American Institute of Aeronautics and Astronautics1

New Generation of Rigidizable/Inflatable Composite forSpace Use

Yuki C. Michii* and Arthur L. Palisoc†

Koorosh Guidanean ‡

Gordon Veal §

Billy Derbes¶

Constantine Cassapakis#

L’Garde, Inc., Tustin, California 92780

Rigidizable/inflatable composites combine traditional reinforcement materials with anew generation of thermoplastic resin. The resin enables structures comprised of thesecomposite materials to achieve high-compaction ratios as well as low mass and offers theseadvantages for those applications that are very constrained in these areas. Laminatesfabricated using this resin system are fully cured but allow for flexibility or rigidity througha change in molecular bond strength in the resin, which occurs at the glass transitiontemperature. Cylindrical booms have been one of the most recent applications forrigidizable/inflatable composites with structures upwards of 48 m in length having beenmanufactured. The fabrication methods that have been developed produce high-qualitylaminates with very good impregnation of the resin in the reinforcement material and lowvoid content. The effect of packaging cycles on stiffness and strength was determinedthrough testing of tubular specimens. Results show these specimens continue to retain goodperformance even after several packaging cycles. Laminate properties predictions throughmicromechanics using commercially available software were compared with test results.However, a combination of the non-traditional weave of the reinforcement fabric layer aswell as effects from the fabrication method makes precisely predicting the materialproperties difficult. In addition to the rigidizable/inflatable laminates, interior- or exterior-mounted thin electric resistance heater elements that are needed to heat the laminates abovethe glass transition temperature for unpackaging or deployment and which are a necessarycompanion component for rigidizable/inflatable laminates were also developed. Therigidizable/inflatable laminate system has been used in applications from antenna booms tomain structural members in large trusses such as the Innovative Space-Based RadarAntenna Technology and Space-Solar Power structures.

NomenclatureTg = glass transition temperature

I. Introductionnflatable structures technology has existed for more than 30 years. One class is the rigidizable/inflatable (R/I) thathas been maturing in that time and is coming into use in space applications. Structures comprised of these

materials offer advantages over traditional, rigid structures in applications that are constrained in mass and volume

* Structural Engineer, Analysis and Structures Department, L'Garde, Inc., AIAA Professional Member.† Supervisor, Analysis and Structures Department, L'Garde, Inc., and AIAA Professional Member.‡ Materials Scientist, Engineering Department, L'Garde, Inc., and AIAA Professional Member.§ Vice-President of Engineering, L'Garde, Inc., and AIAA Professional Member.¶ Consultant, L’Garde, Inc. and AIAA Professional Member# President, L'Garde, Inc., and AIAA Professional Member.

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48th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference<br> 15th23 - 26 April 2007, Honolulu, Hawaii

AIAA 2007-1812

Copyright © 2007 by L'Garde, Inc. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

American Institute of Aeronautics and Astronautics2

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Figure 1. Typical modulus as function oftemperature for R/I resin systems. Td is thedeployment temperature limit. Tg is the glasstransition temperature. Tr is the rigidizationtemperature

but require performance in addition. Composites combining reinforcement materials with sub-Tg resins areinflatable and rigidizable. The sub-Tg resin is the component that enables the material to be very compliant abovethe glass-transition temperature. Rigidization then occurs below the glass-transition temperature when the finalconfiguration is attained. The rigidized composite material will then provide the structural performance that isrequired.

II. Rigidizable/Inflatable Composite MaterialThe rigidizable/inflatable composite material is comprised of two components similar to traditional composites.

The reinforcement materials which provides the structural capability are traditional materials such as Kevlar orcarbon graphite. The matrix or resin component which provides the rigidizable capability is a proprietary aromatic-rich polyurethane resin developed by L'Garde, Inc. These resins have glass transition (Tg) temperatures that aretailorable over a wide thermal range to suit the specific conditions of a given application. A typical curve ofmodulus as a function of temperature that is characteristic of these R/I resin systems is shown in Figure 1. The resinremain flexible enough to facilitate deployment while the temperature remains approximately 20 degrees above theTg. Once the temperature reaches and cools below the Tg, the resin becomes stiff enough that it provides structuralperformance.

Laminates are manufactured through the generalmethod of heating a sandwich of reinforcement materialbetween two resin layers past the resin melting point,which is significantly higher than the glass transitiontemperature. The resin then easily flows through thereinforcement layer resulting in very good wetting. Thelaminate is then cooled resulting in a fully curedlaminate. In this process, there are two importantfactors required to fabricate uniform laminates withgood fiber wetting properties and to achieve the desiredfiber/resin ratios. First, sufficiently high temperaturesare required to melt the resin so that it can flow freely.Second, enough time at these post-melting pointtemperatures (dwell time) must be allowed for the resinto diffuse into the tows to achieve good impregnation ofthe reinforcement material.

