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Syntactic foams are strong, lightweight materials that can be useful in marine and aeronautical applications. However, these materials are brittle, which can limit their usefulness. The fracture toughness of several epoxy based syntactic foams, as affected by foam composition, was evaluated. The parameters of interest were the curing agent type (anhydride and amine) and elastomer type and level. For the anhydride curing agents studied, the addition of elastomer improved the fracture toughness, and no reduction in flexural strength was observed upon elastomer addition. Examination of the fracture surface showed that the failure mechanism varied with foam composition, from crack propagation around the microballoon filler to crack propagation through the filler, possibly explaining the strength results. For a syntactic foam cured with a cycloaliphatic amine curing agent, the addition of elastomer did not significantly affect the fracture toughness. Examination of the fracture surface showed that the failure mechanism for the syntactic foam compositions with and without added elastomer was crack propagation through the matrix rather than through the microballoon filler. Polym. Compos. 25:229–236, 2004. © 2004 Society of Plastics Engineers.

A standard method for the determination of the shear strength of a polymeric foam is a lap shear geometry. The accuracy and validity of this test was evaluated by means of finite-element modeling. Model results show that the shear stress is not constant over the length of the sample, with shear peaks at the ends, so that the average shear stress in the sample can be quite different than the maximum shear stress experienced. Normal stresses are also present at the sample ends giving rise to a combined stress field. Experimental results for a polymeric foam bonded to steel edge plates show that the measured shear strength can vary significantly with the sample geometry (length). Maintaining a minimum length/thickness ratio, as specified in the ASTM C 273 standard, does not necessarily result in a sample with the most uniform shear field, and therefore the most accurate measure of the true shear strength.