Quantitative assessment of microstructure and its effects on compression behavior of aluminum foams via high-resolution
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I. INTRODUCTION
HIGHLY porous metal foams have received much recent attention as structural materials on account of their superior mechanical energy absorption during deformation at a relatively low stress. This is attributed to the existence of a flat and long stress–strain relationship at a low stress called the plateau regime just after an initial elastic deformation. A series of localized collapses percolates through the structure during the plateau regime, thereby resulting in the stable and progressive fracture behavior. Such behavior is well documented in the literature.[1] As is well understood, microstructural features of metallic materials have, in general, principal effects on their mechanical properties. However, although metallic cell materials in the metal foams exhibit microstructural features similar to fully dense metals, such as precipitates, dispersoids, inclusion, micropores, grain boundaries, etc., current assessments of the deformation in the plateau regime are entirely based on rather macroscopic or mesoscopic aspects. For example, it is commonly claimed that the plastic buckling and bending of cell walls and struts dominate the deformation behavior in the plateau regime.[2] Complementary model analyses have been used for structural designing by idealizing the cellular structure. The most famous one is the Gibson–Ashby H. TODA, T. KOBAYASHI, and M. NIINOMI, Professors, T. OHGAKI, M. KOBAYASHI, and N. KURODA, Researchers, and T. AKAHORI, Assistant, are with the Department of Production Systems Engineering, Toyohashi University of Technology, Toyohashi Aichi 441-8580, Japan. Contact e-mail: [email protected] K. UESUGI, Researcher, is with the Japan Synchrotron Radiation Research Institute, Mikazuki, Sayo-gun Hyogo 679-5198, Japan. K. MAKII, Head of the Laboratory, and Y. ARUGA, Researcher, are with the Materials Research Laboratory, Kobe Steel, Nishi-ku, Kobe Hyogo 651-2271, Japan. Manuscript submitted July 15, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
model, which represents bending, buckling, and fracture of a simplified unit cell structure.[2] Such models appear to be useful to some extent as a first approximation. Numerical analyses based on a regularly periodic array of cells might give similar fundamental insights into the mechanical properties.[3] Notwithstanding these efforts, it has been claimed that most commercially available cellular materials yield at a relative strength appreciably lower than those anticipated by such model analyses.[2,4,5] The majority of the commercially available metallic foams have closed cells that exhibit a wide variation of cell size and cell shape, with diameters typically ranging one to several millimeters. Indeed, in the case of closed cell foams, the reduction factor sometimes reaches 3 to 10[5] or more.[2] Extensive efforts have been made to ascertain the cause of the discrepancy from various aspects. One of the macroscopic interpretations for this, which might be widely accepted, is the influence of inhomogeneities.[5–10] Past observation
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