Compressive Characteristic Analysis Of Graded Foam Under Impact Loading

Porous foam metal with excellent physics properties, are widely used in sports, traffic, aviation and other fields. Compared with properties of uniform foam, recently, many scholars put forward a new structure material, which has different density and structure, and it is also defined as graded material. Then graded material is analyzed by using the theory and the numerical simulation. It is shown that the gradient model presents good mechanics performance in certain aspects, such as energy absorption and the ability to resist impact, etc.In this paper, we generates closed-cell foam model by using the 3d-Voronoi technology, combining with the finite element method simulation, the compression properties and deformation mechanism are investigated in different impact velocity. The results showed that the mechanism of foam model is presented almost the same under low impact velocity, it means that the first deformation occurred in low density area. With the improvement of impact velocity, the negative graded foam presents the different deformation pattern on both sides. Through the analysis by nominal stress-strain curve of negative gradient model and the characteristics of deformation on both sides, we propose a new definition local densification strain method, and studied the relative density and the density gradient effects on it. By introducing densification factors, we identified the critical impact velocity. In the process of high speed impact load, different kinds of foam metal are presented the same deformation characteristics, the first local densification is generated at the impact end, then the local densification gradually spread to support end until the final compaction. And based on uniform foam model, the graded model is deduced graded shock wave model under high strain rate, and compared it with the simulation results, in order to prove the feasibility of the theoretical derivation.In the end, the comparison between the initial peak stress and energy absorption ability at different working conditions are dissussed. At the beginning of the deformation of foam metal,the positive density gradient foam(density becomes bigger and bigger from the impact end to supported end) can effectively protect the impact objects, and the negative density gradient foam(density becomes smaller and smaller from the impact end to supported end) can effectively protect supported objects by contrasting the nominal stress-strain curve and deformation modes. The foam which the two sides of the model had a smaller density and the middle of the foam had a bigger density inherited the merits of the positive gradient and negative gradient foam not only can protect supported objects, but also can protect the impact object. Finally, the effect of the different structural foam on energy absorption ability was discussed.