notch root [11]. Fig. 5 shows COD5 and CTOD (crack “tip” opening displacement, which is determined 5 mm behind the actual crack tip; in our case this corresponds to 1.5 times the mean cell diameter) versus crack extension curves for two foams with different densities. It was found that the COD5 curves show a characteristic kink at low crack extensions (about ½ of the mean cell diameter), which can be associated with an “initial” fracture toughness value. The CTOD curves that deliver an actual fracture toughness value did not show a decrease at larger crack extensions, which is in contrast to the K vs. ∆a and J vs. ∆a curves. It seems that the possible softening mechanisms do not influence the resistance against crack propagation in terms of CTOD or crack tip opening angle, but have an impact on the general mechanical behaviour of the CT-specimens. Further detailed investigations are needed to identify the softening mechanisms and to find an approbate method for the determination of fracture toughness values in terms of J. Figure 5 COD5 and CTOD versus crack extension curves for two foams with different densities, 0.25 g/cm3 (left image) and 0.40 g/cm3 (right image). Figure 6 SEM micrograph from the region in front of the notch root of an in-situ cracked CT-specimen after a crack extension of about 6 mm (left image) and local in-cell-wall strain map from the first cell in front of the notch root at crack initiation, showing the plastic zone (right image, 4000 pixels are equivalent 3.3 mm). Strains are given in loading direction, ε=x100%, and the black lines mark the boundaries of the cells. In-situ fracture tests With a small loading device in-situ fracture tests in the scanning electron microscope (SEM) were performed to investigate the fracture processes. Loading of the specimen results in a very early, inhomogeneous plastic deformation of the foam. The strains are localised on different length scales. On the lower level, the cell walls, only some small regions in the walls are deformed, whereby the rest remains nearly undeformed. An
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