Impact Damage Resistance of Aluminum Alloy Foams

Abstract

While some polymers are engineered to improve strength and endurance under elevated temperatures, these same materials are costly both economically and environmentally with the latter of the two stimulating the interest for this study. Polymers, more specifically foam cells are generally flame retardant. When ignited, toxins such as fluorine, bromine and other metallic salts are given off in the air. This poses potential environmental hazards. However, metallic materials (Aluminum) with their high strength, stiffness and ductility are much more environmentally friendly. Even if alloyed with appropriate compounds, the resulting core material could be melted down, separated then cast into new stock. Moreover, the use of an alloyed material can generally enhance the strength and stiffness of sandwich composite structures so essential in aerospace applications. United Technologies Research Center provided a plate of Aluminum alloy foam for impact testing by North Carolina A&T Researchers and graduate students. The material was provided by Austrian Metals Co. (AMAG) to UTRC under ONR Contract # N00014-95-C-0231, Thompson & Renauld (1997). All specimens were cut from one sample of nominal dimensions of 20 inches by 20 inches by 0.65 inches in thickness. The sample mass was 3142 grams and the apparent density was 0.737 g/cc. The chemical composition is close to that of 6061. The sheet sample was formed by AMAG and heat treated to T5 specifications consisting of 14 hours in a furnace at 160 °C. Generally the bending stiffness and failure mechanisms were substantially different from those of polymeric foam sandwich cores made of Rohacell, a polymethacrylimide (PMI) foam in a prior study, Craft et al (1997). Rohacell is an easily machined, but a hygroscopic form with low shear strength and stiffness, but it and many other organic foams have a relatively uniform cellular construction in a wide variety of densities.