Coaxiality between the principal directions of stress tensor and the principal directions of the plastic strain increment tensor is assumed in conventional plasticity models. In order to investigate coaxiality, or not, between these two principal directions, a series of drained tests on dry Hostun sand was carried out using a precision Hollow Cylinder Apparatus (HCA). The applied stress path includes large Principal Stress Axes Rotation (PSAR). Two of the three principal stresses are kept constant. So, among the three principal stresses, only the intermadiate principal stress which is the confining pressure (same pressure outside the hollow cylinder for internal and external lateral surfaces) changes during loading. During these tests, at different stress levels, the elastic (or quasi-elastic) properties are also investigated considering small amplitude quasi-static cycles. These small cycles are performed in two different directions by changing successively only the axial stress σzz or the shear stress σθz. The elastic experimental properties are well simulated using the DBGS hypo-elastic model, which takes into account PSAR. For each test, the elastic part of deformation is calculated using the DBGS model and removed from the global strain to focus only on the irreversible part (plastic part). Then, the principal directions of stress and the principal directions of plastic strain increment are compared. The experimental results show that there is no coaxiality between these directions. This observation attests the existence of a non-coaxial plasticity. In addition, coupling between coaxial and non-coaxial part is clearly shown. Experimental results reveal that the plastic strain part is very important for the first large amplitude cycles and remains greater than the elastic part even after 20 cycles.
