Fabricationandstructuralperformanceofperiodiccellularmetal sandwichstructures HaydnN.G.Wadley a, *,NormanA.Fleck b ,AnthonyG.Evans c a Department of Materials Science and Engineering, University of Virginia, Charlottesville, Virginia, 22903, USA b Engineering Department, University of Cambridge, UK c Department of Materials Science and Engineering, University of California at Santa Barbara, Santa Barbara, California, USA Abstract Metallicsandwichpanelswithperiodic,open-cellcoresareimportantnewstructures,enabledbynovelfabricationandtopology designtools.Fabricationprotocolsbasedonthesheetformingoftrussesandshellelements(egg-boxes)aswellastextileassembly have allowed the manufacture of robust structures by inexpensive routes. Topology optimization enables control of failure mechanismsatthetrusslengthscale,leadingtosuperiorstructuralperformance.Analysis,testingandoptimizationhavedemon- strated that sandwich panels constructed with these cores sustain loads at much lower relative densities than stochastic foams. Moreover, the peak strengths of truss and textile cores are superior to honeycombs at low relative densities, because of their superiorbucklingresistance.Additionalbenefitsofthetruss/textilecoresoverhoneycombsresideintheirpotentiallylowermanu- facturingcostaswellasintheirmultifunctionality. # 2003ElsevierLtd.Allrightsreserved. 1. Introduction Numerous techniques have been developed for syn- thesizing inexpensive metal foams with stochastic, closedcells [1,2].Otherprocesseshavebeendevisedfor manufacturing open cell, stochastic structures [3–10]. Someutilizeopencellpolymertemplatesforinvestment casting [4,5], chemical vapor deposition [6] or slurry coating [7]. Others utilize hollow spheres [8–10] or aggregatesofsolubleparticlesintowhichmetalscanbe injectedandsolidified [11].Aleachingprocessremoves the template leaving behind a cellular solid. While potentially less expensive to fabricate than benchmark honeycomb systems, their mechanical performance is greatlyinferior,becausethemetalligamentsexperience bendingwhenloaded [12].Moreover,mosthaveanas- cast ligament microstructure that contributes to low ductility [13]. Besides their load bearing function,most cellular metals have multifunctional potential. (i) They can be good energy absorbers during impact loading [1]. (ii) Some closed cell systems afford thermal isolation opportunities [1] andotherscanbegoodsounddampers [14]. (iii) Open cell materials have been used as cross flow heat exchangers [15], as current collectors for nickel metal hydride batteries [6] and as catalyst supports [16]. Theinferiorstructuralperformanceofstochasticsys- tems has spurred interest in periodic cellular metals (PCMs)thatnotonlycompetewithhoneycombs(Fig.1), 0266-3538/$-seefrontmatter # 2003ElsevierLtd.Allrightsreserved. doi:10.1016/S0266-3538(03)00266-5 CompositesScienceandTechnology63(2003)2331–2343 www.elsevier.com/locate/compscitech Fig. 1. Low relative density metallic honeycomb cores for sandwich panelscanbemadebyasheetcrimpingprocess.Thestackedsheetscan be bonded by resistance welding (shown) or with an adhesive.The corescanbecutandadhesivelybondedtofacesheetstocreatesand- wichpanels. * Correspondingauthor.Tel.:+1-804-982-5670;fax:+1-804-824- 2032. E-mail address: haydn@virginia.edu(H.N.G.Wadley).