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NASA TECHNP, GAL HASATM X-72771MEMORANDUM

,- (N_SA-TH-X-72771) CO _._IT;_ IA_TIC!_ N75- J2160l.,.r,, ST"OCTfI_F (N_SA) 2@ [ IIC $]?5 CSCT. 22Fr,-i>( _lllclas

I,,-.

'_ COMPOSITELATTICE STRUCTURE:a::ey

Marvin D. Rhodesand

Martin H. Mikulas, Jr.

September, 1975

(X_ ,

'%,Thli Informal documentation medium Is .sod to Novlde accelerated orapelll rellaee of t=chnlcal Inlormatie_ to selected users. The contentsn_y not meet NASA formal editing and pMblicatlon ut_mderd|, my be re-vlIIKI, or may be Incorporated m another publl(:'ztion.

NATIONAL|IOHAUTR!NDSPACEDMINGTRAgONLAN6LEYESI_4ifCHENTER,,AMPTON,BGINIA,tLH_ _

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1. Repot! Nn, 2, Government AcclMlan No_- 3, Ra'J.ipimf_t'sCatalog No.NASATMX-7277l4, T_*:_l!al_dSubtitle _ ReportD_10

COMPOSITELATTICESTRUCTURE _P_.HBEI(197_6. Performin@Orglnizatmn Code73.620

7. Author(s1 8 P_rl(JrlniNg Or,j_nl/dl,o,) I|e|Jr_TtNO.Marvin D. Rhodesand Martin M. Mikulas, Jr.

............... 10. W,)rk Un;t No,9. I_tf_rmingOrganiLazior= N,,me and AddressNASALangley Research Center 11. ( ;ontr_ct or _i'ctlrt NIJ.Hampton, Va. 23665

,,, 13. Type of Report and Period Covered12.spo._=,i.__.=y_,,_.=,_A,_d,_, TechnicalMemorandum

National Aeronautics and Space Administration t4 spo._,,n_A_e,cVC_d_Washington, D. C. 2054615, Supplementary Note_

16. Abstract .....

A lattice type scructural panel concept is described which exploits theunidirectional character of filamentary advanced composite materials. Thislattice has potential for applications where stiff lightweight structures areneeded such as large area panels for space satellites. Formulae are presentedto calculatethe panelweight and platebendingstiffness. This analysisindicatesthat structure_with significantlylighterweight than conventionalminimumgagesandwichconst,'uctionan be fabricated, A suggestedfabricationprocedureisalso presentedalongwith photographsof some typicalpanels.

17. Key Words (Su_lf_!_tudby Author(s)) lB. Distribution 5tate.lU,ltspace strv,._ures, composite materials, Unclassified - Unlimitedsandwich structures, fail safe design

_'9--_Sec_'r_tYla"_" ('this'ePr"_....nclass, fled _ 2o._.,ity,.,,i,,o|his..lnclassified _I---_o.fe.,6 _: r';_"_-$}.?,........

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COMPOSITELATTICESTRUCTURE

By

Marvin D. Rhodes

andMartinM. Mikulas,Jr.LangleyResearchCenter

!_ INTRODUCTION

Contemporaryand future vehiclerequirementscontinueaerospace to5 providechallengesto the structuraldesignerfor reducingstructuralweight.I In aircraft,fuel econon_yis puttinga premiumon structuralweight (ref.I)

while in spacecraftthere are anticipatedneeds for very large area spacestructures(ref.2) that will severelytax the load carryingcapabilityof

', any conceivableboostersystem. Relativelyrecentadvancesin filamentaryL_L,_ comp :itematerials(ref. 3) have providedthe structuraldesignerimprovements =)_ of a factorof 2 to 3 in strengthto densityratiosand a factorof 3 to 4 in

modulusto densityratioswhen comparedwith commonlyavai]ablemetals. Totake full advantageof these improvements,however,structuraldesignconceptsmust be originatedwhich considerfrom the outset,the highly unidirectional inature of the advancedfilamentarycompositematerials.

