Failure Mechanisms during Periodic Cellular Metal Fabrication by Perforation Stretching
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INTRODUCTION
METAL foams are attractive for application in environments requiring high strength-to-weight and stiffness-to-weight ratios at low density. These materials can be created by methods such as injecting gas or adding gas-forming particles into molten metal, or by investment casting of polymeric foam templates (References 1 and 2). However, the stochastic cell structure of foams can limit their overall mechanical performance. Periodic cellular metals (PCMs) are a more structurally efficient class of materials composed of repeating unit cells that can make use of stretchingdominated deformation as opposed to the bendingdominated deformation employed by conventional metal foams. As a result, PCM weight-specific properties can reach greater values than those of foams.[3] Several methods have been developed to produce PCMs, including textile layup, investment casting, and deformation forming approaches (review by Wadley).[4] In the textile layup approach, PCMs are formed by stacking and joining precursors such as woven metal sheets,[5] hollow tubes,[6] or composite filaments.[7] Investment casting methods first form a three-dimensional (3-D) polymeric pattern in the desired PCM architecture, and then use its ceramic negative to cast the PCM.[8,9] In deformation forming approaches, sheet precursors are plastically deformed into 3-D PCM architectures.[10–16] These methods are particularly attractive, because they are based on simple sheet forming operations. B.A. BOUWHUIS, Postdoctoral Candidate, and G.D. HIBBARD, Assistant Professor, are with the Department of Materials Science and Engineering, University of Toronto, Toronto, ON, Canada M5S 3E4. Contact e-mail: [email protected] Manuscript submitted February 28, 2008. Article published online October 15, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
Two broad classes of deformation forming approaches have been used to produce PCMs. In the bending brake method, perforated or expanded precursor sheets are placed in a bending press and corrugated into a 3-D architecture.[10–12] The resultant plastic deformation is localized to the ends of the supporting members (i.e., hinges).[12] The perforation-stretching method[13–16] places periodically perforated sheet metal in an alternating pin jig. The pins apply force out-ofplane, plastically deforming the sheet metal into a trusslike array of struts (i.e., metal supports) and nodal peaks (i.e., strut intersections). The out-of-plane deformation is advantageous because it can allow truss cores of any height to be stacked with the undeformed precursor into a multilayer PCM. The work hardening accumulated during forming is also beneficial, because it raises the yield strength and inelastic buckling strength of the PCM members.[16] In addition, a continuous range of truss core architectures can be fabricated from a single precursor geometry, allowing the structure to be tailored for specific applications and loading conditions (e.g., shear, uniaxial compression, or both). The range of perforation-stretched PCM architectur
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