A series of true triaxial compression tests were performed on Castlegate sandstone, a high porosity reservoir analog. Tests were run under five stress states ranging from axisymmetric compression to axisymmetric extension. The tests were run under constant mean stress conditions at five different mean stress levels, chosen to span the failure modes observed in the material: shear band formation, compaction localization, and bulk compaction. The constant mean stress condition was chosen to investigate dependence of constitutive response and failure on the third invariant of deviatoric stress.;A novel jacketing procedure was developed which allowed strains to be measured in all three principal directions and acoustic emissions generated by the specimen to be recorded. Acoustic emissions, which represent damage within the specimen, were used to track the development of deformation bands within the specimen. Failure was determined to be the onset of localization, which consistently occurred prior to the peak in the stress-strain curve. A plane-fitting algorithm was used to determine the angle of the deformation band that formed in the specimen at the onset of localization.;Three distinct responses were seen. First, at low mean stresses (30--90 MPa), a stress peak and drop followed by a plateau in the stress-strain curve was observed. This response was associated with the formation of a shear band. Typically a shear band was expressed on the specimen jacket; this was confirmed with acoustic emission locations. For all of the specimens that formed shear bands the constitutive response was dilatant at failure. The second response, a knee in the stress-strain curve followed by a flat or slightly increasing plateau, is seen at intermediate mean stresses (90--150 MPa). This response was found to correlate with compaction localization, a diffuse feature that forms roughly perpendicular to the direction of maximum compression. These features did not express themselves on the specimen jacket due to a lack of shear strain. Prior to failure, specimen constitutive response showed either no change in volume strain at failure or compaction. The third response was only observed at high mean stresses (120--150 MPa). These specimens demonstrated no localization, characterized by a stress-strain response similar to that seen for compaction localization; however, the stress plateau for these specimens was always increasing. All of these specimens were compactant at failure.;Acoustic emissions were used to track damage within the specimen. By fitting a plane through the locations of these events, band angles were determined and compared to the angles expressed on the specimen jacket. Band angles decrease with increasing mean stress, transitioning from shear bands to compaction localization and then to no localization. However, no trend is seen relating band angles to deviatoric stress state. Additionally, the rate that at which a specimen generates acoustic events can be used to determine when the specimen localized; therefore, the onset of localization can be determined without locating acoustic emission events.;As the stress state moves from axisymmetric compression towards axisymmetric extension, shear bands and compaction bands are formed at higher mean stresses; this indicates that the nature of the failure feature is dependent on the third invariant of deviatoric stress. Additionally, for low mean stresses (30--90 MPa), the shear stress required to induce failure decreased as the stress state moved towards axisymmetric extension. This indicates that, at low mean stresses, failure is dependent on the third invariant of deviatoric stress.;A new process for separating elastic and plastic strain was developed to determine inelastic constitutive parameters: the dilation coefficient and the local slope of the yield surface. These parameters were used to calculate predicted band angles, which were then compared to those found through acoustic emissions and visual observation. Acoustic emission results correlate well with bands observed on the exterior of the specimen. However, agreement with predicted band angles is inconclusive; some predictions show excellent agreement, while the error for some was 30° or more. (Abstract shortened by UMI.)
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