DYNAMIC FRACTURE SURFACE ENERGY VALUES AND FRUSTRATED MICRO-BRANCHES DURING RCP IN AN IMPACT MODIFIED POLYMER C. Fond and R. Schirrer Institut Charles Sadron, 6, rue Boussingault, F67083 Strasbourg. fond@ics.u-strasbg.fr, schirrer.ics.u-strasbg.fr, http://ics.u-strasbg.fr/ ABSTRACT The fracture behaviour of materials for which the fracture energy decreases near the Raleigh wave speed, cr, is investigated using an impact modified polymer. An experimental device based on strip band geometry has been designed to explore the brittle behaviour of such polymers during rapid crack propagation (RCP). The macroscopic crack speed is found to be quasi-constant along an entire rubber toughened polymethylmethacrylate (RT-PMMA) specimen, even in the case of crack branching and until arrest, if any. As the material behaviour tends to accelerate the crack – the fracture energy decreases near cr - whereas mechanical inertial effects tend to limit the rate of crack propagation – due to crack branching -, the macroscopic crack speed stabilizes at approximately åmb = 0.6 cr, which is the macroscopic crack branching speed for RT-PMMA. Consequently, at the macroscopic crack branching velocity, the experimental fracture surface energy and the fracture surface roughness have no single value in such materials. In fact, the macroscopic fracture surface energy value increases with the number of instabilities or frustrated micro-branches. INTRODUCTION While the dynamic fracture behaviour of materials for which the fracture energy increases near the Raleigh wave speed, cr, has been largely explored and is now relatively well understood – the speed of the cracks decrease at crack branching – the case of materials for which the fracture energy decreases near cr is still to be investigated on real materials. Most polymeric materials exhibit a brittle fracture mode at high crack propagation speeds and experiments have revealed that crack propagation in rubber toughened-polymethylmethacrylate (RT-PMMA) is unstable between approximately 0.001 and 0.6 cr, where cr is the Raleigh wave speed. As the fracture surface energy decreases with increasing crack speed, the propagation velocity jumps from 1 to about 600 m/s when an increasing load initiates propagation of a crack. Crack branching occurs, as expected, at a crack tip speed of nearly 0.6 cr. However, unlike in many other polymers such as for instance PMMA, the macroscopic crack speed (åm) does not change after branching during rapid crack propagation (RCP) in RT-PMMA. In fact, at nearly 0.6 cr, inertial effects modify the crack tip stress field and induce branching [1]. It is noticeable that branching is made easier by the rubber toughening particles, which perturb the micro-mechanical fields. Subsequently, as the material behaviour tends to accelerate the crack whereas mechanical inertial effects tend to limit the rate of crack extension, the crack speed stabilizes at approximately åmb = 0.6 cr, which is the macroscopic crack branching speed for RT-PMMA. 1
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