The Mechanism of Strain Rate Strengthening during Dynamic Compression of Closed-Cell Aluminum Foam

Abstract

A series of compression experiments were conducted on closed‐cell aluminum foams at strain rates ranging from 10−5 s−1 to 2600 s−1. Compression tests were conducted at ambient temperature and pressure, and in vacuum (25 torr). A split Hopkinson pressure bar system was used for experiments at high strain rates; low strain rate tests were performed with a servo‐controlled hydraulic machine. Closed‐cell Alporas foam demonstrated a strain‐rate effect over the range of strain rates investigated. Tests on samples with holes machined through the cell walls to permit intercellular gas flow confirmed that the strain rate sensitivity of the Alporas foam is related to sequential rupture of the cell walls, which allows the gas to exit the structure. Comparison of results for tests conducted in air and in vacuum showed the strength increase at high strain rates is attributed to the stabilizing influence of the gas (i.e., air) pressure within the closed‐cell structure. Evaluation of sectioned microstructures following interrupted high strain rate testing at a series of increasing strains, and high speed imaging of selected samples during dynamic testing, provided further insight on the deformation mechanism.