Creep of anisotropic clay: New microplane model

AS a simpler alternative to a previous microplane model, a new microplane model is presented, in which the relative slipping of clay platelets is characterized by normal'rather than shear strains on the microplanes. As is the case for Batdorf and Budianski's slip theory of plasticity, the microplanes are constrained statically, i.e., the stress components on a microplane are the resolved components of the macroscopic stress, while the previous model used a kinematic constraint. This different type of constraint is needed to model correctly material anisotropy. The distribution function of normal strain rate intensity for microplanes of various orientations is calculated from the frequency distribution function of clay platelet orientations, which was approximately determined by other authors from X-ray diffraction measurements. The 6 x 6 fluidity matrix is calculated from the principle of complementary virtual work on the basis of deformations of individual microplanes and the current values of the stress components. The activation energy approach, validated in previous works, is used to quantify the dependence of the normal strain rates on the microplanes of all orientations as a function of the stress level and temperature. A numerical algorithm to calculate the fluidity matrix is given, and typical test data from the literature are analyzed. With only two free material parameters, good fits of the data are achieved, including their anisotropic features. The modeling is limited to deviatoric creep, and volumetric response is left for subsequent work. The proposed model could be used in finite element programs.