Page 98 - Applied Petroleum Geomechanics

90 Applied Petroleum Geomechanics

Figure 3.3 Stressestrain diagram obtained from a single uniaxial compression test for
Lac du Bonnet granite showing the definition of crack initiation (s ci ), crack damage
(s cd ), and peak strength (s ucs )(Martin and Chandler, 1994).

(4) Crack damage and unstable crack growth (from s cd to peak strength
s ucs ).
Crack propagation is either stable or unstable. Under stable conditions,
crack growth can be stopped by controlling the applied load. Unstable
crack growth, also referred to as the crack damage stress threshold, s cd , has
been associated with the point of reversal in the total volumetric strain
curve (Fig. 3.3). Under such conditions, crack propagation will continue
until failure even if the applied loading is held constant.
(5) Failure and postpeak behavior.
Once the applied load exceeds the peak strength (UCS), rock starts
failure, and the rock loses to its partial or all capacity to bear loading
dependent on the plasticity of the rock. For a brittle rock, it may lose the
capacity; however, for ductile (plastic) rock, it may still have capacity to
partially bear the applied load.