Shock Compression of Foamed Graphite

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Abstract
Conventional high‐explosive techniques have been used to determine the principal Hugoniots of two graphite foams for stresses up to about 190 kbar. The initial densities of the foams were 0.55 and 0.68 gm/cm3. Hugoniot data for each foam fall into two distinct domains, depending on the amplitude of the input shock wave. Continuous free‐surface velocity measurements on the foam surfaces indicate that shock waves in the lower stress domains have two‐wave structures. The inherent irregularity of the surfaces of the foams, however, prevented exact measurement of the properties of these two‐wave structures. Results of further experiments which were designed to study loading and unloading from shocked states in the foam are consistent with the two‐wave picture in the lower stress domains and also show that single shock waves exist in the higher stress domains. Data in the higher stress domains show an increase in specific volume with increasing stress. This ``bending back'' of the Hugoniot is attributed to increases in thermal energy which occur in the compression process. Estimates of Grüneisen's parameter have been made for the higher stress domains, where voidless states are believed to exist. The values obtained for Grüneisen's parameter as well as the general behavior of the Hugoniots suggest that the graphite‐to‐diamond phase transition occurs at the higher stresses.

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