The Forman equation accounts for the stress ratio, and can be written in the following form
where 
is a critical level for the Stress Intensity K,
corresponding to unstable fracture. This is called the fracture
toughness. 
is a level below which no damage occurs;
for some materials this is zero, but for many ferrous materials it
is positive.
Figure: Graph of 
versus 
. Forman
Equation.
The graph in Figure 
of this function shows that
there exist two limits such that
and
It is clear from this that there is a lower limit to ,
below which no crack growth takes place, and an upper limit,
around which growth is explosive.
Finally, with regard to the Forman Equation, it has been shown to give good results for crack growth in aluminium alloys and steels, but not in many other materials.
Other empirical models appear in the literature; for example [44] via the ``Famous Robder Law'' demonstrate, partly in jest, how common practice it is to propose new models to fit data within this field. There are very many adaptations to well known laws which are not mentioned here.