In ti(epresentpaper a latticetypestructuralpanel is describedinwhich an attemptis made to fullyexploitthe unidirectionalcharacteristicsof filamentarycomposites. The main motivationfor the developmentof such ia conceptis the requirementfor largearea spacestructureswhich are designedon the basis of stiffnessratherthan strength. Althoughsandwichconstruc-tion is veryefficientfrom a weight standpointfor moderateto heavilyloadedstructures,severeweight penaltiesresultfor very lightlyloadedsandwich

_' structuresdue to minimumgage constraintson skin thickness,glue weightand Jcore density. In an attemptto circumventtheseminimumgage problemsa very J

'J i

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i

2

sophisticated elcctroformed, hollow core aluminum structure was developed inreference4. The conceptdevelopedin reference4 has a resultantweightperunit area that Is impressivelylow, however,the cost for producingsuchstructuresmay be very high. lhe latticeconceptof the presentpaper hasthe potentialof beingas low or even lowerin weightper unit area than theelectroformedconceptand shouldbe at leastan orderof magnitudecheaperto produce.

Althoughthe latticeconceptwas originallyconceivedfor stiffnesscriticalstructures,it also appearsto have weightadvantagesfor structureswhich are designedby moderateloads. In this papera descriptionis givenof the concept,and the constructiontechniquesare discussed, Carpetplotsare presentedwhich give the weightsand bendingstiffnessesof latticepanels for a wide rangeof 9eometricvariablesand a weight comparisonis madewith san_iichpanels. The materialsystem consideredis graphite/epo_ andthe latticenetworkis taken as isotropic. For specificapplicationsother"compositematerialsystemscouldwell be appropriateand the geometryof thelatticenetworkcould be tailoredto suit the stiffnessor loadingrequirements.

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i1

3

SYMBOLS

A a Peaa width of lattice strips (see sketch a)D plate bending stiffness, see equation 5E elastic modulus

spacing of lattice strips (s_e sketch a)t thicknessW mas_v Poisson's ratiop density

Subscripts1,2 indicate size relation, see equation ga adhesiveC cotee effective or smearedf sandwich face sheetsp total platet composite materlal tape

Basic ConceptThe basic _:on_ostte lattt,.e concept is shm_ in Figure 1. It consists

of a regular geor_etrtcal arrangement of strips of advanced composite materialon an open grid honeycon_ core. It resembles a h_gh]y redundant trussnetwork which generally have _tgh structural efficiency. The lat,)tce conceptrelieves designers of might penalties imposed by nintmom gage requirementsbecause two additional paramel;ers are introduced. These parameters are thespacing and width of the lattice strips. The lattt(:e network shown inFigure 1 has a one ply, (0, _ 60) pseudo-tsotropic f_ce sheet.

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Other concepts of a similar nature can also he considerad such as theone _hown in Figure 2. The pattern shown in this sketch is also a pseudo-isotropic laminate of (0, _ 45, gO) orientation. _lthough both conceptsshown have pseudo-isotropic face sheets, composer:, )_ttice structures are notlimited to isotropic type laminates. Panel design may be tailored to suit aspecific application by using me,"iple thickness laminates in any specificdirection or on selected strips. Sometypical examples are shown in Figure 3for the (0, t 60) face sheet. In addition, the (0, t 45, gO) pattern couldhave somediagonal strips (_ 45) omitted or _ combination of multiple thicknesslaminatesa_idomitteddiagonalstrips and stillmaintaina regularnetworkpatternas shown in Figure4. For applicationsrequiringthe structuretosupport a low pressurelateralload a fil_ could be bondedto one face sheetas in Figure5a or one side of the panelcould be a full multi-plylaminate(Figure5b) with theoppositeside and core of latticeconstruction. Thesketchesshown in Figure 3, 4, and 5 13Iustratethe design latitudeavailableusing the compositelatticeconcept.

