Stress Path Tests with Local Deformation Profile in Flexible Boundary Plane Strain Device

Stress path tests in plane strain (PS) conditions with varying stress ratios have been carried out using a newly developed flexible boundary (FB) multiaxial testing device. It employs electro-pneumatic control and allows the application of independent principal stresses on each face of a 96-mm cubical soil specimen through a closed-loop feedback control system. Different stages of testing have been automated for both deformation and stress-controlled loading. It also has provisions for local deformation field estimation from the images acquired on the transparent rigid PS boundary. The friction angle for different stress paths in compression and extension mode is found to have minimal variation in PS conditions. The experimentally observed yield points match well with a simplified two-dimensional (2D) yield criterion resembling the shape of a "tear-drop."Local shear strain profiles under FB-PS conditions imply nearly uniform material behavior, while the emergence of instabilities is found to be minimal and mostly at the specimen corners.