Investigation On Stress/Force Enhancement Of The Cellular Metallic Materials

Cellular metallic materials have enormous specific bending strength and stiffness, strong energy-absorption ability, high permeability and good thermal conductivity, which satisfy their wide applications in many fields, such as aerospace, automobile, construction, packaging, ships, and protection projects. In the extensive research of their static and dynamic mechanical properties, many significant results have been achieved especially in the study of the mechanism of strain rate effect in their dynamic deformation process. However, the possibility of stress/force enhancement in cellular solid in the propagation of shock waves has been neglected, and so far, the observed stress/force enhancement are mainly in two aspects:1) under the action of impact velocity/load, the critical collapse stress and plateau stress increase with increasing of the impact velocity/load;2) the impact effect on the protected object is much more than the imposed load, viz. the output stress/force is enforced, which brings worse damage to the protected object and reverses the purpose of using the cellular metallic material as crashworthiness material. Therefore, a comprehensive and systematic study on the generation and mechanism of the stress/force enhancement is demanded.In this paper, by adopting the modified Split Hopkinson Pressure Bar, a lot of stress/force enhancement experiments were performed with three kinds of cellular metallic material (closed-cell aluminum foam, open-cell aluminum foam and hexagonal aluminum honeycomb) in two experimental configurations. The first configuration is that the bullet impacts the sample and the output bar, and the other is that the bullet and sample together impact the output bar. Next, by applying the one-dimensional stress wave propagation theory and using the rate-independent rigid perfectly-plastic locking stress-strain model and elastic-perfectly-plastic-rigid locking stress-strain model, the dynamic stresses behind the shock front in two experimental configurations were deduced and the conditions for the existence of shock wave were provided too. Thirdly, the corresponding finite element models were created by adopting the ETA Femb PC Pre-Processor.V29(ETA.Com, USA); the numerical simulation analyses were undertaken with LS-DYNA3D.970on the HP-J6750workstation; the experimental results, theoretical analyses conclusions and numerical simulation results were compared and analyzed; and the relevant parameters of the cellular metallic material were researched. Finally, the2D Voronoi random models were established to research the effects of random factors on the stress/force enhancement, because the numerical simulation was undertaken with continuous medium crushable foam and that cannot reflect the real properties of cellular metallic foam, such as the shape irregularity of cellular cell, the random variation of cell wall thickness and the random deleted cell walls.From the above studies, conclusions are drawn as follows:For a certain kind of closed-cell aluminum foam with a given relative density and cell diameter, when the impact velocity is greater than or equal to a critical speed, shock front is formed and the dynamic stress behind shock front is greater than the stress ahead, so that the stress enhancement appears. Theoretical analysis and numerical simulation indicate that the influence of the bullet's mass on the stress enhancement can be ignored when the mass ratio of the bullet and the specimen is greater than a certain value,the stress, strain and partical velocity jump exist in the open-cell aluminum foam in the wave propagation to form the shock wave, because the stress-strain curve of the open-cell aluminum foam has the characteristics of incremental hardening material and the velocity of the plastic wave with high-amplitude perturbation is greater than that with low-amplitude pertubation. Thereby, stress enhancement appears when the output stress is greater than the input stress, which brings serious damage to the protected object.For the open-cell aluminum foam, the occurence of stress enhancement defers with the increase of their thickness under the same impact velocity, but the velocity of stress enhancement remains constant. Also it is revealed in the experimental research, theoretical analysis and numerical simulation that their dynamic stress behind shock front increase with the impact velocity, just as the closed-cell aluminum foam.For the hexagonal aluminum honeycomb material, when the impact velocity is greater than a certain critical speed, stress enhancement also appears and that is related to the impact velocity and the thickness of the honeycomb material.The stress/force enhancement of'real'cellular materials under impact loads was studied with the2D Voronoi random models. By analyzing the effects of the irregularity of cell shape, the uniformity of wall thickness and the randomly deleted percentage of cell walls on the existence of stress/force enhancement and the peak stress, it is indicated that there is a critical load and stress/force enhancement appears when the impact load is greater than it; stress/force enhancement is closely related to the intensity of impact load, the irregularity of cell shape, the uniformity of wall thickness and the randomly deleted percentage of cell wall; and stress/force enhancement appears earlier than expected with the increase of impact load, and there may even be a second stress enhancement.Numerical simulation analysis by using the2D Voronoi random model reveals that when the rectangular impact load is small and irregular degree of the material is1.0, stress enforcement appears in the non-dense phase with the lasting of computation time, which is mainly caused by the random distribution of the cells in the cellular metallic foam.