Impact behaviour testing of aluminium foam

There is strong interest in employing lightweight metallic foams for automotive and aerospace applications, where weight reduction, and mechanical energy absorption and acoustic damping capacity are required. Knowledge of the mechanical behaviour of these materials, especially under dynamic loadings, is thus necessary. This work presents an extensive study of strain-rate sensitivity of a specific aluminium foam using a Split Hopkinson Pressure Bar technique adapted to perform tests on low strength/low impedance materials. All dynamic tests have been performed using an SHPB which adopts a long, pre-stressed steel bar to generate the compressive wave and aluminium tubular bars to load the foam specimen. The advantages of this configuration are that the loading pulse reaches a duration of almost 4 ms and that it allows tests to be carried out on specimens with a sufficiently large representative volume (usually SHPB tests on metallic foams have been performed on less representative specimens due to lower specimen/cell volume ratio). The stress-strain curves of all specimens at different strain-rates have been obtained by adopting the classical SHPB theory and some analytical procedures to compensate for dispersion phenomena. Experimental tests have been performed on aluminium foams with two different densities (approximately 150 kg/m3 and 300 kg/m3) using specimens of diameter 50 mm and a length varying between 15 mm and 50 mm. The influence of loading direction (x, y, z) has also been assessed to check the hypothesis of isotropic behaviour. Because of the oscillatory behaviour of the curves, especially in dynamic tests, to readily compare the experimental results obtained at different strain-rates an average stress operator has been introduced. The results indicate no appreciable effects for strain-rates up to 300/s.