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Mechanical Properties of Strand PET Foams at Different Length Scales

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. The combination of both honeycomb- and wood-like morphology (manifested as elongated cells through the panel thickness) results in strong anisotropy in the mechanical properties. Two fundamentally different foam cell deformation mechanisms were observed: a strong axial deformation response followed by plastic buckling of the cell walls when loaded in the out-of-plane direction, and a weaker plastic bending response when loaded in the in-plane direction. 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. Furthermore, 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.

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Author
 Amir Fathi

Amir Fathi
Fakultät für Ingenieurwissenschaften
Universität Bayreuth

Information

Free keywords: Polymer Foams, Strand Foams, PET Foams, Compression properties, Anisotropy, Cell morphology, Digital Image Correlation (DIC)
Institute / chair: Lehrstuhl für Polymere Werkstoffe der Universität Bayreuth
Language: English
Technical consultant for expert services: Professor Dr.-Ing. Volker Altstädt, Professor Dr.-Ing. Alois K. Schlarb
Publication year: 2018
Provider: Wissenschaftlicher Arbeitskreis Kunststofftechnik (WAK) / Kunststoffe.de

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