These laminates are manufactured throughmodifications of different established industrialmethods. L’Garde has developed large-scale fabricationprocesses both with conventional vacuum-bagging androll-to-roll techniques. In the conventional style

method, the laminates are vacuum-bagged in flat sheets. For longer sheets, an entire roll of material is placed insidethe vacuum system. An unrolled section of reinforcement material is then impregnanted as a flat section. Once thatsection is fully cured, the laminate section is rolled up and more bare material is unrolled. This process is repeatedin step-wise process until the entire roll is impregnated with sub-Tg resin. The final laminate length is determinedonly by the length of the roll. These techniques have been used to fabricate large laminates up to 15m in length orlonger. The fabricated length depends only on the size of the vacuum-bagging system. Laminates up to 60-m inlength have been manufactured. Figure 2 shows a 48-m continuous tube fabricated using these processes.

Fabrication of the tubes, such as those in Figure 3, from the R/I laminates required additional technicaldevelopments that include the addition of Kevlar spiral wrap for some tubes, a layer of Kapton in the laminate, andseam bonding techniques. The use of Kevlar tow spiral wrap is to provide hoop strength while the tube ispressurized under temperature in preparation for final rigidization. Since the tubes are constructed from flat, rolledlaminates, a seam is needed to create a butt-joint over the adjoining edges. A method was developed where a stripof the same material is placed over the length of the seam and locally heated past the resin melting point. As aresult, the resin from the seam and the rest of the laminate flow together creating a continuous resin layer whicheliminates the discrete boundary due to dissimilar materials if traditional adhesives were employed. Additionally, aseamless method was developed for fabricating tubes in which the reinforcement is a composite fabric materialwoven into tubular form. The reinforcement is then impregnated with resin in the same method as with flat

American Institute of Aeronautics and Astronautics3

ConditionCompression

Modulus [GPa]Unpackaged / Pristine 70After 12 packaging cycles 56.71% Percent Retention 81%

Table 1. Compression modulus of tubular testspecimens before and after packaging.

Condition

Apparent Modulusof

Rigidity [GPa]Before Proton Exposure 2.80After Proton Exposure 3.06% Change 9%

Table 2. Apparent modulus of rigidity before andafter exposure to 5 MeV protons with a totalabsorbed dose of 260 Mrads at –40 C.

laminates but results in a laminate that already hasa circular cross-section which eliminates a numberof steps required for fabricating tubes from flatmaterial.

For very long laminate pieces (>100m) theroll-to-roll fabrication method is used. In thistechnique, the structural fabric with the Sub-Tgresin on both its top and bottom surfaces is madeto pass between two rollers that apply bothpressure and temperature. This method oflaminate fabrication is commonly used byindustry.

Completely fabricated laminates are fully cured and no other processing is needed for thefinal application. Only heating to above the glass transition temperature is required so that thelaminate becomes soft and pliable. This occurs simply because, above the glass transitiontemperature, the heating weakens the covalent bonds of the resin molecules. In this condition,the laminate can be folded and packaged or unfolded and deployed. Cooling back below theglass transition temperature, the laminate becomes structurally capable as the covalent bonds

between the resin molecules become stronger, much more rigid, and essentially locking into place. This process ofheating above and cooling below the Tg is completely reversible in both directions.

III. Structural Performance

A. Tube StructuresThe most recent applications of the sub-Tg R/I composite materials have been for tubular structures. These

include structural members of the Space Solar Power truss (SSP), the main structural longerons for the DARPA-sponsored Innovative Space-Based Radar Antenna Technology (ISAT) program, and boom components for the AirForce Research Laboratory-sponsored Demonstration and Science Experiment (DSX). The DSX tubes use Kevlarfabric as the reinforcement material and are conically packaged. The ISAT tubes use a heavily longitudinally-biasedcarbon graphite fabric as the reinforcement material and is flat-fold packaged. Some of these structure are shown inFigure 4.

