Distribution of non-plane strain in experimental compression of short cylinders of Solnhofen limestone

Axi-symmetric compression experiments on short cylinders (radius>length) have great potential for the investigation of different non-plane strain geometries. In such experiments strain geometry and intensity are heterogeneously distributed throughout the specimens due to the effects of friction at the contact surface between the pistons and the specimen. We ran a series of tests on Solnhofen limestone at constant confining pressure (200 MPa), temperature (600 °C) and displacement rate (5.2×10−6 m s−1), conditions at which this material deforms predominantly by crystal plasticity. Using shape and crystallographic preferred orientation (SPO and CPO) patterns of calcite grains, the latter measured by electron back-scattered diffraction techniques (EBSD), we can discriminate areas in the deformed specimens with distinct non-plane strain geometries. SPO and CPO patterns record differences in the orientation of the finite stretching direction with respect to the external reference frame. It is radial in the vicinities of the rigid loading pistons, but circumferential outside the periphery of the pistons. As shortening progressed, the middle part of the specimen was extruded with respect to the contacts with the rigid end pieces, imparting a non-coaxial component to the flow with a circumferential vorticity axis in both upper and lower halves of the specimen. This component was recorded in the microstructure as an asymmetric CPO. The aim of the experiments is to improve the interpretation of naturally deformed rock bodies when strain departs from plane strain conditions by using the CPO as a 3D kinematic indicator, providing not only shearing direction but also information about the shape in 3D of the strain ellipsoid.