Among the formulas used to represent the cracking speed with respect t
o the stress amplitude, the Paris formula: da/dN = C Delta K-m is cert
ainly the most widespread, Even though many studies have tried to find
a relationship between the coefficients m and C of this equation, con
troversies still exist. We show that this relation exists but does not
have the general character given to it in the literature, and that it
depends on the material, We then study the influence of various facto
rs: temperature, heat treatments and the load factor, before trying to
make a physical interpretation and to show the practical interest of
this approach.