In-plane dynamic crushing of honeycomb. Part I: crush band initiation and wave trapping

In this paper the dynamics of crush band initiation and wave trapping that result from in-plane impact on a honeycomb are analysed using finite element simulations. The honeycomb structures were loaded in compression at the top surface with a prescribed velocity. Two different boundary conditions were considered; these produced an approximation to a state of uniaxial stress or uniaxial strain at an early state of deformation. The simulations proved that elastic wave propagation had an important effect on crush band initiation, since loading and unloading parts of the compressive wave can reinforce or delay crushing, respectively. Stress enhancement with increasing impact speed is mainly due to translational micro-inertia and not due to micro-rotational inertia. This effect is stronger for uniaxial strain than for uniaxial stress. For the specific case examined here, stress enhancement begins for uniaxial strain at an impact speed of about 1.0 m/s whereas for uniaxial stress it begins at about 10.0 m/s. For the analysed honeycomb the simulation showed for both boundary conditions the existence of a critical impact velocity, so that for impact speeds larger than this critical speed a crush band initiated at the impact surface. For much smaller speeds the location of the initial crush band was determined by the distribution and extent of initial imperfections. In the case of uniaxial stress an analytical estimate of the critical impact speed was derived using the theory of wave trapping. The critical speed depends only on the ratio of the length of the ribbon wall to the length of the thinner inclined wall and the material properties of the constituent material. For the aluminium honeycomb investigated in this paper, wave trapping is predicted to occur for impact speeds greater than 7.6 m/s which is close to the critical speed calculated with the finite element simulations.