Plastic failure analysis of an auxetic foam or inverted strut lattice under longitudinal and shear loads

Auxetic materials possess negative Poisson's ratios. As such, they can be applied in situations where traditional materials perform poorly or cannot perform. We investigate the plastic failure of a 3D auxetic strut lattice under uniaxial and transverse loads in order to complement ongoing research in miniaturized strut-based sandwich cores. The chosen lattice is also representative of an auxetic foam. Plastic failure models derived with respect to two physical parameters (packing parameter and relative density) which control the negative Poisson's ratio compare well with numerical data. Microscopic failure modes differ depending on the loading state: shear failure is due to global plastic yielding while plastic localization occurs under uniaxial loads. This observation suggests among others that it is advisable to use auxetic cores when structural softness under normal loads and hardness under transverse loads are both critical design conditions.