Full-field laboratory imaging of localised deformation in sandstone

Rocks are neither truly homogeneous nor are laboratory test boundary conditions perfect plus at some stage of loading deformation in a specimen generally localises into shear/compaction bands, tensile/shear cracks or fractures. In the presence of such localised strains and heterogeneity point-wise measurements at a boundary do not well characterise the mechanics of the system. Therefore Full-field measurements are needed to better study the behaviour. In this paper full-field measurements, including x-ray tomography, 3D-volumetric digital image correlation and ultrasonic tomography, are used to characterise localised deformation phenomena in a sandstone deformed under triaxial compression. X-ray tomography allows 3D characterisation of the internal structure of laboratory specimens and the consequences of deformation are visible where there has been sufficient change in material density. Complementing x-ray tomography by 3D-volumetric digital image correlation (DIC) techniques allows quantification of 3D displacement and strain fields between different stages of loading by comparison of the corresponding x-ray tomography images. In addition ultrasonic tomography provides full-field measurement of ultrasonic velocities, and thus elastic properties, in laboratory specimens. DIC and x-ray tomography can provide information on structural changes due to deformation and the associated kinematics and strains, whilst ultrasonic tomography gives insight into damage (degradation of elastic properties), which are to be expected, e.g. due to compaction and related porosity reduction or grain crushing.© 2010, Society of Petroleum Engineers.