Stress non-uniformity in a hollow cylinder torsional sand specimen

The hollow cylinder torsional apparatus (HCTA) facilitates more generalized stress path testing although it has been criticized on account of the level of stress non-uniformity that may develop because of the curvature of the test specimen wall. The stress non-uniformity which may develop in dense sand specimens (inner radius, 35.5 mm; outer radius, 50.0 mm; height, 200 mm) over small to medium strain levels in the HCTA was studied using an isotropic linear elastic stress analysis. The stress non-uniformity, quantified in terms of the stress non-uniformity coefficient β_R, increased in proportion to the stress ratio R, and was generally acceptable throughout the stress space for R < 1.5. Regions where unacceptable stress non-uniformity may develop were identified in the vicinity of (b,α_σ), equal to (1,0°) and (0,90°) and for all values of the parameter b in the vicinity of α_σ = 45°. It was found that placing restrictions on the difference in the confining pressures which may be applied across the specimen wall thickness reduced the stress non-uniformity but also limited the regions in stress space that could be probed.