Closed-cell bead foams are excellent materials for lightweight applications with specific demands in energy dissipation during impact and crash events. Their use in numerous general engineering, automotive and safety equipment applications inevitably leads to loading scenarios with repeated loading and unloading. Closed-cell bead foams are characterized by a hierarchical geometric structure that presents challenges for statistical reconstruction and finite element modeling. Understanding the complex interaction of micro-structure, and base material regarding stiffness, energy dissipation, damage, and failure under static and cyclic loading is in focus of ongoing research. To understand the macroscopic deformation behavior and to validate micro-scale numerical models, the local microscopic deformation behavior must be analyzed.
A versatile and powerful non-destructive method for digitizing, visualizing and analyzing the three-dimensional internal foam structure even under deformation is X-ray computed tomography (xCT). At TU Dresden ILK an in-situ x-ray computed tomography system is located, consisting of a screw-driven material test rig with a maximum capacity of 250 kN and 2000 Nm and a Finetec micro-focus xCT installed between the columns of the test rig. This setup allows mechanical tests on specimens and structures in tension, compression and torsion or processes (e.g., joining techniques, processing of materials) and tomographic imaging is performed in parallel.
In this work, three EPP bead foams with closed cells and different densities are investigated. Cylindrical specimens are extracted from steam-chest molded plates. Circular 3D-printed pads are bonded to both sides of the specimens to ensure parallel and repeatable load introduction. The specimens are loaded to selected deformation states and scanned with xCT while holding the stage. After reconstruction, the resulting image stacks are analyzed regarding the cell morphology using standard algorithms provided by the open-source package distribution FIJI. From the statistical analysis of the cell structure global and selected local deformation mechanisms are revealed. It is shown that morphological parameters such as cell volume, sphericity, mean axis orientation, etc. are influenced by compression deformation. Due to the structure-property-relationship, these results also suggest changes in anisotropy, stiffness, and damping behavior, supporting experimental results from mechanical tests.