Crack development in transversely isotropic sandstone discs subjected to Brazilian tests observed using digital image correlation

Abstract

Digital image correlation (DIC) is used to investigate crack initiation and propagation in discs of transversely isotropic Hawkesbury sandstone subjected to the Brazilian test. To verify DIC's validity and precision a simple calibration method is presented. It involves adjusting the size and spacing of subsets of pixels within images of a specimen's surface from which local deformations and strains are quantified, achieving agreement between strains measured by DIC and gauges attached to the surface. Using DIC the tensile and shear strain fields, crack opening displacements (CODs) and displacement vectors are determined at all stages of loading. The development of the fracture process zone is also identified prior to a major crack initiation. Also, a range of load contact configurations is considered, including the loads being applied via flat rigid platens and across finite strips with widths controlled by small (3 mm wide) and big (5 mm wide) wooden cushions placed between the load applicators and discs. It is observed that, for 0° and 90° anisotropy angles (representing layering normal to and parallel to diameters connecting load contacts) and for both the flat platen and small wooden cushion, cracks initiate far from the disc centres. Also, for the 45° anisotropy angle and all load configurations, cracks initiate at the disc centres. Furthermore, when the loads are applied via big wooden cushions, cracks initiate at the disc centres regardless of the anisotropy angle. Also, specimens loaded via big wood cushions exhibited the largest COD while the specimens loaded via small wood cushions exhibited the smallest. Also, specimens loaded via flat platens failed at smaller loads compared to those using wooden cushions. This study confirms transverse isotropy has strong effect on crack development. It also demonstrates important parameters including time and location of initiation, as well as COD, may be determined using DIC.