Now that we know the macroscopic modes of rock failure we can investigate again the actual mechanisms of rock inelasticity and failure. First, uncemented rocks cannot hold tensile stresses and failure takes place through internal shearing (grain to grain friction and rotation) and grain crushing at high mean compressive stress. Fig. 4.22 shows experimental evidence of grain rotation in the shearing region of a sand specimen loaded axially (see warm-colored shear band). Second, cemented rocks can hold tensile and shear stresses. Most rocks have some level of internal microfracturing or defects that act as fracture tips. Fractures propagate in three modes: opening, in-plane shear, and out-of-plane shear.
Shear and tensile stresses amplify at fracture tips. Therefore fracture propagation usually starts at fracture tips. The images in Fig. 4.23 show the propagation of fractures after applying a vertical stress on Tuffo carbonate samples with a pre-existing crack (thick dark line in the middle).
The coalescence of multiple microfractures can form a macrofracture that defines a macroscopic failure plane (Figure 4.24).