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Yuki Sugimura and Anthony G. Evansb) Division of Engineering and Applied Sciences, Harvard University, 29 Oxford Street, Cambridge, Massachusetts 02138 (Received 10 July 2000; accepted 20 November 2000)

Microscale sandwich beams with cell diameters and wall widths down to 150 and 15 ␮m, respectively, and having both metallic and polymer/metal cores were produced through fabrication methods that combined photolithography and electrodeposition. Various core structures were used, including some with negative Poisson’s ratio. The bending response was investigated and compared with beam-theory predictions. Most of the cores evaluated had sufficient shear stiffness that the bending compliance was relatively high and dominated by the face sheets. Two of the core configurations were “soft” and exhibited behavior governed by core shear. The relative dimensions of the cores evaluated in this study were far from those that minimize the weight, because of fabrication constraints. The development of an ability to make high-aspect ratio cores is an essential next step toward producing structurally efficient, lightweight microscale beams and panels.

I. INTRODUCTION

Sandwich construction is commonly used in structures where strength, stiffness, and weight efficiency are required.1 Low-density, hexagonal honeycombs are preferred as the core material on a performance basis.2 Problems with robustness and affordability, however, have prompted investigations of alternative sandwich constructions based on lightweight metallic foams and trusses.3,4 New microfabrication techniques such as laser stereolithography,5 UV laser ablation,6 and lithography galvanoformung abformung (LIGA)7 provide an opportunity to create microscale analogues of macroscopic structures with useful mechanical properties. In this article we explore a rapid prototyping strategy consisting of photolithographic, electrodeposition, and face-sheet bonding steps to fabricate microbeams consisting of a range of metallic and polymer/metallic composite core configurations sandwiched between two sheets of nickel. We also investigate the structural response—load, flexural rigidity, and failure load—of the sandwich beams in

a) b)

Address all correspondence to this author Present address: Princeton Materials Institute, Princeton University, 70 Prospect Avenue, Princeton, New Jersey 08540 J. Mater. Res., Vol. 16, No. 2, Feb 2001

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detail. For the cores we used regular hexagonal honeycombs, structures with negative Poisson ratio (NPR), structures based on various tilings, and Flexcore (Fig. 1). The objectives of the present study are (i) to explore convenient fabrication procedures for microstructures such as sandwich beams, (ii) to measure the bending performance of fabricated microbeams, and (iii) to compare the measured load capacity and stiffness with beam theory predictions. The configurations chosen have not yet been optimized for minimum weight, wherein several failure modes would operate simultaneously.1,2,14,15 I