The essential work of fracture method (EWF) to evaluate the fracture toughness of ductile materials in plane stress with prevalent gross yielding conditions was originally developed by the Cotterell-Mai research group in Sydney University more than ten years ago. Since then, the EWF method has been proven independently by many researchers to be a valid tool for a range of ductile metals, polymers, paper sheets and fibrous composites. The current interest in the technical community of the plastics industry is to extend the EWF methodology to high-rate impact testing of polymers and their blends and to determine the impact specific essential work of fracture. Since the literatures related to the applicability of this methodology under dynamic loading conditions appear controversial, our purpose in this work is to clarify under what conditions the impact EWF method is applicable. The EWF methodology was applied to the energy impact data obtained on two PP copolymers and one ABS polymer at different specimen thickness and geometry. The results showed that the EWF method can be applied to high-rate testing provided the ligament is fully yielded and the plastic zone is scaled with the square of the ligament length, When these conditions are fully satisfied, the impact specific EWF is a material constant independent of specimen geometry for a given sheet thickness in plane stress; and it is also invariant with specimen thickness when plane strain conditions are met.