Rock Salt - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 116

s is referred to as the “secondary creep”,
“steady state creep” or “stationary creep” component. It increases linearly with time
( = const., Fig. 5.1) and is mainly governed by the dislocation density and the pattern
of dislocations in individual crystals (Langer 1993).
If the applied stress is larger than the uniaxial yield stress

primary creep is also called “transient creep”. ε

σ f , the creep curve has a point
of infl ection. After the infl ection point is reached, an accelerated creep process starts
which fi nally leads to creep failure (Fig. 5.1). This behavior can be interpreted by a ter-
tiary creep component

t increasing with time and being superimposed onto the elastic,
primary creep and the secondary creep components:

ε

el + p (t) + s (t) + t (t) if

ε (t) = ε ε ε ε σ ≥ σ f . (5.3)

Figure 5.1 Primary, secondary and tertiary creep of rock salt (Kiehl et al. 1998, Wittke 2000b)

The σ eff diagram in Fig. 5.2 forms a basis of the understanding of the complex

constitutive behavior of rock salt, and other salt rocks containing halite, in three di-
mensions. In this plot two domains can be distinguished denoted as the “compaction
zone” and the “dilatancy zone”, also called the “damage zone. The mean stress

σ m -

σ m is an

invariant of the stress tensor defi ned as

(5.4)

σ eff denoted as the “von Mises stress” and defi ned as