Brittle Deformation
Due to growth of fractures or sliding on preexisting fractures (non-recoverable strain)
Cataclasis: involves crushing and frictional
sliding of grains and rock fragments (fault
gouge).
Crack or joint: Fracture due to tensional stress
Shear crack or joint: fracture due to shear stress
parallel to surface.
Shear zone: A wide zone of ductile
deformation(not necessarily high Temp.)
Vein: Fracture filled with mineral from hydrous
fluid.
Tensile cracks
Rocks are very weak under tension (~< 10
MPa), but are v. strong under compression
(>100 MPa). Why?
Griffith crack: cracks have v. elliptical shapes.
Remote stress
High stress concentration at crack tip depends on
crack length.
remote
= (2E/c)
0.5
E: Young’s modulus
: new surface energy
c: crack length
Hydraulic fractures (tensile cracks)
Form under high pore pressure and high
confining pressure.
Form parallel to 1
3 types of cracks
Mode I: Tensile parallel to 1.
Mode II: Sliding mode perpendicular to fracture
front.
Mode III: Scissors mode, parallel to fracture
front.
(shear cracks cannot grow in their own plane)
Faults
Type of shear fracture with displacement
Form by coalesence of small cracks during
failure.
Fault failure criteria
Coulomb failure criterion (empirical):
 = C +n
shear stress
n = normal stress
C = cohesion (= 0 for dry sand)
 = coefficient of friction
Do 3 experiments with increasing 1 and 3 until
failure. Repeat experiments with new rock.
#
1
2
3
1
175
240
355
3
25
50
100
Plot data on Mohr diagram:
Shear
stress
Normal
stress
Coulomb line is tangent to Mohr circles
Slope of Coulomb line = (coeff. of friction)
Line from center to point of tangency = 2 =
twice angle fault makes with normal to .
120/2 = 60; 90-60 = 30 degrees to 1.
Why does failure not occur at 45o to s1 (max.
shear).
Because the normal stress is too high.
Normal stress becomes low enough when angle
is 30o.
Combined Mohr-Coulomb envelope
At higher confining pressures, Coulomb line not
straight – becomes curved at higher pressures.