Rigid polymer foams are used in a broad range of structural applications. The mechanical behavior of foams is governed by the properties of the base material, the relative foam density (ρ), and the morphological features of the cellular structure . Clearly, the first two, namely material type and relative density, have the highest impact on the mechanical properties. Nevertheless, the cellular morphology, too, can have a strong influence on the resulting macroscopic properties. Foams with complex cell morphologies usually have anisotropic mechanical responses. Understanding the relations between cell morphology, cell deformation mechanisms, and the mechanical properties in such foams will enable future foam generations with even better performance to weight ratios. In the present thesis, structure-properties correlations at different length scales are established for strand PET foam in a density range of 80 to 200 kg/m³. Strand PET foam can be described as a foamed honeycomb, filled with foam. It is shown that the cell morphology of strand PET foam has huge analogies to that of wood, since a fraction of cells are highly elongated and oriented through the panel thickness. The combination of both honeycomb- and wood-like morphology results in strong anisotropy in mechanical properties. Following quantitative morphology analysis, the multi-scale compression response of strand PET foam (T92.100) is identified and discussed. The full-field crush patterns of the foam specimens were correlated to the local cell deformation mechanisms. Two fundamentally different cell deformation mechanisms were observed. When loaded in the out-of-plane direction, the interlocked elongated cells activate a strong axial deformation response, followed by local plastic buckling of the cell walls. This behavior was manifested as several irregular shear bands throughout the specimen thickness. On the contrary, the cell deformation mechanism in the in-plane loading direction is governed by a weaker plastic bending of the cell walls and struts. The full-field deformation maps in the in-plane loading were orderly and regular, with geometrical similarities to hexagonal shapes. A two-stage plastic yielding of the foam cells located in different strand regions caused this regularity in strain fields. Different cell level deformation mechanisms (buckling or bending) also left distinct signatures on the global stress-strain curves. For example, the axial deformation and buckling of the elongated foam cells cause a post-yield softening response in the out-of-plane stress-strain curve. In the next step, the findings are extended to other foam densities. The fitted curves to the property-density data had different slopes for the out-of-plane and in-plane loading directions. This also confirmed, from a completely different experimental approach, that the cell deformation mechanisms in the out-of-plane and in-plane loadings are stretch-dominated and bending-dominated, respectively. The findings from both density-scaling approach and the in-situ deformation measurements agreed well and verified the hypotheses. Last but not least, it is demonstrated that in the high density PET foam (200 kg/m³), the measured mechanical properties are lower than the predicted values using the density scaling laws. The reduction of cell aspect ratio at this density creates a shift in the deformation mechanism. This makes the macroscopic measurements deviate from the theoretical predictions.
|School:||Universitaet Bayreuth (Germany)|
|Source:||DAI-C 81/4(E), Dissertation Abstracts International|
|Subjects:||Mechanics, Materials science, Polymer chemistry|
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