[American Institute of Aeronautics and Astronautics 49th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference 16th AIAA/ASME/AHS Adaptive Structures Conference

Long-Term Structural Variation of Deployable Membrane Space Antennae

Johanne C. Heald* and Xin Xiang Jiang† Spacecraft Engineering Directorate, Canadian Space Agency, Montréal, Québec, Canada J3Y 8Y9

This paper presents an investigation into the properties of membrane structures for space-based antenna applications when subjected to tension in their deployed state. The membranes described here are preliminary to the development of prototypes for a possible on-orbit flight experiment in thin structure deployment, where such an experiment would address necessary questions related to the use of this technology for membrane synthetic aperture radar. In the work described in this paper, the magnitude and time variation of material creep and the stress relaxation expected of various membrane materials is examined. Various experiments are conducted on candidate polymer materials being considered for use as substrate structure for the crystalline electronic components. Included among these materials are polymer film materials, as well as liquid crystal polymer (LCP) braided substrates. Early experimental results with Kapton E indicated that both material creep and stress relaxation are significant contributors to deployed membrane position uncertainty, which may dictate how useful membranes may be to antenna applications. Experiments detailed here have used a Dynamic Mechanical Analyzer (DMA) instrument to examine the elastic and microplastic behaviour of the membrane material in ground laboratory conditions. Two types of thin film polymer material are considered as possible membrane structures for antenna applications, as well as a braided LCP material. The goal of this investigation is to be able to better predict the variation in boundary conditions of a flat, taut membrane, and therefore the flatness variations that would result due to constant tensioning of the membrane in its post-deployed state. The effect of this slowly time-varying plant on the control that must be exerted on the membrane in its space environment must be taken into account.

* Structures Research Scientist, Space Structures Group, AIAA Member † Space Environment Research Scientist, Thermal and Materials Group © Government of Canada, 2008

American Institute of Aeronautics and Astronautics

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49th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference <br> 16t7 - 10 April 2008, Schaumburg, IL

AIAA 2008-2214

Copyright © 2008 by Government of Canada. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

I. Introduction HILE there has been a great deal of discussion in the past few years about the challenges posed by membrane structures in space, that discussion has centred on a limited number of design concerns. The characterization of basic mechanical properties of the thin films membranes has been neglected in favour of

issues such as membrane wrinkling and the deployment of the flexible structure. The membrane sunshield of the James Webb Space Telescope1 was the application that was driving much of the research in membrane wrinkling. Given the design of the deployment system that loaded this sunshield boundary, the consideration of stiffness losses, the required nonlinear numerical modeling, and the non-uniform heating of a membrane structure due to wrinkling at membrane corners were critical issues. However, there are other architectures that could make use of membrane structures that do not have the same boundary constraints. Particularly, for science applications in which the membrane structure supports data collection devices, the accuracy and stability of the membrane structure is integral to the accuracy of scientific data. Using membrane structures as an inseparable part of a scientific instrument, with more design choices available in the boundary conditions, requires that membrane mechanical and material properties be examined from a different perspective.

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The Canadian Space Agency (CSA) is currently studying several applications that might make use of thin-film membrane structures. Particularly, these applications include C-band antennae for synthetic aperture radar (SAR) used in low-Earth or planetary orbit2 and antennae for sea-surface height altimeters. Multiple membrane layers would be required in order to support an antenna radiator plane physically separate from its feed network and ground plane. Additional thermal multi-layer insulation (MLI) and shielding layers would likely also be required. Polymer materials Kapton E or HN, Pyralux AP, and Liquid Crystal Polymer (LCP) are the candidates being considered for the thin-film membrane antenna structure. In order to have confidence in the on-orbit deployment sequence, control of the membrane position when subjected to disturbances, and long-term electrical performance of such antennae, the structural behaviour and material properties of the membrane providing the structure must be known. Very few of the mechanical properties of membranes have been verified experimentally. Several different methods of modeling membrane structures as two-dimensional continua in tension3-5 and as thin shells with non-zero bending stiffness6, have been proposed as ways of tackling the problem numerically. These methods were introduced and developed specifically in order to address the varied stress states of partially wrinkled and partially taut membranes. Experimental work on membrane structures has focused on macroscopic behaviour, examining wrinkle amplitude and area under varied loading conditions7,8 quantifying dynamic behaviour9 and determining deployed shape accuracy.10 The Canadian Space Agency has done some preliminary work on membrane microplasticity, and on the influence on the behaviour of the structure due to degradations caused by the space environment on membrane material.11, 12 This paper will re-examine some of the more puzzling results previously achieved, as well as fundamental issues involved in membrane mechanics. The elastic, viscous, and plastic nature of the membrane is explored using samples of different candidate membrane materials. The results are compared to current models, and the effect of nonlinearities is examined for the stowage, subsequent deployment, and on-orbit control of membranes for space applications.

