Dynamic compressive behaviour of auxetic and non-auxetic hexagonal honeycombs with entrapped gas

This work aims to study the uniaxial dynamic compression response of hexagonal honeycombs with different cell morphologies in the presence of an entrapped gas. A theoretical model is proposed to estimate the dynamic crushing strength of non-auxetic honeycombs while including the effect of the entrapped gas on the crushing process. The theoretical predictions are shown to agree well with the finite element (FE) simulations. From the numerical simulations, Hugoniot relations between the shock velocity and the impact velocity are obtained for various honeycomb geometries. It has been observed that shock velocity varies almost linearly with impact velocity. Using this fundamental relation, we derive the stress-impact velocity Hugoniot from the conservation law of momentum. The dynamic stress-strain states of a regular hexagonal honeycomb obtained from the FE simulations show a good agreement with the Hugoniot predictions. It is shown that the dynamic stress-strain states for various impact velocities lie on a unique curve, which is different from the quasi-static stress-strain response. The local strains behind the shock front are significantly lowered in the presence of an entrapped gas, and the stresses behind the shock front are higher as compared to the case where there is no entrapped gas. The variation of the plateau stresses with the cell morphology has been explained, and correlated to the energy absorption capacity of the honeycombs. A new method to characterize the energy absorption capacity of honeycombs is proposed, and the performance of various honeycombs has been compared through the dissipation performance parameter.