where W is the width. The stress near the hole is something
like: 
For a hole of length L (very ellipsoidal), with radius of curvature
at the ends, R, the energy of the distorted stress region is 
. Then
. The minimum R is on the level of the atomic size. This means 
.
This is too low. creation of cracks involves surface tension. propagation
condition is 
. Then 
= griffith crack length. 
. This means 
. Surface
is actually jagged, increasing the real crack length. 
=work of fracture
which is typically, 
. Then 
sometimes, but the crack does not propagate.
propagation condition: 
.
, then 
, then 
There should really be a kinetic term, because the cracks can propagate at the speed of sound.
for 
you get the griffith crack length.
. Crack can accelerate over time. Stresses tend to be perpendicular
to the crack. Instability in crack growth is onset at 
.