Shear behavior of polymer micro-scale truss structures formed from self-propagating polymer waveguides Alan J. Jacobsen a,b, * , William Barvosa-Carter a , Steven Nutt c a HRL Laboratories, LLC Malibu, CA, USA b Department of Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, CA, USA c Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, CA, USA Received 10 September 2007; received in revised form 11 November 2007; accepted 14 November 2007 Available online 9 January 2008 Abstract The shear behavior of micro-scale truss structures formed from a three-dimensional interconnected pattern of self-propagating poly- mer waveguides was investigated. These structures had sub-millimeter size features and comprised a repeating octahedral-type unit cell designed to suppress bending in the truss members prior to initial failure. For a typical shear loading method, the experimentally mea- sured shear moduli deviated from predicted values as the relative density decreased. However, the mechanical behavior changed when the shear loading condition was altered to distribute the load more uniformly between all truss members and the measured shear modulus was aligned with predicted values. The shear strength and total plateau strain of these structures were strongly dependent on the mode of failure and the uniformity with which the truss member failure was distributed through the thickness of the structure. Furthermore, pla- teau strains of up to 60% were achieved when all truss members in compression buckled uniformly. Thermo-oxidation reaction of the polymer micro-truss structures caused an increase in the shear modulus and maximum shear strength (up to 2.9 MPa), although oxidized structures fractured at the maximum shear strain. Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Mechanical properties; Cellular solids; Polymer waveguides; Lithography; Rapid prototyping 1. Introduction Shear loading conditions are commonly encountered in engineering applications, and thus understanding the shear response of new materials and structures is essential. The shear behavior of cellular materials, i.e. materials with sig- nificant porosity, is governed primarily by the cellular architecture and the uniaxial properties of the solid mate- rial of which they are comprised [1]. For a cellular material with a randomly oriented architecture, Gibson and Ashby [1] have shown that the cell struts tend to exhibit bending under mechanical load. However, for ordered, truss-like cellular architectures, bending of the cell struts can be sup- pressed, improving the modulus and strength of the bulk cellular material [2–4]. Investigations of the shear behavior of truss-type cellu- lar materials that exhibit stretch-dominated deformation modes are fairly limited, partly because of the shortage of suitable synthesis methods. One technique for creating cel- lular materials with micrometer-scale truss features involves folding a two-dimensional grid (formed using soft lithography and subsequent electrodeposition) into a three- dimensional truss structure. Brittan et al. [5] tested a beam formed through this technique under four-point bending, which exerted a constant shear force on a portion of the truss structure. Although, the bending stiffness and strength were less than a simple box beam of equivalent weight, the truss structure afforded the opportunity for optimization, which they reported could greatly enhance the mechanical performance. 1359-6454/$34.00 Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2007.11.018 * Corresponding author. Address: HRL Laboratories, LLC Malibu, CA, USA. Tel.: +1 310 317 5398; fax: +1 310 317 5840. E-mail address: ajjacobsen@hrl.com (A.J. Jacobsen). www.elsevier.com/locate/actamat Available online at www.sciencedirect.com Acta Materialia 56 (2008) 1209–1218