Response of Castlegate sandstone to true triaxial states of stress

Abstract

[1] A suite of true triaxial tests were performed on Castlegate sandstone to assess the influence of the intermediate principal stress on mechanical response and failure. Five independent deviatoric stress states were employed, for which the intermediate principal stress ranged from equal to minimum compression (axisymmetric compression) to maximum compression (axisymmetric extension). For each deviatoric stress state, five constant mean stress tests were conducted, covering mean stresses ranging from brittle to ductile failure. At low mean stresses, shear bands formed, and the peak stress required to induce failure decreased with increasing intermediate principal stress. Thus, failure at low mean stresses depends on the third invariant of deviatoric stress. Shear bands formed under all deviatoric stress states and over a wide range of mean stresses. The band angle (defined as the angle between the band normal and the direction of maximum compression) decreased with increasing mean stress. There was no clear trend in band angle with respect to intermediate principal stress; however, a small trend would be obscured by data scatter due to specimen variability. At higher mean stresses, no localization was observed. The upper bound mean stress at which shear localization occurred increased with increasing intermediate principal stress. Therefore, the mean stress that demarcates the brittle-ductile transition depends on the third invariant of deviatoric stress.

Failure characteristics of two porous sandstones subjected to true triaxial stresses: Applied through a novel loading path

“…The strong dependence on Lode angle is also consistent with the true triaxial results of Ingraham et al . [] on Castlegate sandstone in which they keep both Lode angle and σ oct,f constant. As in the results here they find that τ oct,f increases with Lode angle Θ for fixed σ oct,f in the brittle regime with the trend reversing in the ductile regime.…”

“…Our test results are also consistent with the observations by Ingraham et al . [], although as mentioned earlier they use loading paths that maintain both a constant octahedral normal stress and Lode angle.…”

“…Two exceptions are the work of Ingraham et al . [] and of Makhnenko and Labuz [] [see also Makhnenko et al ., ; Labuz et al ., ]. The latter uses a plane‐strain apparatus with passive constraint: the intermediate principal stress σ 2 maintains the condition of minimal deformation in the out‐of‐plane direction and, hence, varies with increasing σ 1 .…”

“…Ingraham et al . [] control changes in all three principal stresses so that both Lode angle and mean stress are kept constant while increasing the octahedral shear stress to failure. A disadvantage of the common loading path is that all three principal stress invariants, mean stress, octahedral shear stress, and Lode angle, change throughout the test.…”

“…The majority of earlier true triaxial tests (also in recent modeling efforts by Duan et al [2016]) were programmed so that σ 2 and σ 3 were maintained constant [Haimson, 2006;Mogi, 2007] (referred to here as "common" loading path). Two exceptions are the work of Ingraham et al [2013] and of Makhnenko and Labuz [2014] [see also Makhnenko et al, 2015;Labuz et al, 2016]. The latter uses a plane-strain apparatus with passive constraint: the intermediate principal stress σ 2 maintains the condition of minimal deformation in the out-of-plane direction and, hence, varies with increasing σ 1 .…”