Failure behavior of closed-cell polymer foams - the necessity of advanced failure prediction

The requirement of advanced failure prediction and the limitation of the validation of classical failure criteria become apparent from the investigation of closed-cell polymer foams. The most common criteria, for example the von Mises stress hypothesis, neglect the influence of the hydrostatic stress. This assumption is obviously not suitable for closed-cell polymer foams, which fail under hydrostatic load situations as well. To understand the material response of the foams, the micro scale has to be analyzed. The excellent material characteristics, like a low density combined with a high weight-specific stiffness and strength, result not at the least from the complex microstructure of the foams. Furthermore, it is well known that imperfections of the micro scale influence the effective material response crucially. Consequently, a finite element model (FE model) on the basis of a tetrakaidecahedron has been implemented and adapted to the results of an image analysis. A strain-energy based homogenization concept has been utilized to numerically calculate the effective properties. The FE model also provides a prediction of the effective strength of the material. For the investigation, different failure criteria on the micro scale such as the von Mises and Pisarenko-Lebedev hypothesis are discussed in this approach. The numerical results for different load situations have been validated with experimental data. The considerable number of conducted experiments includes standard uniaxial and multiaxial tests and provides stress-strain curves and critical failure stresses. The present approach focuses on the material RO-HACELL® IG-series (industrial grade), produced by Evonik Industries AG, Germany.