Mechanical behavior of steel and aluminum foams at elevated temperatures: local buckling based approach toward understanding of the material system behavior.

Abstract

The objective of this study is to understand and quantify the thermo-mechanical behavior of hollow sphere (HS) steel and powder metallurgy (PM) aluminum foams over a broad range of elevated temperatures. The behavior of both the HS steel and PM aluminum foam is tested under compressive loading at ambient temperature (24 ∘C) and elevated temperatures of 100 ∘C, 150 ∘C, 200 ∘C, 300 ∘C, 400 ∘C, 550 ∘C and 700 ∘C, and results for the two foams are compared by their rates of degradation in mechanical properties. To link the cell geometry and base metal properties with the global mechanical performance, the experimental work is underpinned by a computational micro-model, consisting of an assembly of hollow spheres. The computational model shows that plastic buckling of cells with progressive plasticity of the contact area is the key local failure mechanism. As expected, due to the plastic buckling of the unit cells, thermal degradations of the tested metallic foams follow similar trends as does the yield stress of their bulk metals. The HS steel foam exhibits only minor elevated-temperature-induced degradation in stiffness and strength at or below 400 ∘C, while still maintaining 69 % of its compressive strength at 550 ∘C. Comparatively, the PM aluminum foam begins degrading at an elevated temperature of only 150 ∘C. Interestingly, the HS steel foam oxidized between 300 ∘C and 400 ∘C, resulting in corresponding increases in the quasi-elastic modulus of elasticity. Future work might explore how to take advantage of oxidation reactions at the surfaces of the cells in the design of components using HS steel foams. Our computational study also revealed a possible new regime of cellular structures made up of ultra-thin-walled spherical cells that are predicted to fall within the elastic buckling regime at the local level. Thus, their deformations would be reversible even under high strains and their thermal behavior would be only controlled by thermal deterioration in the elastic constants, rather than plasticity parameters such as the yield stress.