Some of the design parametersior latticestructureare given in Table !.These parametersfail into two grou_s,thoseassociatedwith the latticeface

_ sheetsand those relatedto the hor,eycombore. By appropriateselectionof) the desig_lparameters,the weight,strengthand stiffnessof a panel can oe

,_ selectivelytailored.

The compositelatticepanelconceptwas concelvedfor lightlyloadedapplicationssuch as spacestructureswhere stiffnessratherthan strengthisthe predominantdesignpara_.ter.However,it may have applicationsfor more

Mmoderatelyloadedstructures. Also, due to the high degreeof redundancy

I and diagonalsupports(trussframework)the conceptmay have appiicatlonswhere fail-safetyis considered.

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5

ANALYSIS

In the pr_vtous section two concepts were proposed. The first was(0, _ 60) and the second was a (0, _ 45, 90) face sheet latttce. Althouohboth are pseudoisotroptc laminates and are viable lattice panel candidates,only the (0, 60) is examined in this section. Furthemore, only thisconfiguration with uniformly spaced single thickness laminates is considered.

Hass - The massof composite lattice panels can be determined using atypical triangular element as shown in sketch a.

, Sketch a - Typical triangular elementConsidering on|y the material within the dashed line the mass of a typica|triangular ele:nent can be found to be

N " 3 a* Pttt + (Pctc + 2 Pata)(_ 3_1_a2) 1

The a_ fQr this sam,)element i_ given byA _ "_3_2 2q.

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:= 6i .Therefore the unit mass of a panel is given by

&!:_ W/h = 27_ ,_.a2 ott t .:-Pctc + 2 Petal - 3(_-)2 (Pctc + 2 Petal 3

These equations apply only when the spacing _t _, a -it'3'. if the spacing_; a't_', the lattice wi]l hove no triangular cutouts ,n the honeycomb

r3re or adhesive, however, it will also not have adjac_at strips of composite); material touching as if a full three ply laminate. FGr this configurationthe unit mass is given by

7- "

:_ N/A = 4 1/3_ Pitt +Pctc + 2%t a 4i_ where

!- 2a_ Whenthe spacing becomesequal to _ tim latti,,, is reduced tostandard sandwich construction with face sheets of three continuous larntna.Shownin Figure 6 is the u, it mass of" lattice panels as a function

; of spacing (_L) for 'two thickne'_ses of tape materl_l. The 0.0L)55 in. thick_ tape is the standard commercial prepreg materta! while the 0.003 in. thick_ m_terial is also commerciallyavailablebut is _ot as commonlyused. The

propertiesof a11 materialsused to ca'lculatehe structural.sss are giveni _ in Table2 and are considev'edo be representativeof availablecommercial

products. It can be noted thatwith a largespacing (_) a very low massstructure can be fabrica"-ed using the lattice concept. However, as thespacing is reduced the _tructura] ma:_sincreases to the mass of standardrsandwich construction which 1_ app_'oxtmately 0.25 and 0.37 ibs/ft 2 for the

) two thicknesses of tape consider,.,d. It should also be observed that if designrequirementsdictate;Jspecificcrosssectionalarea of material,then athickertape rather than a wider 'latticestrip (a) will yield the lowermass

structure because t,,e wider latti,:e strip has more.honeycombweight included.

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' I I I ' IL

Plate Bend]d Stiffness - The b_ndlng stiff'hess of sandwich structurescan be calculated from the equattm. (reference 5)

D = F';(tp 3 " tc3) 512 (1 - vf 2)

when Ef >> Ec If the latttce facings are considered to be _mea_edoverthe entire face sheet an effective face sheet thickness can be found to be

