Mechanics of Notched Izod impact testing of polycarbonate

Abstract

Polycarbonate is widely used as a transparent protective material because of its low density and excellent mechanical properties. However, when defects such as cracks or notches are introduced, it is subject to catastrophic brittle failure at relatively low loads. Notched Izod testing is a common qualitative measure of toughness of a material, measuring energy absorbed prior to failure under high triaxiality and high rate loading conditions. Much research has been done using Izod testing to compare the fracture energies of blends of Polycarbonate and rubbery materials; however the specific yielding and fracture mechanisms associated with each blend are rarely analyzed. This study presents detailed images, fracture energies, and time durations of the deformation and failure processes actively occurring during the Notched Izod testing of 3.23mm and 6.35mm thick Polycarbonate specimens, as well as of a quasi-static version of Notched Izod bending. The thin specimens were found to yield in a ductile manner followed by tearing across most of the ligament width, resulting in a final failure including a small plastically-deformed ligament hinging the two failure surfaces in both the Notched Izod impact and Quasi-Static tests.

(cont). The thick specimens exhibited slight yielding followed by catastrophic failure, where the crack initiated ahead of the notch and then propagating back towards the notch root as well as across the remaining ligament.. In the thick Izod tests local pre-failure yielding was evident at the notch root resulting in extensive blunting of the notch. The fracture energies per unit thickness for the thin specimens were almost a full order of magnitude larger than those for the thick specimens. A finite element simulation for the Notched Izod Impact test was developed using the Arruda and Boyce(1988) constitutive model of polymers as modified by Mulliken and Boyce(2004) for high rate deformation. The 3.23mm Notched Izod impact test was successfully modeled from initial contact of the pendulum through initiation of failure and early tearing. The yielding patterns and failure occurred along the same lines as in the experiment where diagonal shear bands and lobes initiate plastic deformation from the notch tip and tearing progresses in a horizontal manner across the specimen width. An extensive shear yielded region is observed ahead of the propagating tear. The 6.35mm thick model shows the beginning of the formation pressure concentration which causes brittle fracture, but further refinement of the mesh needs to be performed for more accurate modeling.