Experimental study of the deformation band network at brittle-ductile transition

We analyzed strain localization initiation and evolution at a confining Pressure P_c =P_bdt corresponding to the transition from brittle faulting to ductile deformation. To do this we performed axisymmetric compression test with the samples made of a low-strength synthetic granular, cohesive, frictional and dilatant rock analogue material, known as GRAM1. GRAM1 behaves very similar to real rock, but at much weaker magnitude. This allows using transparent pressure cell to visualize the evolution of the deformation, including the moment and place of initiation of the first band and then of the others by using digital image correlation technique DIC. In each test the deformation bands do not form simultaneously but sequentially and evolve with loading. The inelastic heterogeneous deformation initiated at well positive hardening modulus h. During shearing (hardening modulus h close to zero), the individual bands exhibit jerky propagation and thickening. Series of sub-parallel strands which form sequentially during loading are supposed to contribute to the band’s final thickness. The deformation bands pattern on a post-mortem sample corresponds very well with DIC full-field strain results.