Generally, reliable mechanical investigations are required for the evaluation of quality and durability of fracture sensitive components. In this context, strain energy release rate and J-integral are standard descriptions to investigate samples based on nonlinear elastic fracture mechanics. In the present publication, a novel approach to determine the required parameters is introduced. This formulation is implemented as an effective pure post-processing procedure of a finite element simulation. The approach is not restricted to elastic properties and the extension to inelastic behaviour of rubber, i.e. softening, viscoelasticity and elastoplasticity, is an active field of research.