= at e 2 "1_ _ t t 6Plate bending stiffness in terms of effective face sheet thickness can bedetermined by substituting

tp = 2t e + t c 7

into equation 5 and expanding tp 3. If te

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i I _ ' I

8

A carpet plot giving the mass of a panel as a function of plate bendingstiffness for sandwich panels and lattice panels having various _.atios ofa/ is shown in Figure 8. Also shownon the figure are curve_ indicatingconstant honeycon_core depth. Based on these calculations, lattice construc-tion offers significant weight saving potential when comparedwith sandwichpanels of equal bending stiffness. For a given ratio o_ a/ and tapethickness, bending stiffness increases due to increases in core depth.However, for very thick cores the _'Jss can be reduced by increasing thethickness of the face sheet instead of the thickness of the core. T!e pointat which face sheet thickness should be increased instead of increasing thethickness of the core can be determined from the simultaneol:s solution ofthe equations for mass and stiffness. The relation between tape and corethickness has been found to be

= t _t_tt2c 1c.!t2t 9

tic = 4pt (t2t - tlt)IPc (2 -_'3 al) (I _ 10"|t-_-tl

where t2t tltTherefore. all design variables need be considered whenexamining require-ments for a particular application.

FabricationCempostte sandwich lattice can be fabricated from commonlyavailable

m_tartals and adhesive systems. The following dtsc,sstnn suggests one methodof fabrlcatic, however, manyother methods, or variations of the proposed_etho_Jare possible.

oFaceSheets.- The face sheets are fabricated from unidirectionalp_Jmpregnated tape that is cut into strips of the desired width. The strips_re laid in (, mold machined from aluminum and overcoated with a re]easeagent. A m_ctHnedmold such as the one shown in Figure g permits accuratealJgnme_it _nd positioning of the tape with minimumeffort.

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9

lloneycomCore.--lhe trJanqularpattern illtPleh(ineycomboremay her_lllOVedeither beforem' after bondlngt(_the lac_,FJ1eets.li tllf_yare

.. r(:iiiovedefore,bendingcaremust l_etakent()accl4raLely()_,itiunbe core onthe latticestrips. If they are removedafter L.o'_(linghereis dangerofdamageto the faces_hen the core if_heing cut.

In order to removethe triangularsectiom_,._romthe core a novel cutterwas devised. A photographof this cutter is ._tlownn Figure I0. The cutteris fabricatedfrom aluminumwith conventiona'_ingleedge razorbladesasthe cuttingedge. A springloadedplunger _,_otin the centerream)yesthetri-egul_rsectionfrom the cutter.

_he panels fabricatedto dat_ have beenmade by precuttingthe honeycombcorp. A specialtemplatedesigned for t:,isp_rpose is shown in F_gureII anda photo showingthe templateand cutter In use is shown in Figure12. Thetemplatewas fabricateJfrom thin alumhmm sheet stock and the grid patternwas cut on a numericalcontrolledmil!h_gmachine.

Curin_and.Bondin_Face__heets.- Panels fabricatedin this investigationwere made by using the tea,)platend cutterto precutthe honeycombcore.Siliconrubberblanketswere also cu'_with the templateand placed iu thebase of the mold. The composite,aaterialface sheetswere the_plac_.dinthe mold followedby the honeycombcore whose faces had been coatedwith anepoxy resinsystemto permitbondingto the core. The molds were closedandloadedwith leadweightsto get good contactpressurebetweenthe face sheetand the honeycoil_.The totaIsy'_temwas heatedto a low temperatureto curethe bondingresin. The low temperaturealso allowedthe resin in the graphiteto fl(,wand give good joint co_,paction.The latticewas then removedfromthe mold, vacuumbagged ar, placed in an oven to cure the resinin thegraphite. It was necessaryto cure the graphiteprepregoutsidethe aluminummold due to the differentialthermalexpansionbetweenthe alumi_:umnd thegraphitecomposite. Molds machlnedfrom a materialwith low thermalexpansioncharacteristicssuch _s graphiteor a ceramicwould allow the latticeto hecompletelybonded ars_cured in the a_Id i_ ow_eopc,_'ationithout d,_mag().completedpanel fabricatedby the._thod outlinedabove is shown in Iiq_re 13.This panel has a stripwidth of 0.25 inch _nd a latticespacinqof 1.5 inches.

