Test of the Effective Stress Law for Semibrittle Deformation Using Isostatic and Triaxial Load Paths

For brittle friction and rock deformation, the coefficient α in the general effective stress relation σ e  =  σ  −  αP p can be approximated as unity with sufficient accuracy. However, it is uncertain if α deviates from unity for semibrittle flow when both brittle and intracrystalline‐plastic deformation is involved. We conducted triaxial and isostatic compression experiments on synthetic salt‐rocks (∼300 ppm water) at room temperature to test the effective stress relation in the semibrittle regime using silicone oil and argon gas as pore fluids. Confining and pore pressures were cycled while their difference (differential pressure) was kept constant, such that changes in the mechanical behavior would indicate deviation of α from unity. Microstructural observations were used to determine the dependence of α on true area of grain contact from asperity yielding. In triaxial compression experiments, semibrittle flow involves grain boundary cracking and sliding, and intragranular dislocation glide and cracking. Flow strength remains constant for changes in pore fluid pressure of more than two orders of magnitude. In isostatic compression experiments, samples show combined processes of microcracking, grain boundary sliding, dislocation glide, and fluid‐assisted grain boundary migration recrystallization. Volumetric strain depends directly on the differential pressures (i.e., α equals one). Analysis of grain‐contact area in both experiments indicates that α is independent of the true area of contact defined by plastic yielding at grain boundaries. The observation of α effectively equals one may be explained by operation of pressure‐independent intracrystalline‐plastic mechanisms and transmission of pore pressure at grain boundaries through thin fluid films.