Full-field measurements on low-strained geomaterials using environmental scanning electron microscopy and digital image correlation: Improved imaging conditions

Full-field strain measurement at microscale on geomaterials by means of hydromechanical in situ testing and imaging in an environmental scanning electron microscope and digital image correlation techniques is a challenging task because of both low magnitude of investigated strains (typically of the order of 10^-3) and unfavourable imaging conditions. In view of improving strain measurement accuracy, three major sources of measurement errors are evaluated, and methods to minimise their effects are proposed. First, a fast and simple procedure to accurately quantify image noise is proposed and subsequently used to optimise various environmental scanning electron microscope setting parameters, such as dwell time, spot size, working distance and chamber pressure. Second, a specific procedure to limit magnification fluctuations to a sufficiently low level is described. Finally, digital image correlation systematic errors are quantified on real images, and several ways to reduce their amplitude are compared. The combination of these improvements finally allows us to reach an appropriate accuracy for overall strain measurements and characterisation of microscale heterogeneities.