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10

SUMMARY

Future space missions may require larg_ area structures which aredesigned on the basis of stiffness rather tllan strength. Sandwich structuresare generally very efficient, however, for lightly loaded applications severeweight penaltiesresult fromminimumgage constraints. A latticetypestructuralconcepthas been describedwhichexploitsthe unidirectionalcharacterof filamentarycompositematerialswhile relievingthe designerof conventionalminimumgage considerations.Althoughconceivedfor stiffnesscriticalstructures,the conceptmay have weightadvantagesfor structuresdesignedby moderateloadsor applicationswhere fail-safetyis important.

Formulaeare presentedto calculateboth weight and plate bendingctiffnessof the latticestructure. Carpetplots which give the weightandbendingstiffnessfor a wide varietyof geometricvariablesare also presented.The compositematerialsystem consideredis graphiteepoxyand the latticenetworkis isotropic,however_the analysismay be modifiedto considerothermaterialsystemsor nonisotropicnetworkswhere appropriate. A suggestedfabricationprocedureis describedalongwith photographsof some typicalpanels.

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RLI:LRrNCII

I. Nagel, A. L.; and Alford,W. ,}.,Jr.: FutureLong-RangeTranspogt-Pro;;pect_()rImprovedFuelEfficic'ncy.NASA TM X-12659,Feb. 1975.2, (iia,_(,r,. I...;t al: l'easibilitytudyof a SatelliteSolarPower

Station. NASA CR-2357,F(.d).()74.3. Jorles,R. M,: Mechanicsof ComvositeMateria'Js.Srript_Bc.._ke., _'_)75.4, Carlson,J. A.: Developmentof t.ightweightolarPanels. NASa._;RF,38325. Ashton,J. E.; llalpin,J. C.; an_ Petit, P. H.: Pr_cr c,,I_',,..=iteMaterials: Analysis. TechnomicPublish'ingo.. 'r..,196_;6. _lilliams,. G.; and Mikulas,M. M., Jr.. _,nalyti_.aind Experir.;ental

Studyof StructurallyEfficientComposiC.el&t-Stiffenedanels L'j_.declin Axial Compression. AIAA Pel)erNo. 75-/._4,ay 1975.

ACKNOWt.E_tRENI'S

Appreciationis expressedto IVlessrs;.oseph Power and E1vinAhl ofthe CompositeModel Developmet_thcp for their a;d in developingfabricationtechniques,and to _lightStroupeof the MechanicalDevelopt_ntSectionforhis suggestionson the molds and honeycombcutter.

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12

Table 1._ LatticeDesignVariables

LatticeComponent DesignVariab'leHoneycombCore DensityThicKnessShear Modulus

Face Sheet DesignConfigurationMaterialModulusMaterialStrengthStripWidthStripThicknessStripSpacing

Adhesive DensityBond Strength

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t i! I13

Table2.- MaterialPropertiesUsed to CalculatePanelMassand StiffnessParameters

ComponentMaterial Property

Honeycombcore % = 3.0 Ib/ft3t = 0.2 inc

Graphiteepoxy tt = 0.003 inFace sheet* Pt = 0.0563 Ib/in3

vf --0.326Et = 1.96 x 107 psiEf = 7.84 x 106 psi

L

Bonding % = 0.208 Ib/in3Adhesive t = 0.001 inEL

See reference6 for propertiesof graphiteepoxytape material.

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J

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' I J

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a. Composite lattice with bonded plastic film

:, b. Lattice with multiply latl_hlate far, o sheet

Figuz'e 5.- _etches tlhtetrating l_8,_tble nzz_digiz'ldi,zn t_) the lat!i('!, f_et, _h(,(,t I!) ,_,l)l_ttt)res_ure ()r ()ther dl._triimted I_ancl l_vd_,

i II I 1

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