DEFORMATION CHARACTERISTICS OF DRY HOSTUN SAND WITH PRINCIPAL STRESS AXES ROTATION

Coaxiality between the principal directions of the stress tensor and the principal directions of the plastic strain increment tensor is assumed in conventional plasticity models. In order to investigate coaxiality, or non-coaxiality, 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. Therefore, among the three principal stresses, only the intermediate principal stress, which is confining pressure (same pressure outside the hollow cylinder for internal and external lateral surfaces), changes during loading. During these tests, at different stress levels, elastic (or quasi-elastic) properties are also investigated, using small amplitude quasi-static cycles. These small cycles are performed in two different directions by successively changing only the axial stress sigma(zz) or the shear stress sigma(theta z) Elastic experimental properties are well simulated using the DI Bendetto-Geoffroy-Sauzeat (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 global strain so that it is possible to to focus only on the irreversible part (plastic part). Then, the principal directions of stress and plastic strain increment are compared. Experimental results show that there is no coaxiality between these directions. This observation attests to the existence of a non-coaxial plasticity. In addition, the coupling between tge 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.