II. Membrane Prototype Development at the Canadian Space Agency
y Canada has developed an expertise in Synthetic Aperture Radar antenna, and it is from this expertise that
Canada's interest in membrane antennae stems. This experience is based on rigid-panel SAR, however, and synthetic aperture radar has several requirements that complicate the design of a membrane structure to replace the rigid panel. First, a C-band SAR antenna must be large in order to obtain the required accuracy. Plans for a future membrane antenna have been fluctuating in terms of size over the course of the early planning stages, although it has always been on the order of tens of meters. The current design calls for a membrane of the order of approximately 4 m by 10 m, which would be deployed in two wings of 4 m by 5 m each. These two wings would then be held in tension on-orbit. Secondly, due to the fact that the membrane structure constitutes part of the data-gathering instrument, the requirement on the level of flatness of the membrane is rather stringent, on the order of a maximum of a few millimeters of out-of-plane deflection for a C-band antenna. This flatness has to be maintained


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Figure 2. Tensioning of prototype membrane layers

despite thermal cycling, vibration disturbances, membrane stress relaxation, or wrinkling of the membrane. Thirdly, the dimensional stability of the membrane is also critical, since the locations of electrical elements on the membrane

cannot be allowed to physically drift from one another if the membrane were to undergo stress relaxation or a change in properties over time. It is this quality of stability of the membrane over time that is addressed by the examinations of the effects of the material stress relaxation in the current paper.

As the Canadian Space Agency is still in the early stages of its research into membrane structures technology, it

he suitability of these structures for Synthetic Aperture Radar, as well as determine the technical difficulties involved in their use. While the CSA has developed a variety of membrane mock-ups and scale research models in the past,13 it is developing its experience with a fully and autonomously deployable prototype. For these reasons, the CSA is currently

deployable membrane structure prototype in-house, which is described in detail in a previous and current paper.14,15 The material to be used in the membrane prototype has not yet been selected, and it is expected that

this decision will rest on both mechanical considerations such as those presented in this paper and the electrical considerations described elsewhere.2

wishes to judge t

developing its first

The current prototype of a SAR membrane antenna in C-band being developed at the Canadian Space Agency is

sized at a quarter-scale of the full structure required for a full space mission. See Figure 1 for the sequence of deployment of one wing of the prototype. The prototype design calls for 2 to 5 membrane layers, interconnected with copper-coated membrane material carrying electrical signals from one layer to another. Figure 2 shows the tensioning of multiple membrane layers while maintaining a constant distance between the layers. Layers distributing the power will be almost completely covered with copper, whereas layers carrying only the radiating elements will be sparsely covered. Several separate layers of membrane must be tensioned and kept at a constant distance from each other. Currently, this separation distance between the top-most layers in Figure 2 is estimated at 14 mm. The amount of copper on each of these layers varies, and the membranes will not only be subjected to different thermal environments, with some layers exposed to the sun while others are at least partially hidden from it, but the varying amount of crystalline material will cause each membrane layer to react differently. Figure 3 shows the membrane prototype with several layers peeled back during manufacture. The candidate materials being considered for the membrane prototype are thus being examined both with copper coating and without at this point. The results presented here are crucial to future CSA decisions concerning membrane structures.

a b

c d Figure 1. Deployment frame for one wing of two-wing membrane


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Figure 3. Prototype membranes during manufacture

III. Previous experience with the investigation of thin-film membrane materials

When we first began working with membrane materials, it was noticed that tension applied to membrane layers changed continuously over time, although the applied tension never varied. It was suspected that the membrane materials were experiencing some creep, although given the variety of the material, it was never determined how much creep could be expected or what variables were affecting this creep. These initial tests were conducted in a High Bay environment, which was not particularly well isolated from the surroundings in terms of temperature and humidity variation. So it was not clear what variables were affecting the membranes.

