Testing Apparatus Stiffness Variations With Application to Rock and Sediment Deformation

Frictional slip instability, resulting in intermittent stick-slip rather than continuous sliding, is a phenomenon that depends on the frictional properties of the sliding area and the stiffness of the surrounding material. For geomechanical rock and sediment testing, the stiffness of the testing apparatus partially controls the occurrence of stick-slip sliding behavior. Under a wide range of conditions, we directly measured the shear loading stiffness of five direct-shear apparatuses in the Marine Geotechnics laboratory at MARUM, University of Bremen. Under constant normal stress, the shear loading and unloading curves are non-linear and exhibit significant hysteresis. Shear stiffness values generally increase with increasing normal and shear stresses. Absolute values of stiffness as well as their dependency on shear and normal stress vary amongst the apparatuses despite the same basic apparatus design. For the application of stiffness concepts to stick-slip sliding in the Earth, for example, earthquakes, the most appropriate stiffness value is obtained at a shear stress value comparable to the sample strength, and measured during stress unloading. Well-characterized apparatus stiffness under a wide range of testing conditions is recommended to optimize analyses of laboratory friction data.

Automated summary

Frictional slip instability, which results in intermittent stick-slip behavior, is influenced by the frictional properties of the sliding area and the stiffness of the surrounding material. By directly measuring the shear loading stiffness of five direct-shear apparatuses under various conditions, it was found that shear stiffness generally increases with increasing normal and shear stresses, although the values and their dependency on stress vary among the apparatuses.