Fracture mechanical methods describe the local failure behavior very precisely and allow a simple comparison of materials and their molecular structure. One of these methods, derived from the field of post-yield mechanics, is the essential work of fracture (EWF). The investigation of the EWF in combination with the digital image correlation is so far mostly limited to an one-sided camera setup and the verification of the complete plasticization of the fracture process zone. In this work, the fracture behavior of polyethylene terephthalate (PET) and glycol modified polyethylene terephthalate (PETg) was investigated using the EWF method in combination with digital image correlation. Using a novel four-camera setup, it is now possible to combine information from two independent two-camera systems, which do not have an intersecting field of view and therefore no information from each other, to obtain a three-dimensional evaluation of the deformation of the specimens. PET has demonstrated a higher specific essential and non-essential work of fracture compared to PETg. Furthermore, it was possible to establish a more accurate method for separating the non-essential work of fracture and the associated form factor by the image correlation. Thus, it could be shown that, in the case of polyethylene terephthalate, the process of crack initiation is less affected by the molecular structure than the process of crack propagation. It could further be shown that the deformed volume was significantly higher in the case of PETg compared to PET and therefore the non-essential work of fracture showed lower values.