The effects of confining pressure and stress difference on static fatigue of granite

Abstract

Samples of Barre granite were creep tested at room temperature at confining pressures up to 2 kbar. The time to fracture increased with decreasing stress difference at every pressure. The time to fracture increased with increasing pressure, even when the stress difference was normalized to account for the increase of strength with pressure. At a load equal to 87% of the short‐term fracture strength, for example, the time to fracture increased from about 8 minutes at atmospheric pressure to longer than one day at 2 kbar of pressure. The inelastic volumetric strain at the onset of tertiary creep, Δ, was roughly constant, independent of the applied load at any particular pressure, but increased with pressure in a manner analogous to the increase of strength with pressure. The creep strains were best fit by power functions of both stress and time, but the functional dependence on pressure remains uncertain. At the onset of tertiary creep the number of cracks and their average length increased with pressure. The crack angle and crack length spectra were quite similar, however, at each pressure at the onset of tertiary creep. Two theories of static fatigue are shown to explain the data adequately. Both suggest that the activation enthalpy for the stress corrosion process controls the creep rate and increases with pressure. Purely mechanical effects of pressure are not, however, discounted. The fatigue time may be a function of the rate at which corrosive agents reach crack tips and the rate of crack linking. Risking the extrapolation to longer times, the data suggest that creep rupture in the upper crust is possible if deviatoric stresses of several kilobars can be maintained for millions of years. Pore fluids and temperature can counteract the effect of pressure and substantially lower the fracture times and required stress, but not enough data presently exist to evaluate this properly.