In order to examine the structural behaviour of thin-film membranes in a more controlled environment, small 30 mm long by 6.5 mm wide coupon samples were examined in a Dynamic Mechanical Analyzer (DMA). The DMA instrument is used to verify physical properties under controlled temperature conditions. The sample is mounted on a clamp assembly for the application of tension, with its temperature monitored by a chromel/alumel thermocouple. While the furnace assembly can control the temperature over a wide range, the experiments done up until the current investigations were all conducted at the temperatures of +35 ± 2 °C. A coupon sample (Kapton E in this instance) is shown mounted in the DMA clamp in Figure 4.

The experiments were performed on small coupon samples of Kapton® Type 100 HN Film (25 µm or 1 mil thickness), copper-clad AP Pyralux® of 25 µm total thickness, including the two 7 µm copper coating layers, and

braided 25 µm LCP material with no
coating. Prior to mounting in the DMA, the samples were conditioned to the test environment for a period of no less than 24 hours. Once the samples are mounted, the sample test section consists of the full width of 6.5 mm and a length of approximately 15 mm, with individual variations associated with the clamping of each individual sample. Prior to the load/unload cycles of the experiment, a small tensile force of 0.005 N was applied to the sample in order to extend it to its full length. This preload is considered the zero-point for the determination of any subsequent strain.
The results of the coupon testing showed considerable creep at the scale of the deformations being considered. Figure 5 shows the creep experienced by a sample of Kapton film after exposure to UV radiation. In this case, the coupon is sealed

in the DMA furnace. Thus, it is not exposed to humidity changes in the surrounding environment, and is kept at a constant temperature of +35 ± 2 °C. Naturally, the best-fit line shown here could be a steady variation in the strain of the coupon due to temperature equilibration, but it seems to be completely unaffected by variations in the small tensions being periodically applied to the sample. For samples of this size, even the largest applied tensile force of 0.040 N in Figure 5 is only 0.123 MPa or approximately 17.8 psi for a 6.5 mm-wide coupon with a working

Figure 4. Kapton sample in DMA clamp assembly


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Figure 5. Best-fit creep line in Kapton sample exposed to UV radiation

thickness of 50 µm (2 mil). It is unclear from these experiments whether the creep is due solely to temperature change or whether it is also due to the Kapton material, and what kind of variation in creep could be expected for larger applied tension.

Similarly, in Figure 6, a coupon of LCP demonstrates very similar behaviour, for similar levels of tensile force applied. Note the resulting deformations are correspondingly small. For a coupon working length of 13.6 mm, a 1 µm deformation is the equivalent of 0.0074 % strain.


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Figure 6. Best-fit creep line for LCP coupon

For a more precise determination of creep (of “drift” or stress relaxation) effects on membrane coupons,

experiments were again run on the DMA that consisted of loading the coupon to a variation in applied tension at a particular fixed temperature. The stress on the sample is increased from its initial preload value up to three different operational points, and then held constant for a period ranging from one hour to one day. The strain of the sample is measured while this constant stress is applied, to determine if any relaxation can be detected.

IV. Discussion of results In order to hold the material creep constant in these experiments, it was decided to examine only samples of the

copper-clad AP Pyralux material. A copper-clad polymer is the material that is most likely to be used in the multiple membrane prototype, so it makes sense to examine this material first. In order to examine the bahaviour of this material at varying conditions, it was subjected to three levels of applied tensile forces: 0.05 N (0.154 MPa or 32.3 psi), 0.08 N (0.246 MPa or 35.7 psi), and 0.12 N (0.369 MPa or 53.5 psi). to observe the effect of temperature, the operating temperatures of the test were also varied, between +35 ± 2 °C and +120 ± 2 °C.

