Dynamic behavior of anisotropic polystyrene foams

Microstructural anisotropy plays a crucial role in determining the qualitative and quantitative mechanical properties of polymer foams. Moreover, their behavior at high strain rates remains an open question due to the limited studies on the topic, which poses a limit in the design of impact absorbing foam-made components (e.g., helmets). In this work we study the influence of microstructural anisotropy on the dynamic behavior of transversely isotropic closed cells polystyrene foams. We first produce isodensity foams with different degrees of microstructural anisotropy by using the gas foaming technique. We characterize the microstructure of the cells of the produced polystyrene foams by analyzing 2D images of the microstructure and computing the statistics of the shape indicators of the cells. We then characterize their dynamical behavior on split Hopkinson pressure bars at an average strain rate of 835 s^-1. Dynamic results are compared with quasi-static ones and confirm similar findings to the literature. We report a different strain rate sensitivity for the different foams. In particular, a higher sensitivity for foams when cells are aligned with the loading direction. Therefore, the strain rate sensitivity of the foam can be associated with the microstructural distribution of cell and the strain rate sensitivity of the base material.