Abstract:
Creep/static fatigue experiments were conducted on samples of highly-welded, non-lithophysal Paintbrush Tuff. The tests were conducted at a confining pressure of 5.0 MPa, a pore water pressure of 4.5 MPa, and a temperature of 125°C. The specimens were rapidly loaded to a constant stress ranging from 57 to 100% of their estimated fracture strength. Strain was measured parallel and normal to the core axis. Samples loaded to stresses of 80 to 100% of the estimated fracture stress failed in times of 2 to 3.6 × 105 sec. Samples stressed to less than 80% of their estimated fracture strength did not fail. All of the specimens tested showed primary and secondary stages of creep; those that failed exhibited tertiary creep. The time to failure decreased with increasing stress, although there is considerable scatter in the results. Samples that did not fail exhibited very low secondary radial creep and no dilatancy. The radial strain decreased with time suggesting that the cracks parallel to the loading axis are closing. The cracks may well reach the equilibrium state proposed by Costin (1983). He argues that at low strain rates, stress corrosion at crack tips is so low that the tensile stresses that drive the crack are relaxed.
Introduction
One of the most striking characteristics of brittle rocks, when subjected to a constant load at temperatures well below the melting point, is a continuous inelastic increase in strain with time. This time-dependent deformation is termed creep. Typically, creep is reported in terms of three distinct phases: primary or transient creep, secondary, and tertiary or accelerating creep. Transient creep has been reported for a variety of rock types over a wide range of environmental conditions (Wawersik, 1972; Kranz and Scholz, 1977; Kranz, 1979; Kranz, 1980; Lajtai et al., 1987; Lockner, 1993; Martin et al., 1997). The strain rate immediately after loading decreases rapidly. The transient creep strain is often reported as the logarithm of time, because both the axial and lateral strains exhibit a logarithmic time dependence. At low stresses, transient creep may account for most of the total observed creep strain. However, at stresses greater than 60% of the fracture stress, secondary creep is often observed. For secondary creep, the strain is proportional to time. If secondary creep continues, the strain rate increases and the rock fails (tertiary creep). The onset of tertiary creep has been attributed to crack coalescence (Scholz, 1968; Kranz, 1979). All three stages of creep have been observed in granite, quartzite, and tuff (Martin, 1972b; Wawersik, 1972: Kranz and Scholz, 1977; Kranz, 1979; Lockner, 1993; Martin et al., 1997).
