Research On Dynamic Mechanical Response Of Cellular Metals And Temperature Dependency

The cellular metal is a type of structural material. In research of its mechanical properties, usually there are some difficulties existing in the experimental data, such as data scatter, non-apparentness of the regularity of results, and the confusion of concept. In this paper, the strain rate effect of cellular metal under different ambient temperature, relevant factors, and stress heterogeneity caused by axial inertia effect are studied in detail from experimental method with improved SHPB equipment and numerical methods.The influence of density dispersion of the foam material on the experimental results is analyzed in the experiment. The experimental results show that the experimental data curve with approximate density have better repeatability when the specimen density distribution is similar to normal distribution. In addition, the technique of large diameter (φ37mm) quartz crystal slice is used to inverstigate the stress uniformity of low impedance and large cell material in SHPB experiment. The results indict that the stress non-uniformity of sample increases and the wave effect is more apparent with the increase of the thickness.Using ABAQUS finite element software the strain rate effect of two typical structures are investigated. It is can be concluded that for both Type I (ring) structure and Type II (folded plate) structure if there exist buckling of the structure, it must have the strain rate effect. The static and dynamic experimental results present that the mechanical behavior of this material shows the phenomenon of’stress drop’. For this large cell foam materials this phenomenon is caused by the collapse of cell aperture which lead to instability. While the collapse and instability must relate to rate sensitive.The change of the mechanical properties of aluminum foams are obtained with the change of temperature from the experiments. The experimental results show that aluminum foam has a soften effect, that is the mechanical properties of the material change from hard to soft, and from frangility to ductility. In addition, the strain rate sensitivity of aluminum foam also increases with the temperature increases. At the low temperature range (-50℃-200℃), the properties of deformation of matrix material closed to that of solid. The stain rate effect is not significant variation with the variation of temperature. While at the higher temperature range, the properties of deformation of matrix material closed to that of fluid, the strain rate effect is more apparent.At the same time, a type of visualization high temperature furnace based on SHPB device is designed. And then the deformation characteristics of aluminum foam under high temperature and high strain rate is observed with a high-speed photography. At low temperatures, there are more buckling, tearing and other deformation of cell structure in the sample. While at high temperature, the deformation characteristics is mainly the plastic bending of cell wall.Using improved Hopkinson bar experimental device the stress-time curve of impact end and support end of aluminum foam under the impact process. And the deformation process of specimen at different impact velocity is observed by high-speed photographic. The stress at both ends have same magnitude under low velocity. The deformation mainly is random collapse of cell structure and shear collapse, which correspond to the quasi-static model. While under the high velocity, the deformation of specimen starts the impact end and propagate from impact end to support end with compaction wave speed. Under this condition, the stress of impact end significantly greater than that of the support end, which correspond to the impact model. The stress uniformity of two ends became worse with the increase of impact velocity.The experiment results revealed that the stress-time curves of two ends are unrelated to the thickness of specimen in the impact model. But it related to the density of specimen. The stresses of two ends are closer with the increase of the density under the same impact velocity.At last, a single impact Hopkinson is preliminarily investigated. The stress of impact end and support end at high temperature and high speed (≤26m/s) is detected successfully. The experimental results indicated that the worse of the stress uniformity is obtained at the two ends under same impact velocity and the high ambient temperature. In addition, this phenomenon is validated by the numerical simulation of SHPB model. And the increase of temperature and impact velocity has the similar influence on the stress heterogeneity.