13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- Assuming a dilatant behaviour of the joints as defined by an associated plastic flow rule, the volumetric plastic strain increases during the deviatoric loading phase. This increase is used to update the local permeability in plastified joints according to 0 0j(1 k PNR YR ) (23) The set of parameters used in the computations is defined in Table 1. The applied loading follows a typical triaxial test with a strong confinement. A confinement of 100 MPa is first applied in all directions, followed by an increased of the axial stress, the other stress components remaining fixed. This high confining stress is used to avoid any convergence problem in the mechanical problem, and to keep the local strain distribution consistent with a small strain description. Table 1. Material parameters used in mechanical and fluid transport homogenisation Material E ν c c [ 0j B GPa MPa ° GPa (m²) Grains 60 0.1 - - - 10-13 0 Joints 30 0.3 40 17 1 10-13 5.104 Based on these material and loading parameters, the simulation of the triaxial test is performed. Fluid transport homogenisation is then performed for the mechanical degradation states of the RVE corresponding to successive deviatoric stress levels. The mechanical response of the RVE is depicted in Fig. 6a. (a) (b) Figure 6. Homogenised response of the RVE: (a) Mechanical response under triaxial conditions depicting the average deviatoric stress applied as a function of the axial and transverse strains, (b) Axial permeability evolution as a function of the applied deviatoric stress The corresponding vertical averaged permeability evolution as a function of the deviatoric stress is depicted in Fig. 6b. A permeability increase of almost one order of magnitude is obtained based on the chosen value of the parameter k for the strong confinement applied (100 MPa). The volumetric strain distribution, that directly controls the local permeability evolution inside the RVE, is depicted in Fig. 7. As can be noted from these plots, local permeability can be magnified by a factor up to 500 at a macroscopic deviatoric stress of 350 MPa. The averaged permeability increase for the RVE however remains more restricted as a result of the absence of continuous pathways with high local permeability values inside the microstructure.
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