Rate Dependent Reinforcement of Liquid Nanofoam on Thin-Walled Tubes

Abstract

Thin-walled metal tubes have been widely used as energy absorbers to mitigate adverse effects of impact and protect structures and facilities. However, once the initial buckling stress of the tube is reached, the post-buckling plateau of the tube has a much reduced average stress which determines the energy absorption efficiency of the empty tube. As a result, the real energy absorption efficiency of the thin-walled tube is much lower than the theoretical limit which is proportional to the value of initial buckling stress. We hypothesize that by filling thin-walled tubes with the novel liquid nanofoam (LN), (i) the energy absorption efficiency of the hybrid structure can reach the theoretical limit, and (ii) the main working mechanism is the effect of solid-liquid interaction on tube buckling. To test these hypotheses, we have characterized the energy absorption efficiency of LN filled steel tubes by using quasi-static compression tests and dynamic impacts. The quasi-static behavior of LN filled tubes is characterized by an Instron 5982 universal tester. Results show that the gravimetric and volumetric energy absorption efficiencies of LN filled steel tubes are 20% and 220% higher than the values of empty tubes, respectively. This is due to the changed buckling mode and the promoted post-buckling stress of the hybrid structure by the highly compressible LN. The dynamic behavior of LN filled tubes is characterized by a dynamic impact test (∼3 m/s) with a lab-customized drop tower apparatus. It is found that both the gravimetric and volumetric energy absorption efficiencies of LN filled tubes are further increased by 16%. The strain rate dependent behavior of LN filled tubes must be attributed to the solid-liquid interaction between the LN and the steel tube wall, which is further verified by comparing the mechanical behaviors of LN filled tubes with solid foams filled tubes. Our experimental results have demonstrated that the energy absorption efficiency of thin-walled tubes are significantly improved by the LN filler especially at higher strain rates. This hybrid structure may have a potential for future use in the design of light-weight and small scale cellular structures for vehicle safety and crashworthiness.