The essential work of fracture method is widely used for the determination of fracture toughness of ductile metals and polymers under plane stress conditions. Nevertheless, this method has numerous prerequisites, which are easy to fulfill in metals, but are less certain in polymers. The aim of this study was, therefore, to present a simple, empiric "displacement-criterion" that helps the determination of valid ligament lengths for polymers-one of the biggest sources of error of this theory. This criterion not only facilitates the definition of a lower bound, but also helps the description of an upper ligament limit. Its other advantage is, that combined with a stress criterion, it helps the data validation not only until crack initiation, but also over the entire deformation process.