On the valid ligament range of specimens for the essential work of fracture method: The inconsequence of stress criteria

The Essential Work of Fracture (EWF) method is widely used for the toughness determination of thin, ductile sheets under quasi plane-stress conditions. To define the valid ligament range of tests empirical role of thumbs and theoretical approaches are generally used. The lower ligament limit is typically ascribed to the “plane-strain/plane-stress transition”, which is accompanied by an increase in maximum net-section stress. Nevertheless, during the measurements neither the assumed plastic-rigid behavior nor true geometric similarity exists, which unfits the theoretical stress criteria based on these approximations. In the present study several materials were considered to explain the experimental results and to clarify the possible ambiguities related the observed maximum stresses.