Digital image correlation analysis (uniaxial compression testing)

Abstract

Digital image correlation was used in parallel with longitudinal extensometer-based measurements during the uniaxial compression test under continuous humidity variation, with the digital images acquisition system outside the climate chamber, free from climate interference. DIC analysis was undertaken using the open-source 2D digital image correlation software Ncorr. It tracks surface deformation by correlating the best fit between pixel values within a defined search window in images taken during hold steps to those in the reference image acquired immediately prior to the first load step. --> In order to validate strains measured by a mechanically-mounted extensometer during a uniaxial compression test on a Herrnholz granite specimen exposed to continuous humidity variation, digital image correlation (DIC) analysis was used in parallel with longitudinal extensometer-based measurements. We analyzed 425 images using the open-source MATLAB programme Ncorr. It tracks surface deformation by correlating the best fit between pixel values within a defined search window in images taken during hold steps (0.1 MPa axial stress) to those in the reference image acquired immediately prior to the first load step. The search window was set to be 50 pixels in radius, providing a measured displacement field at a resolution of 1 mm. Due to the distortion in cylindrical projection, we focused on displacement within a 1 mm wide region in the middle of the projected surface, where the distortion is very pale. We tracked the DIC-based axial strain by averaging the vertical strain within a 1 × 100 mm (width: height) window (50 × 5000 pixels) corresponding to the position of the longitudinal extensometer. The DIC-based radial strain was estimated by averaging the horizontal strain within a selected 1 × 1 mm (50 × 50 pixels) region at the midpoint of the specimen.