A significant amount of analysis and testing were performed on the ISAT tubes since they comprise a significantportion of the primary support structure. The R/I tubes fabricated for ISAT used a graphite fiber-reinforced polymer(GFRP) highly-unbalanced, plain-weave fabric as the reinforcement component. Some of the characterization testsperformed on these tubes included determination of the compression modulus, measurement of local thin-walledbuckling strength, and measurement of resin voids and fiber wetting in the laminates. The effects of packagingcycles were measured for the compression modulus and the buckling strength. These laminates, when packaged andunfolded properly, retain a high percentage of their initial mechanical properties over repeated packaging/ unfolding

cycles. A typical ISAT laminate (Graphite/L5.5 resin)retained 81% of its low temperature compressionstiffness after 12 packaging cycles. Table 1 lists themodulus before after packaging.

Axial compression tests for local buckling of thin-walled cylindrical columns at rigidized temperaturewere performed for a set of 1.3 m tubes. Figure 5ashows a typical axial compression tube test set-up forthe tubes shown in Figure 5b in the packaged state.Figure 6 plots the local buckling strength as a functionof packaging cycles for up to 6 cycles with a linearregression curve. For each packaging cycle, theaverage buckling strength of all the tubes is plotted.The least-squares fit linear regression curve is based onthe averages at each package cycle. Based on thiscurve, the buckling strength decreases byapproximately 8% of the unpackaged strength for eachpackaging cycle. Buckling occurred with the classical

Figure 2. 48 mlong R/I tube.

Figure 3. Hoop-direction Kevlar spiralwrap support for longitudinally fiber-biased tube.

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(a) (b)Figure 4. (a) Folded 1.3 m tube display, (b) Space-Solar Power truss

diamond buckling pattern, asshown in Figure 5c, and visualinspections of the tubes did notreveal any obvious damage to thelaminate after each test. Thedecrease in buckling strength ishypothesized to be the result ofpackaging as opposed tobuckling pattern damage. Byminimizing the required numberof packaging cycles for anapplication in conjunction withan appropriate factor of safety,R/I tubes such as these providegood strength even afternumerous packaging cycles.

Radiation effects on laminatecoupons were measured for the modulus of rigidity using the ASTM D1053 testing standard. Radiation consisted of5 MeV protons for total absorbed doses of 224 MRads at the entrance and 462 MRads at the exit. Analysisdetermined that for the MEO environment over a 10-year period the expected radiation dose to be 61 to 257 MRadsat different through-thickness depths. Table 2 shows that the apparent modulus of rigidity increased after exposurewhich is likely the result of further cross-linking in the material's polymer chains which is an effect that has beenobserved in other materials.

Impregnation of the resin in the laminate was measured using pictures taken through scanning electronmicroscopy (SEM). Figures 7a and 7b show pictures of the cross section of the laminate indicating good/efficientwetting of the graphite tows and fibers in the laminate that was made by a vacuum bagging process. The measuredvoid content was less than 1%. Measured fiber-resin ratio by volume is 48%:52%.

B. Predictions and AnalysisPredictions of the ISAT tube's longitudinal modulus of elasticity and coefficient of thermal expansion (CTE)

were computed using commercially-available CompositePro and other methods. Using the Fabric Builder Utility inCompositePro, values for void content and the fiber-resin volume ratio close to the measured values were input.The laminate was modeled with three layers. A reinforcement layer sandwiched between two thin resin layerswhich is similar to the method of manufacture. From a survey of the electron microscope pictures, a thin layer ofresin atop the reinforcement layer can be seen in a number of the photos although this layer is thicker in some areasthan in others. Table 3 compares predictions with measured results.

Among the possible factors affecting the predictability of the properties is the non-traditional characteristics ofthe laminate. The first is thehighly one-directional nature ofthe reinforcement material.Over 90% of the tows are in thewarp direction. The sizes ofthe tows used in the warp andfill directions are different.The weave pattern is aconventional plain weavepattern but only the warpdirection tows undulate; the fillfibers are straight. And, asmentioned previously, themethod of fabrication whichresults in top and bottom resinlayers is also a factor. Thecombination of all of thesecharacteristics demonstratesthrough the measured

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Figure 5. (a) Folded 1.3 m tube display, (b) Packaged 1.3 m tube, (c) Localthin-walled cylinder buckling strength as a function of packaging cycles for1.3 m tubes

American Institute of Aeronautics and Astronautics5

Local Thin-Walled Cylinder Buckling Strengthat Rigidized Temperature

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Parameter Measurement Prediction % DifferenceLongitudinal Modulus [GPa] 70 67.5 -3.6Axial CTE [ppm/C] 2.3 1.53 -33.5

Table 3. Compression modulus of tubular test specimens before and after packaging.

Figure 7. (a) Scanning electron microscope picture of the laminate cross section between two pieces ofmounting material. (b) Scanning electron microscope picture of the laminate cross section at 500 timesmagnification. Note: black regions are not voids but fiber defects.

properties that better accounting for the effects of theseparameters are needed for more precise predictions.

