Investigation of mechanical behavior of closed-cell aluminum foam reinforced with carbon nanotubes under dynamic loadings
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Aluminum foams represent a unique class of solid cellular light metals that can undergo large deformations at a nearly constant stress known as Plateau Stress. With relatively greater energy absorption and better weight saving as compared to polymeric foams, closed cell Al-foams still suffer from low strength. Thus, it is important to enhance the strength of closed cell aluminum foam by using nanomaterials without compromising the energy absorption capability. Closed cell aluminum foam reinforced with carbon nanotubes (refer to CNT hereinafter) was fabricated using the liquid metallurgy route with different CNT concentrations: 0, 1, 2 and 3 wt percent. Quasistatic and dynamic compression tests have been conducted to study the mechanical properties of Al-foams. Split Hopkinson Pressure Bar (SHPB) apparatus has been used for high strain rate compression with a proper specimen aspect ratio and a perforated pulse shaper to achieve constant strain rate and stress equilibrium in the specimens. It was observed that closed cell aluminum foam composites are strain rate sensitive. The mechanical properties of CNT reinforced Al-foams such as compressive stresses (peak stress and plateau stress) and energy absorption capacity are significantly higher than that of monolithic Al-foam under both low and high strain rates. It was determined that 2 wt percent concentration produces the highest peak stress, plateau stress and energy absorption among 1-3 wt percent CNT reinforced Al-foams. Also, dynamic high temperature compression behavior has been studied over varying strain rates and temperatures (298 K to 623 K). Peak stress and plateau stress of CNT reinforced Alfoam increase as the strain rate increases under increasing temperatures. Compared to unreinforced Al-foam, the CNT reinforced foam shows an increase in peak and plateau stress depending on the CNT concentration and temperature. Among the different CNT concentrations investigated under different strain rates, the 2 wt percent CNT Al-foam shows the highest peak stress, plateau stress and energy absorption over the temperature range studied. Furthermore, the effect of relative density (0.16 to 0.30) of 2 wt percent CNT reinforced Al-foam has been studied under quasi-static and dynamic loading. Compressive stresses and energy absorption increase as the relative density increases. It is noted that relative density 0.30 exhibited the highest peak stress, plateau stress and energy absorption. Additionally, deformation mechanisms and failure modes of cell walls and cell membranes of closed-cell CNT reinforced Al-foams have been experimentally studied under quasi-static compression loading. Crush bands were observed and fully developed at less than 10-20 percent strain. Two types of crush band were found: V-shaped and inclined crush bands and the crush band angles varies in the range 14- 31 degrees.
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