Effect of Lateral Constraint on the Mechanical Properties of a Closed-Cell Al Foam: I. Experiments
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THE mechanical behavior of metal foams has been extensively investigated in the recent past.[1–6] The plastic response of closed-cell metal foams is largely determined by collective cell collapse.[1] Progressive cell band collapse, from one band to another, results in a long plateau in the compressive stress-strain curve after an initial elastic regime. This phenomenon continues until all the cells have collapsed, which results in a steep rise in the stress with further strain. The strain at which this transition occurs is referred to as the densification strain, ed. This large plastic plateau up to a ed of ~60 to 70 pct in aluminum foams has two important practical consequences. First, it allows for a large amount of energy absorption, making Al foams attractive candidates for impact energy absorbing applications. Second, by suitable design in blast amelioration systems, it is possible to limit the stress experienced by the protected components to that of plateau stress. In the unconstrained uniaxial compression tests, the collapse of the first cell band often leads to the shear displacement of one part of the specimen with respect to the other, resulting in the loss of uniaxiality. This becomes an issue particularly at large strain. A lateral constraint is expected to prevent such a shear displacement and could lead to strain hardening. However, this possibility has not been given due consideration yet, as the plastic Poisson ratio of metal foams is presumed to be equal to zero that would result in insignificant transverse strains during uniaxial compression. M. KOLLURI, formerly Master of Engineering Student, Department of Materials Engineering, Indian Institute of Science, Bangalore 560 012, India, is Doctoral Researcher, Mechanical Engineering Department, Netherlands Institute for Metals Research, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands. S. KARTHIKEYAN, Assistant Professor, and U. RAMAMURTY, Associate Professor, are with the Department of Materials Engineering, Indian Institute of Science. Contact e-mail: [email protected] Manuscript submitted November 20, 2006. Article published online July 18, 2007. 2006—VOLUME 38A, SEPTEMBER 2007
In most practical applications of metal foam containing parts, there will be a reasonably thick face sheet covering the foam, which will act as a barrier for unconstrained shear displacements. A similar scenario exists for the foam filled columns and boxes, which are potential candidates for sacrificial energy absorbers under low velocity impact as well as in blast amelioration applications. Here, the design is such that the protected part sees the plateau stress at the maximum. If there is any strain hardening in the foam due to the constrained deformation, the protected part may experience higher stress. The fatigue response of metal foams is also likely to be affected pronouncedly. Accurate description of the constitutive response of the metal foam under constraint is essential for designing foamfilled components. Despite its significan
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