Micro and meso-scale modelling of the response of transversely isotropic foam to impact — a structural cell-assembly approach

Foam materials can exhibit transverse isotropy in their mechanical behaviour if their cells are elongated in the foam-rise direction during the fabrication process. Such transversely isotropic foam displays different deformation patterns depending on the loading direction. Since the deformation of foam cells is extremely difficult to observe experimentally due to their small size, a structural cell-assembly model is established via Finite Element (FE) modelling to examine all-level deformation details. A representative volume element (RVE) comprising an appropriate number of cells is established via convergence analysis, and the simulation results are analysed and compared with experimental data [1]. The cell-assembly model is able to capture the mechanical response of PU foam for compression at various strain rates and loading angles to the foam-rise direction, in terms of both the overall stress-strain response and deformation mode. Analysis of the deformation modes of cells via FE simulation indicates that loading in the foam-rise (0°) direction induces: (i) unstable buckling of cells for quasi-static compression; (ii) cell rotation+buckling in addition to cell buckling for quasi-dynamic and dynamic compression. Loading in the transverse (90°) direction generates uniform deformation via stable bending of cell struts for quasi-static, quasi-dynamic and dynamic compression. Loading at angles between 0° and 90° induces a combination of stable cell bending and cell buckling; the ratio of the amount of cell bending to that of cell buckling increases with both loading angle and strain rate. Rate-dependence of the mechanical behaviour of PU foam can also be captured by incorporating rate-sensitivity into the mechanical properties of the cell strut material.