Figure 7 shows the effect of applying a preload of 0.005 N and a constant temperature of +80 °C to a pristine coupon sample for 30 minutes. As expected, the effect of applying a higher temperature to the coupon results in the coupon increasing its length. After the 30-minute isothermal period, a constant tensile force of 0.12 N is applied to the coupon, and the resulting strain in the sample is measured for 120 minutes. The resulting change in strain is measured as the difference between the strain measured at 32 minutes and that measured at 149 minutes. In this example, the change in strain measured over the two-hour period is 0.0021%, or 0.2856 µm. Note that the strain in the sample continues to change even after the full two-hour period is expired.


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Figure 7. Measured strain in Pyralux coupon with 0.12 N tensile force applied at 80°C

Figure 8. Twenty-four hour exposure of Pyralux coupon to 0.12 N tensile load at 80°C


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Figure 9. Changes in strain over 2-hour period at applied tension of 0.05 N for varying temperatures

Given that the strain continues to change with time, an additional test was applied to see how the coupon would behave over a longer time period. Figure 8 demonstrates the effect of the same tensile force and the same temperature being applied to the same material but for a much longer 24-hour period. In this case, a much greater 0.01632% strain can be detected, or 0.00223 mm. Most importantly for this coupon, while the initial creep appears to be contained within the first 200 minutes, there is some variation in the level of strain throughout the test period, and it has not completely died away at the end of the test window.

Finally, in Figure 9, the creep of the coupon with an applied tensile load of 0.05 N is compared at various temperatures. As expected, a higher temperature dictates a high level of expected creep. Interestingly, the slope of the line representing strain with time is significantly steeper for 120°C than it is for the next highest temperature, 100°C. The same cannot be said for the lines representing 100°C and 80°C, although the variation between the two temperatures is exactly the same in either case. Finally, note that the slope of the line representing the data measured at 50°C is the least steep, which corresponds to the idea that creep increases with increasing temperature.

Finally, to compare the creep measured at the three levels of applied tensile force, refer to the varying levels of

creep in Table 1. Note that, once again, the creep follows a logical, but not necessarily a linear, trend. The error in these measurements is quite high with respect to the strains measured: ± 0.0007. The creep levels measured do increase with increasing temperature in each case. However, comparing the results of one tensile force to another, the creep does not necessarily increase with increasing applied tensile force. It is important to note that measurements within each level of tensile force were made sequentially, so it is possible that there are external variables at work in these results.

Previous work16, 17 on strain deformation in polymers have noted the nonlinear nature of those deformations. However, they have concentrated on high strain rate responses, such as reactions to impact loading, which is not the case under consideration here.


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Table 1. Creep (in % strain) for varying levels of applied tensile force Temperature (degC) F = 0.050 N F = 0.080 N F = 0.12 N

50 0.0016 0.0010 0.0018 80 0.0038 0.0038 0.0021 120 0.0075 0.0049 0.0040

V. Conclusions and future work Creep has been a factor in test made on all three membrane materials discussed here, Kapton, copper-clad

Pyralux, and braided LCP. The results obtained in the most recent tests presented here, on the copper-clad Pyralux, are consistent, but not necessarily linear. Nevertheless, the data measured here for that single membrane material clearly demonstrates that the amount of creep in the material will vary, not only with the applied tensile force, but also with the temperature applied. Further, the results have shown that the variation with temperature, as well as the variation with applied tensile force is not a linear one. This nonlinearity makes it difficult to summarize quick design rules that could be applied in any circumstance. However, from the point of view of the prototype SAR membrane currently be developed for deployment in an ambient environment, the effect of temperature can be ignored in favour of the effect of tension. The same will not apply, of course, to a SAR membrane structure deploying on orbit.

Acknowledgments This project is sponsored by CSA research funding under STRP-001.

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[American Institute of Aeronautics and Astronautics 49th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference 16th AIAA/ASME/AHS Adaptive Structures Conference