Field of Expertise: Advanced Material Science
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Field of Expertise: Advanced Material Science | |
Fracture behaviour investigation in polymers by 3D-reconstruction of cracks The fracture of polymers, especially of polypropylene, is an already widely investigated topic. By far the majority of these investigations are however based upon parameters and characteristics resulting from mechanical tests, e.g. stress–strain diagrams from tensile tests. Additional information can be gained from micrographs recorded from the fracture surfaces by light and electron microscopy. But all these experiments and results provide only limited insight into what is happening in the material during the tests at a microscopic length scale. Performing such experiments in a microscope and observing with high magnification what is happening at the crack tip or at the surface of the specimen might be one possibility to overcome this obstacle [1, 2]. Additionally the combination of mechanical tests with other methods like acoustic emission analysis can help to elucidate some of the micromechanical processes [3]. But even this does not give information about the structures forming in the bulk of the specimen during the tests. To this aim tensile tests were stopped at a predefined elongation, the specimens were removed from the stage and sectioned by ultramicrotomy. However, as Figure 1 demonstrates, from a single image a clear-cut determination of the length of the cracks is not possible. Since vital parameters like area of the fracture face or its texture and roughness can help to assess the energy necessary for the creation of the cracks and to gain knowledge about the fracture behaviour at the microscale, 3D-reconstructions of the fracture region are imperative. Isotactic polypropylene (iPP) samples modified with ethylene propylene rubber (EPR) particles of different sizes were subjected to a tensile test, with the test stopped at a predefined strain of around 25% yield. After removal of the sample from the tensile stage part of the fracture region was extracted and stained in RuO4. Thereafter automated slicing and imaging of the fracture region was performed by use of the in situ ultramicrotome 3View® (Gatan, Pleasanton, CA) mounted in an environmental scanning electron microscope (ESEM) Quanta FEG 600 (FEI, Eindhoven, NL). Finally the resulting stack of images was used for 3D-reconstructions of the cracks and particles (Software: Avizo Fire, FEI, Eindhoven, NL). A 3D-reconstruction of a fracture surface can be found in Figure 2. |