Creep of aluminum-based closed-cell foams
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11/7/03
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Creep of Aluminum-Based Closed-Cell Foams M. HAAG, A. WANNER, H. CLEMENS, P. ZHANG, O. KRAFT, and E. ARZT Metal foams creep when loaded mechanically at high homologous temperatures. We have studied the creep behavior of closed-cell aluminum-based foams with relative densities of 0.092, 0.112, and 0.163. Compressive creep tests were performed at 300 °C at strain rates ranging from 109 to 104 s1. Special efforts were made to produce and characterize a bulk reference material exhibiting the same chemical composition. Results show that the foams exhibit a lower creep strength and a higher stress exponent than predicted by the Gibson–Ashby model for regular foams. The possible mechanisms responsible for this deviation are discussed. A semi-empirical rate equation is established which describes the experimental data well.
I. INTRODUCTION
AT elevated temperatures, metal foams offer a better mechanical stability than polymer-based foams or adhesively assembled lightweight constructions. For long-term structural applications, however, the susceptibility to creep deformation must be taken into consideration. While numerous investigations have been performed on the mechanical properties of highly porous materials at room temperature (References 1 through 4 provide an overview), relatively few studies have addressed the time-dependent mechanical behavior at elevated temperatures. Gibson–Ashby have developed a creep model for cellular solids based on a cubic cell.[1] It is assumed that the major contributions to macroscopic creep stem from the creep bending of transversely loaded struts and the creep stretching of the cell membranes. Using a power law for the cell-wall material, # CS # snS [1] # where e is the creep rate, is the applied stress, and CS and nS are the creep constants of the cell-wall material. The # creep rate of a cellular material (*) can be written as follows: # * C* # (s*)nS [2a] where r* 3nS 1>2nS nS 1 nS 2 1/nS C* CS # a a b a b a b rS 1.7 0.6 2nS 1 [2b] nS r* 2 (1 ) b rS 3
* is the stress applied on the foam, */S is the relative density, and is the fraction of solid in the struts (for an M. HAAG, Doctoral Student, and A. WANNER, Akad. Oberrat, are with the Institut fur Metallkunde, Universität of Stuttgart, 70569 Stuttgart, Germany. Contact e-mail: [email protected] H. CLEMENS, Professor, is with the Institut fur Metallkunde, Montanuniversitat Leoben, Leoben, Austria. P. ZHANG, Postdoctoral Researcher, formerly with the Max Planck Institut für Metallforschung, Stuttgart, Germany, is with Lehrstuhl Metallkunde, Universität Cottbus, Cottbus, Germany. O. KRAFT, Professor, is with the Forschungszentrum Karlsruhe GmbH, Karlsruhe, Germany. E. ARZT, Professor and Director, is with the Max Planck Institut für Metallforschung. Manuscript submitted September 30, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
open-cell foam, is equal to 1). According to Eq. [2], the stress exponent of the foam is the same as that of the cellwall material. To assess
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