According to Linear Elastic Fracture Mechanics the stress intensity factor and the energy release rate are two fracture parameters linked by the elastic modulus and Poisson's ratio of the considered material. This concept has been extended to the analysis of linear viscoelastic materials, by introducing time-dependent quantities; it is also used for nonlinear viscoelastic polymers, even if its accuracy in this case is still an open question. In this work the Slow Crack Growth and the Environmental Stress Cracking resistance of two highdensity polyethylene grades were investigated, differing for their molecular weight distribution and fracture resistance. The description of the fracture behaviour of the two materials provided by the stress intensity factor or the energy release rate turned out to be equivalent, despite the nonlinear mechanical behaviour exhibited by the two polymers. Moreover, a time-dependent effective modulus, related to the two fracture parameters, was evaluated: its value was in good agreement with the modulus experimentally determined from tensile tests. A constant relevant effective strain was found despite the different testing conditions (i.e. applied mechanical loading, temperature and presence of an active environment), its value being equal for the two considered polyethylenes.