Compression testing of periodic cellular sandwich cores
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ON
PERIODIC cellular metals (PCMs) are hybrids of space (air) and metal, i.e., an effective material with its own set of properties.[1] Open-cell PCM architectures (e.g., Reference 2) reduce the total material mass by retaining only that which has geometrically-high load-bearing efficiency. The resultant strength-to-weight ratio is improved and relative density can be reduced to as low as 2 to 3 pct.[3,4] Furthermore, these architectures can be used as cores in sandwich structures and provide multifunctionality such as cross-flow heat exchange[5] and shape morphing.[6] The PCMs are also promising candidates for impact energy absorption applications,[4] an area where conventional metal foams have attracted a great deal of interest.[7] Overall, PCM cores and sandwich structures may exhibit superior mechanical properties when compared to their stochastic cellular metal foam counterparts.[2,8,9] The PCMs are typically tested as part of a sandwich panel in which the PCM core is fixed (i.e., confined) between solid face sheets,[2–4,10] imposing a support-thickness dependence as well as a confinement dependence.[11,12,13] During compression testing, failure typically occurs by plastic buckling of the PCM struts.[3,4,11] When a strut buckles, the rotation of its ends (i.e., hinges) is opposed by their bending stiffness[4,12]; the magnitude of this opposition depends on the fully plastic moment (i.e., plastic hinging) of the beam. Therefore, both strut and node properties contribute to the overall PCM performance. In order to investigate these failure mechanisms, this study examines the collapse mechanisms in PCM sandwich cores by compression testing in two limiting conditions. In the first case, the PCM nodes are laterally confined and compressive forces are resisted through both PCM beam buckling and beam bending (i.e., confined compression). In the second case, the PCM cores are placed between smooth compression platens, where the nodes are restricted only by interfaBRANDON BOUWHUIS, Doctoral Candidate, and GLENN HIBBARD, Assistant Professor, are with the Department of Materials Science and Engineering, University of Toronto, Toronto, Canada M5S 3E4. Contact e-mail: [email protected] Manuscript submitted May 19, 2006. METALLURGICAL AND MATERIALS TRANSACTIONS B
cial friction and compressive forces are resisted through PCM bending and plastic hinging (i.e., free compression). II.
EXPERIMENTAL
Pyramidal truss core PCM was fabricated from a 0.81mm-thick (t) square punched aluminum 3003-H14 sheet, purchased from Woven Metal Products, Inc. (Alvin, TX). The 90.82 mm2 (internal edge length 5 9.53 mm) perforations were punched on a two-dimensional square lattice of unit cell size 12.7 3 12.7 mm. The resulting structure is a series of four-rayed nodes with arm cross sections of 3.18 mm (w) 3 0.81 mm (t), having 56 pct open area. The sheet was strain relief annealed at 415 °C for 1 hour to obtain an O temper.[14] A perforation-stretching process was used, similar to that described by Sypeck and Wadley,[10] to fabricate the PCM co
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