Bending Properties of Al-Steel and Steel-Steel Composite Metal Foams
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INTRODUCTION
METAL foams are porous materials with unique combinations of thermal, mechanical, acoustic, and electrical properties desirable in many engineering applications. High stiffness-to-weight ratios and the ability to absorb large amounts of energy at relatively constant stress levels make closed cell metal foams attractive materials for automobile structural parts and crumple zones as well as structural parts for aircraft and spacecraft, where minimizing weight without sacrificing material performance is a priority.[1] Additional applications include components for vibration and acoustic damping, blast protection, boat decks and bulkheads, and prosthetic parts for biomedical applications.[1] Because the melting point of metal foam is similar to its parent material, metal foams can also be used in hightemperature and harsh environments that are unsuitable for other types of foam, such as polymer foams.[2] Although metal foams could provide significant improvements to these applications, most current processing methods produce foams with significant variations in cell size and shape, which makes an accurate prediction of mechanical properties difficult and the performance of the foam less than desirable.[2] Strong variations in the cell structure such as missing cell walls, wiggles in the cell wall, and anisotropic cell sizes cause localized deformation bands mostly around large or JUDITH A. BROWN, Ph.D. Candidate, is with the Department of Mechanical and Aerospace Engineering, North Carolina State University, 911 Oval Drive -3250 EB III, Raleigh, NC 27695-7910. LAKSHMI J. VENDRA, formerly Ph.D. Candidate, Department of Mechanical and Aerospace Engineering, North Carolina State University, is now Mechanical Engineer IV, Baker Hughes, Houston, TX 77073. AFSANEH RABIEI, Associate Professor, is with the Department of Mechanical and Aerospace Engineering, North Carolina State University and Associate Faculty in the Department of Biomedical Engineering, North Carolina State University. Contact e-mail: [email protected] Manuscript submitted September 9, 2009. Article published online July 1, 2010 2784—VOLUME 41A, NOVEMBER 2010
highly elliptical cells and limit the performance of the whole material to a function of the strength at its weakest point.[3,4] Several studies of commercially available closed-cell Al foams show that a large number of such defects are distributed inherently throughout the material, which causes significant reductions in stiffness and strength from those predicted by scaling laws.[5–7] This problem is solved partially by constructing metal foam from preformed hollow spheres, such as those created at Georgia Tech[8] and the Fraunhofer Institute.[9] Because the hollow spheres possess uniform cell size, shape, and wall thickness, they can produce metal foams with more uniform deformation patterns. However, because of the small point contacts between the sphere walls to support the foam structure under load, this type of hollow sphere foam exhibits low strength and low energy absorption capability. Co
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