A New True Triaxial Cell for Field Measurements on Rock Specimens and Its Use in the Characterization of Strain Localization on a Vosges Sandstone During a Plane Strain Compression Test

A new experimental apparatus for rock-specimen analysis has been developed in Laboratoire 3SR (Grenoble) to characterize the evolution under loading of the deformation mode, placing particular attention on the onset of strain localization and the post-localization regime in rocks. The experimental apparatus can be configured to apply true triaxial or plane strain conditions on prismatic specimens, with a confining pressure of up to 100 MPa. The three principal stresses are independently controlled to apply true triaxial conditions. When applying plane strain conditions, the stress perpendicular to the observed specimen's surface is proportionally varied as a function of the loading applied on the specimen, to satisfy the condition of zero strain along this direction. The prismatic specimen can be observed under loading, because its surface, orthogonal with the direction of the intermediate principal stress, is in contact with a hard window (sapphire), thus allowing pictures to be taken throughout the test, as well as enabling full-field measurement. Photographs are analyzed by digital image correlation (DIC) to measure displacement and strain fields. The new true triaxial apparatus is presented here, first with a description of the experimental setup and mechanical characteristics of the cell. Then, the experimental procedure adopted to apply and assess the speckle pattern on the sample's surface, as required for DIC, is detailed. Finally, selected results from a test performed with the true triaxial device on a Vosges sandstone sample, performed in plane strain compression, are shown. The onset of strain localization and the shear bands' pattern evolution are shown by means of incremental strain fields obtained by DIC.