Frictional instabilities in direct shear experiments under different displacement control modes

The frictional behavior of Gabbro samples is examined using Servo-controlled direct shear experiments. We use two different shear displacement control modes: 1) Piston Displacement Control [PDC], and 2) Block Displacement Control [BDC]. In PDC the displacement output from the shear piston is used for servo control whereas in BDC the displacement output from two horizontal LVDT's positioned directly on the sheared interface are used for servo control. The frictional characteristics are studied particularly in relation to initial roughness, displacement rate, and normal stress. Direct shear tests of rough (RMS = 500 μm) and smooth (RMS = 7 μm) surfaces in Gabbro are performed under a constant normal stress of 5 MPa and a constant shear displacement rate of 0.01 mm/s. All four direct shear experiments exhibit stick-slip oscillations once reaching “steady state” sliding. The amplitude of the stick slip events in the smooth surfaces is significantly higher than in the rough surfaces but unexpectedly, the stress drop is also higher in BDC mode, in general. Similar effect occurs under different slip rates and normal loading conditions. In both control modes the ultimate shear strength is similar, with peak friction coefficient ranging from μ = 0.75 to 0.85. PDC mode is characterized by hardening behaviour, compared to BDC mode which presents more classic transition from peak to residual friction. These results prove that the shear response of rock interfaces depends on the shear displacement control mode. The actual control mode that should be applied at the laboratory will depend on the physical application at hand.