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 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.