Figure 1 : Schematic representations of (A) tensile (mode I)
fracture, (B) shear fracture (mode II); in both A and B the crack faces
formed behind the leading edge (crack tip) are stress-free. (C) Shear
fracture with a frictional interface; a frictional residual shear
stress, \(\tau_{R}\), remains in the wake of the fracture tip. In all
three cases the elastic energy flowing into the tip is focused to a
stress singularity of the form \(\sigma=\frac{K}{r^{1/2}}\) where K is
the stress-intensity factor and r is the distance from the
tip. This stress-field is shown schematically in D (for cases A and B)
and in E for case C.