For what kind of polymer is the toughness assessment by the essential work concept straightforward?

The essential work of fracture (EWF) concept seems to be a proper and easy way to determine the inherent fracture behavior of ductile polymers. Unfortunately, all experiments reported until now were performed on polymers which did not meet the basic requirement of this fracture mechanical approach, viz. full ligament yielding prior to onset of crack growth. This problem often resulted in wrong conclusions and useless discussions on the specimen preparation, including notching techniques. By using tensile-loaded deeply double-edge notched (DDEN-T) specimens of amorphous copolyesters (aCOP) it was demonstrated that they are, in fact, the optimum choice to push forward the EWF concept for ductile polymers. Full ligament yielding before crack growth was evidenced by infrared thermographic (IT) frames taken during the loading of the DDEN-T specimens. The yielding ''marked'' with a load-drop in the corresponding load-displacement curves, enabled to split both the specific essential and non-essential work of fracture into their contribution terms: yielding and necking incl. fracture, respectively. It was argued that this EWF approach is most straightforward for the toughness description of such amorphous polymers that undergo necking by (multiple) shear banding without considerable strain-hardening, as aCOPs do.