A constitutive coupled plasticity-damage formulation based on fabric tensor and discrete approach is proposed for modeling inherent and induced anisotropy in cohesive-frictional geomaterials. Fabric tensor is used to characterize the inherent orientation-dependent properties of materials. Macroscopic plastic deformation and material degradation are considered as the result of frictional sliding along weak planes. For each weak plane, damage evolution law and plastic flow are formulated within the framework of irreversible thermodynamics. The coupling problem between the inherent and induced anisotropy is carefully addressed and discussed. A series of numerical simulations are performed in order to check the predictive performance of the proposed model. Comparisons between numerical results and test data show the present model is capable of describing the main mechanical behaviors of anisotropic materials.
In practical engineering applications, the constructions of underground projects are most often carried out in host geomaterials. Mechanical properties of such materials show inherent and induced anisotropy, existing in elastic and inelastic range, respectively. Experimental results have confirmed that the material inherent anisotropy leads to a great independence of the strength on loading directions, whose coupling with the induced anisotropy in the process of plastic flow and damage evolution makes the failure and stability analysis much more complicated. In this sense, a good comprehension and proper description of the inherent and induced anisotropic properties as well as their coupling are of great importance for practical applications.
Various anisotropic models have been proposed to simulate the anisotropic mechanical behavior. A comprehensive review on classical failure criteria for inherently anisotropic rocks is provided by Duveau et al. (1998) in which the anisotropic models were divided into three classes: mathematical continuous approach, empirical continuous models and "discontinuous weakness planes based" models. However, the contradiction between the practicality and mathematical regime is not satisfactorily resolved. In recent years, several alternative approaches are proposed based on the concept of fabric tensor (Pietruszczak et al. 2001; Chang et al, 2007). Compared with other approaches, fabric tensor retains the mathematic rigour, but also can be easily implemented for practical applications.
The majority efforts in these studies have mainly focused on the strength analysis for inherently anisotropic materials. In fact, during loading processes, the material constants are also degraded (damaged) by progressive propagation of microcracks in some preferred directions, implying that induced anisotropy also exists. However, few attentions have been paid on this topic. The main difficulty consists in the description of the induced anisotropy and of its coupling with inherent material anisotropy, which constitutes one of the main objectives of the present study.
In this work, a constitutive elastoplastic damage model is proposed for modeling inherent and induced anisotropy of cohesive-frictional geomaterials, which is formulated by using fabric tensor and discrete approach (Zhu et al. 2008). The damage evolution and plastic flow are determined within the framework of thermodynamic. By considering the coupling between elastic damage and anisotropic plasticity, the influence of induced anisotropy on mechanical properties can be well reproduced.