FRACTURE SURFACE ROUGHNESS The fracture surface roughness displays a sharp change at branching, visible in the left and right parts of Photo 2. Optical microscopy reveals here a relatively coarse surface texture a few millimetres before branching (left) and a finer texture after branching (right). Figure 4 shows atomic force microscopy images of a fracture surface (left) a few millimetres before macroscopic crack branching, which corresponds to a high fracture surface energy and (right) just before crack arrest, which corresponds to the lowest fracture surface energy. It can be seen that also on a microscopic scale the surfaces are rougher prior to crack branching than before arrest at a similar crack speed. One notes further that the fracture surface does not pass through the rubber particles of RT-PMMA. Photo 2: Optical micrographs showing the roughness change at branching: propagation from left to right in an RT-PMMA specimen of thickness 2 mm, (left) 9 mm before the onset of crack branching and (right) 7 mm after crack branching. Figure 4: Atomic force microscopy images of a fracture surface: (left) rough surface before crack branching, åm > 500 m/s, and (right) smooth surface just before crack arrest, åm > 500 m/s. Since the crack forms branches even though the experimental crack speed and the energy released inside the specimen remain quasi constant, two or sometimes three branches must consume the same energy as a single crack propagating at the same speed. The variable parameter at crack branching is thus the roughness of the fracture surface, which means the total surface created (SFt). As this may be approximately the same for two smooth crack branches as for a master rough crack, the usual planar crack surface (crack length increase multiplied by width, B ∆a) cannot be employed. In the case of a smooth crack created at speed åmb, as in Figure 4 (right), the fracture energy is GIDmin. If the energy release rate is greater than GIDmin, the propagating crack may produce microscopic instabilities since the inertial effects are sufficient to allow crack branching. These small instabilities are themselves 4
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