This paper describes an attempt to use fracture mechanics concepts to the problems associated with creep crack growth in shale. Chevron-notched Brazilian disk (CDISK) specimens were used in this investigation. Before the creep tests, the samples were loaded until failure in order to determine the fracture toughness. This critical value was then used as an index for creep-induced rupture. The crack mouth opening displacement (CMOD) was used to calculate the crack length and the stress intensity factor. Unlike load-point displacement (LPD), CMOD varied significantly when the creep test reached the rupture stage. At the same time, the stress intensity factor reached its critical value, indicating that the fracture toughness can be used as a failure criterion in short-term creep fracture tests.
Creep crack growth has received much attention over the past years (Odqvist, 1966; Riedel and Rice, 1980; Saxena, 1980; Swanson, 1984; Atkinson and Meredith, 1987; Costin, 1987; Li, 1987; Geertsma, 1989; Lockher, 1993). It is of interest to observe and understand how this phenomenon develops, especially under a variety of environmental conditions. For petroleum and geological engineers, it has been known for a long time that, even when rocks are loaded below failure strength, they can undergo permanent, time-dependent deformations, especially when subjected to corrosive environments such as water and oil (Locknet, 1993).
In general, the process of creep may be divided into three separate stages (Figure 1): primary, secondary, and tertiary (Odqvist, 1966). It is the same for the process of creep crack growth: at the primary stage, cracks tend to be open, but do not propagate; stable crack growth may occur at secondary stage; while unstable crack propagation happens at the third stage. The active flaws and microcracks may affect the stability of the internal structure in the first two stages due to strain-hardening; however, when the crack density reaches a certain critical level, a local weaken region will be formed around the crack tip due to strain-softening, leading to a macroscopic fracture. Most of past studies have, so far, concentrated on the first two stages of creep fracturing; while fracture mechanics theories make the understanding and prediction of third creep stage possible.
An attempt was made in this study to use such concepts to creep crack growth in shale. This paper presents the experimental set-up and procedure. Some preliminary results are also discussed.
