Crushing of a closed-cell polymeric foam under triaxial loading

The crushing behavior of a polymeric closed-cell foam with a relative density of 0.077 under combined axial compression and external pressure is investigated experimentally. The foam has a nearly monodisperse polyhedral microstructure with mean cell size and wall thickness of 0.50 mm and 0.0348 mm respectively. A custom triaxial apparatus is used to compress cylindrical specimens at different levels of external pressure. A typical axial stress-displacement response exhibits a stiff elastic branch that terminates into a maximum beyond which deformation localizes into a horizontal axisymmetric band of crushed cells. The band then propagates axially with the stress remaining essentially constant. Both the initial stress maximum and the plateau stress decrease linearly as the pressure increases. Micro-computed tomography imaging of the microstructure of a specimen crushed axially at zero pressure confirmed that during the stress plateau, a highly crushed zone of cells with an average strain of about 50 % coexists with zones of essentially undeformed cells. Above a critical pressure, the mode of instability switches to predominantly lateral contraction that evolves into a neck. This is also the mechanism of failure under pure pressure. The localized crushing behavior observed is similar to that of lowdensity open-cell foams and the results should inform and guide further development of homogenized models for this class of materials.