APPLICABILITY OF THE LOAD SEPARATION CRITERION AND THE NORMALIZATION METHOD TO HIGH-RATE J- TESTING OF DUCTILE POLYMERS

The load separation criterion, which assumes that during the fracture process of a cracked body the load can be represented as the product of two separate functions, a geometry function and a material deformation function, was previously studied in stationary crack tests and then extended to growing crack experiments on both metallic and polymeric materials. On the basis of the load separation criterion a single specimen method, labelled normalization method, was developed for determining JR curve of ductile materials. According to this method, firstly used in testing metallic materials and then extended to polymers, JIc is estimated by performing only two tests and, through the experimental determination of the material key curve, JR curve is drawn. This paper examines the applicability of the load separation criterion and the normalization method in determining JR curve of a toughened polyamide 6/6 at high loading rates (1 m/s). The analysis of procedure problems associated to this high experimental rate is performed. The results obtained using the normalization method are then compared with those measured via multi-specimen testing procedures proposed by ESIS Technical Committee 4. The results show that, unlike low loading rate tests, the presence of the oscillations in the load vs displacement traces, due to the inertial effects produced during the impact, complicate considerably the elaboration of the data, with particular reference to the identification of the separable blunting region. The comparison of JIc values obtained according to the different procedures examined indicates that the values of JIc=J0.2 (taken at 0.2 mm crack growth) are in good agreement, whereas consistent differences among the values of JIc=J-blunting (taken at the blunting line) are observed.