17th European Conference on Fracture 2 -5 September,2008, Brno, Czech Republic Influence of the Geometry on the Essential Work of Fracture of Polypropylene Materials Ralf Lach 1,a Thomas Koch 2,b , and Sabine Seidler 3,c 1,2,3 Vienna University of Technology, Institute of Materials Science and Technology, Favoritenstrasse 9–11, 1040 Vienna, Austria a rlach@mail.tuwien.ac.at, b tkoch@mail.zserv.tuwien.ac.at, c sseidler@mail.zserv.tuwien.ac.at Keywords: Essential work of fracture, geometrical requirements, polypropylene materials, structure-property relationships. Abstract. For reasonable application of the essential-work-of-fracture (EWF) concept to polymers some geometrical requirements has to be fulfilled, whereas these conditions, especially that of plane state of stress and self-similarity of the load–displacement diagrams are often handled very non- critical in the literature. A brief discussion of minimum and maximum valid ligament length and the influence of specimen thickness on toughness has been given, therefore, by comparing data empirically determined with the predictions. Furthermore, the applications of small-sized specimens has been shown on example of ethylen-propylene copolymer. Introduction Based on fundamental studies of Broberg [1] as well Cotterell and Reddel [2], the method of essential work of fracture (EWF) as one of the main concepts of the ‘Post-Yield’ Fracture Mechanics has been first applied to polymers by Mai and Cotterell [3]. At present, the EWF methodology is widely used for highly ductile materials, mostly polymers but also other materials such as metals, paper or ductile ceramics, which are prepared in form of films or thin plates. The continuous success of the method, also manifested in a standard draft of the European Structural Integrity Society (ESIS) [4], is caused by its relative simple experimental preconditions compared to other approaches in fracture mechanics as well as the low consumption of materials and time. Recently, well-founded correlations between molecular and fracture mechanics parameters (EWF) have been detected by Chen and Wu [5] for polymers, likewise by Halary et al. [6] as well as Lach and Grellmann [7] in the case of Linear-Elastic and Elastic–Plastic Fracture Mechanics, which can provide the basis of understanding the underlying physics of polymer toughening. The EWF concept is based on the assumption that the total work of fracture W can be divided into the component W e , scaling with the ligament area B·l (B – specimen thickness, l – length of unnotched ligament), dissipated in the inner or fracture process zone, and the component W p , scaling with the volume B·l 2 , dissipated in the outer or plastic zone: 2 p p e Bl w Bl EWF W W W        . (1) After divided W by the ligament area, the specific work of fracture w is obtained: l w EWF w p     , (2) where w p is the non-essential work of fracture and  is the shape factor of the plastic zone. EWF was found to be independent on specimen configuration such as single or double edge notched tensile specimens (SENT or DENT specimens) etc. for given thickness, whereas w p is a function of the plastic constraint. EWF has the meaning of a ‘crack-moving force’ comparable to that of the 2056