A NEW WAY FOR POLYMER CHARACTERISATION USING A COMBINED APPROACH LEFM - PLASTIC ZONE CORRECTED LEFM

摘要

Toughness quantification of polymers is still an open issue, since no unique fracture mechanics method is able to provide intrinsic crack propagation resistance values over the whole range of material behaviours from brittle to ductile. A sensitive way to characterise the mechanical performance of these materials, exemplified with rubber modified isotactic polypropylenes, is to determine their ductile-brittle transitions as key-factor for material ranking. A conventional Linear Elastic Fracture Mechanics (LEFM) is often used for data reduction. Although being strictly reserved for brittle fracture assessment, it constitutes a guaranty for continuity of analysis over several decades of test speeds. Nevertheless, material descriptors such as K, the stress intensity factor, provided by this method are geometry dependent in the ductile range. To overcome this difficulty, an original approach, which takes into account the high amount of plasticity in tough polymers, is proposed. It consists of determining experimentally the size of the plastic zone developed at a crack tip using specimens of different ligament lengths. It was shown to provide intrinsic values over a large range of test conditions, except near the ductile-brittle transitions. A combined approach "conventional LEFM-plastic zone corrected LEFM" is therefore necessary to apprehend accurately the fracture behaviour of the studied blends.