To better predict the mechanical properties in thefuture, a representative volume method (RVM) [6]could be used to try to model the unique construction ofthe laminate. Thus, each direction of the lamina can berepresented separately. These methods have shownsuccess with more traditional composites and weavepatterns. Application of these methods may providebetter results than those that have been employed todate.

C. Packaging and DeploymentFor large structures, inflatable and rigidizable

materials provide a distinct advantage with respect tostowage volume compared to traditional structures.This is accomplished through the compliance of thesub-Tg resin. A number of deployment controltechniques that take advantage of the unique traits ofthe R/I materials have been devised which incur verylittle parasitic mass. For the ISAT structure, R/I

materials allow compact stowage of the main structural components through flattening the cross-section of the tubesand folding near structural joints. Conical stowage through a telescopic packaging method of the booms to be usedon the DSX allows for the required compact stowage. Figure 8 shows a Sub-Tg Kevlar conical boom in the processof deployment which is similar to that to be used on the DSX. Packaging ratios for some of the R/I structures isshown in Table 4.

While stowed, if the material temperature has dropped below the glass transition temperature, the material willrequire heating to approximately 20° C above its Tg (glass transition) before initiating the deployment sequence. Atthis temperature, the material will then be compliant enough to allow it to readily unfold during deployment. Flat,conforming resistance heaters have been developed to provide the necessary thermal conditions to enabledeployment. These heaters are packaged with the R/I structure either on the exterior or interior surface. Figure 9

Mounting Material

Mounting Material

American Institute of Aeronautics and Astronautics6

Structure Packaging RatioISAT FDS / Objective System 100:1/300:1In-Space Propulsion Solar Sail 100:1Demonstration and Science Experiment 30:1

Table 4. Packaging Ratios of Select Structures

Figure 8. Deploying Sub-Tg R/I Conical boom. Figure 9. Packagable conforming resistance heaterassembled with R/I composite boom

illustrates a heater on the interior of a R/I tube. Future development possibilities include integrating the heatingelements into the composite R/I material itself which would eliminate one of the additional separate components thatenables the unique characteristic of the R/I material. Additionally, for certain configurations such as tubes, inflationgas is supplied to generate internal pressure which helps to achieve the desired shape and minimize surfaceimperfections that could have resulted in the folding process for stowage. This gas can then be allowed to escapepassively through the material or through a non-propulsive vent.

IV. Enabling TechnologyInflatable and rigidizable structures have many potential applications. So far, the majority of considered

applications are in space structures, especially large structures, where mass and stowage/packaging volume are at apremium. Some space applications include trusses, reflectors, booms, and decoys. Possibilities exist also for use inlunar and Martian surface structures.

Terrestrial applications for inflatable, rigidizable structures have been paid less attention. However, someemerging applications include wings of unmanned aerial vehicles, airships, and large and compact emergency ormilitary structures.

References

1Cassapakis, C., Thomas, M. "Inflatable Structures Technology Development Overview," AIAA Space Programs andTechnologies Conference, Huntsville, AL, Sept. 26-28, 1995, AIAA-1995-3738.

2Guidanean, K., Lichodziejewski, D. "Inflatable Rigidizable Truss Structure Based on New Sub-Tg PolyurethaneComposites," 43rd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, Denver, Colorado,Apr. 22-25, 2002, AIAA-2002-1593.

3Lichodziejewski, D., Veal, G., Derbes, B.. "Spiral Wrapped Wrapped Aluminum Laminate Rigidization Technology," 43rdAIAA/ASME/ ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, Denver, Colorado, Apr. 22-25, 2002,AIAA-2002-1701.

4Lichodziejewski, D., Veal, G. "Inflatable Rigidizable Solar Array For Small Satellites," 44th AIAA/ASME/ASCE/AHS/ASCStructures, Structural Dynamics, and Materials Conference, Norfolk, Virginia, Apr. 7-10, 2003, AIAA-2003-1898.

5Redell, F., Lichodziejewski, D. "Testing of an Inflation-Deployed Sub-Tg Rigidized Support Structure for a PlanarMembrane Waveguide Antenna," 47th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference,Austin, Texas, Apr. 18-21, 2005, AIAA-2005-1880.

6Gao, X-L, Mall, S. "A Two-Dimensional Rule-of-Mixtures Micromechanics Model for Woven Fabric Composites," Journalof Composites Technology & Research, Vol. 22, No. 2, pp. 60-70.