Bead foams provide low density, excellent energy absorption, good damping behavior, great design freedom and low production cost. For this reason, they are widely used in different industrial applications like automotive, packaging, sportswear and construction. The properties are mainly determined by the morphological structure which is formed during the foaming process. As bead foams belong to the closed-cell materials, the entrapped air has significant impact on the material properties. According to the existing model concepts, the contribution of the cell gas to the stress response in compression-mode is negligible in the initial deformation phase, increases significantly in the plateau region and decreases as soon as the compaction region is reached. Although this concept has gained general acceptance, its experimental verification and its modelling remain open questions.
This contribution summarizes the results of a joint basic research project, which was, amongst others, dedicated to the elucidation of the cell gas role in expanded polypropylene (EPP) bead foams. In order to experimentally quantify and numerically model the influence of the enclosed air, an innovative test concept and a novel simulation strategy were developed. To this end, a servo-hydraulic testing machine was equipped with a custom-designed vacuum chamber with adapted sealing to allow testing under ambient and vacuum conditions. Loading-unloading experiments were performed and the resulting stress-strain curves were evaluated. To enable an in-depth understanding and further parametric studies, an explicit simulation approach was developed. It combines the smooth particle hydrodynamics (SPH) method, which is used to reproduce the fluid-like behavior of the entrapped air, and an FE shell model that was built with the open source software neper. Statistical volume elements were built and subjected to virtual loading-unloading tests. The presented combined experimental-numerical work opens up new possibilities for characterization and analysis of the fundamental principles of polymer foams.