture gradients from stress relaxation, and a high noise sensitivity to SEM parameters. In this paper, it is shown that these sources of error can significantly impact the quality of the results and must be accounted for in order to perform accurate SEM-DIC experiments. An existing framework for distortion corrections is adapted to improve accuracy and the procedures are described in detail. As the results demonstrate, time varying drift distortion is a larger problem at high magnification while spatial distortion is more problematic at low magnification. Additionally, the new use of sample-independent calibration and a method to eliminate the detrimental effects of stress relaxation in the displacement fields prior to distortion correction are introduced. The impact of SEM settings on image noise is quantified and noise minimization schemes are examined. Finally, a uniaxial tension test on coarse-grained 1100-O aluminum is used to demonstrate these techniques, where active slip planes are identified and strain localization is examined in relation to the underlying microstructure.