中文摘要：

运用三维Voronoi技术生成闭孔梯度泡沫模型，结合有限元分析方法模拟负梯度闭孔泡沫金属在不同冲击速度下的力学行为．结果表明，随着冲击速度的提高，得到了与均匀泡沫一样的三种变形模式：准静态模式，过渡模式和冲击模式．通过对名义应力应变曲线和变形模式的研究，提出了一种新的定义局部密实化应变的方法，并研究了相对密度和密度梯度对它的影响．分别建立了相对密度和密度梯度与冲击速度的变形模式图．通过引入密实化因子，确定了三种变形模式对应的临界冲击速度．最后讨论了不同冲击速度下，密度梯度大小对泡沫材料能量吸收能力的影响．结果表明，在高速冲击的变形初期，密度梯度的绝对值越大，泡沫材料的能量吸收能力越强．

英文摘要：

Metal foams are widely used as advanced lightweight construction or kinetic energy absorbers in many industrial fields. Graded metal foam is becoming a research hotspot due to its outstanding designability. The dynamic compressive mechanical behavior of closed-cell aluminum foam with continuous negative density gradient under different impact velocities was investigated using the finite element software ABAQUS. First, the random 3-D Voronoi technique was employed to construct graded closed-cell foam models. Then different impact velocities were applied at the impact end of foam along the negative density gradient direction. Like the uniform foam, three deformation modes, i.e. quasi-static mode, transitional mode and shock mode were observed with the increase of impact velocity. The densification factor was introduced to define the critical velocities of mode transition. A new method was proposed to define the local densification strain by contrasting the nominal stress-strain curves with deformation modes. This method took the effects of relative density and density gradient into account. Deformation maps of impact velocity versus relative density and density gradient were respectively presented for the graded foam. Finally, the effect of density gradient on energy absorption ability was discussed. The finite element simulation results indicated that the first critical velocity was insensitive to the relative density, while the second critical velocity increased with the increase of relative density. The critical velocity decreased with the increase of density gradient. It was found that the smaller the absolute value of density gradient was, the more energy the foam would absorb at the initial stage of deformation under low impact velocity; and the larger the absolute value of density gradient was, the stronger energy absorption ability of the foam material at the initial stage of deformation under high impact velocity would be. These research results could be applied to the design of optimal energy adsorption s