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This section will review the different techniques developed for the structural characterisation of gossamer sail systems and its components.

Material Testing

Gossamer sail system materials normally consist of lightweight structural support elements (booms or masts) and thin films. For the first, normally fiber reinforced plastics are the composite materials chosen given their high specific strength and the ability to tailor lay-ups for specific applications. Also, generally the combination of fiber and polymer matrix are chosen to have high strain to failure properties so that the support elements can be elastically collapsed into the very small volumes required for launch. For the films, high performance polymers that are highly resistant to the space environment are utilised. Their ease of manufacturing into ultrathin forms of just a few microns thick enable flexible membrane structures that can also be folded and packed into small volumes.

• Thin-Flexible Composites

Aside from high specific strength, the main property of thin-flexible composites is the high material deformation allowed that is usually utilised to exploit stored strain energy to motivate self-deployment of the structure from a highly compact coiled/folded state. The results are highly compliant structures, which take advantage of the competitive mass density of their constitutive composite materials, that form rigid structures once the geometric stiffness of the deployed config- uration is achieved. Therefore, recent interest has been growing among space structures researchers

to better understand and predict the mechanics of flexible high strain fibre reinforced composites. These materials are useful for enabling large gossamer systems to be folded in small volumes for launch. For example these materials are constructed into simple hinge component for an articu- lated truss longeron [Pollard and Murphey, 2006], or they are used monolithically to form a rollable slit-tube boom [Fernandez et al., 2014a] or doubly-curved shell [Keil and Banik, 2011].

The key advantages of using thin flexible high strain composites in a flexure loading regime, is that they can endure strains that are much higher than measured by standard coupon tests in pure tension or compression [Yee et al., 2004]. It is well known that fibre stability is the most common compression failure mechanism in composite materials [Jones, 1998]. However, it has recently been discovered that the tension side of these thin flexurally loaded laminates can stabilise the compression side [Sanford et al., 2010, Murphey et al., 2011], a task normally left only to the matrix in pure compression or shear loading. It is this stabilization that allows these laminates to sustain abnormally high compressive strains. By taking advantage of this behavior, new folding schemes are made possible that promise to reduce the complexity of traditional mechanically articulated structures. An improved test fixture and procedure, that minimized potential sources of error, was presented in [Sanford et al., 2011] to quantify the pure bending moment versus curvature behavior of thin composite laminate coupons.

In [Yee et al., 2004] it is shown that the maximum bending strain in plain-weave laminates decreases when the number of plies increase, and that this trend does not happen in triaxial-weave laminates. In [Yee and Pellegrino, 2005] tension, compression, in-plane shear, and bending tests of coupon laminates of interest for deployable structures applications, made from plain-weave T300 carbon fibre with Hexcel’s 913 or 914 epoxy resins, were analysed. The conclusion was that the ultimate strains along the fibres for one-ply laminates were up to 36% higher than those of two plies. Also that the [0/90] laminates allow higher maximum surface fibre strains than the [± 45], though the latter can be folded into much tighter radii. Generally, the maximum surface strains found (1.9-2.8%) are much larger than the failure strain of T300 fibres (1.6%) reported by the manufacturer Toray. Scaling effects could be invoked to justify these results [Fleck and Liu, 2001], which normally increase when the ratio between specimen thickness and fibre diameter is less than 200.

Also the matrix of the composite materials plays a very important role in the final packaging ratio allowed for a given structure element or architecture. A soft matrix will enable a composite structure to be bent to a much tighter radius than is achievable with a normal stiff matrix. This advantage is exploited by for example rigidisable composites, where the matrix is in a softened state during packaging and deployment, i.e by the application of heat, and is then rigidised for use once deployed in space. The micromechanic deformation mechanisms associated with the folding of softened matrix composites was studied in [Murphey, 2001]. It it shown that micro-buckling of the fibres in the soft matrix allows unprecedented compressive strains (of the order of 5%) that are associated with laminate bending of highly compact packaging schemes. However, experiments revealed that the folding process can degrade tensile strength and buckling strength. Also, the softer the matrix is the more elastic relaxation or creep over time it would suffer under e.g. mechanical loading, such as the shear stresses induced during folding.

In general the creep of polymer composites is a function of all service environmental parameters (temperature, stress, physical aging, and moisture). A study on the interactive effect of all four parameters was conducted in [Gupta, 2009], and demonstrated that the creep under combined effect of all four parameters is entirely different and can not be predicted by just adding the individual effect. Recently, extensive research efforts are being focused at studying the effects of creep on lightweight structures formed of thin-flexible polymer composites. In [Gupta and Raghavan, 2010] the creep of plain weave polymer matrix composites under on-axis and off-axis loading was studied. The time-dependent behavior of thin CFRP flexures of a few unidirectional plies was studied in [Santer and Saturni, 2012]. The IM7-8552 specimens were held in a folded configuration by compression-induced bending for a long time. It was shown that tensile surface strain increased over time.

In [Kwok and S., 2012] a micromechanical finite element homogenization scheme to determine viscoelastic properties of woven composite laminae is presented. The solution scheme is employed in numerical simulations of deployment and shape recovery of composite tape-springs, which are shown to agree with experimental measurements. It was also found experimentally that stowage has the effect of slowing down both the short-term deployment and long-term shape recovery of folded tape-springs at elevated temperatures.

In [Makuch and Reynolds, 2012] a method for developing an in situ sensor for health monitoring composite tape-springs was investigated. The initial sensor proposed is able to detect viscoelastic effects, such as exponential decrease in strain due to stress relaxation, and record changes due to temperature.

In [Fernandez J.M., 2012] the creep effects of single-axis-rolled, doubly-curved composite shells, for a new concept of parabolic reflector constructed from thin fibre reinforce composite plies, are assessed. The study investigated over thirty different laminates for low creep, manageable folding, low stored strain energy, low folding radius, and high stiffness for ease of ground-based testing of the rollable shells. One of the findings was that fibers along the two principal axes of stowage (roll axis and normal to it) were necessary for reducing stress relaxation effects, but that fibers along the roll axis should be included in the centre of the laminate rather than in the surface plies because of the otherwise very high strain energy effects. This example shows the difficulty of designing ultralight structures with thin-flexible composites, where sometimes opposing requirements need to be fulfilled i.e. low folding radius for packaging versus low strain energy and creep.

• Thin-Film Membranes

For the sail films, high performance polymers that are highly resistance to the space environment are employed. Their ease of manufacturing into ultrathin forms of just a few microns thick enable flexible membrane structures that can also be folded and packed into small volumes.

Characterisation tests

There are many tests that need to be performed to characterise a given polymer film and to qualify it for space applications. These tests from [Dupont, 2014, Sheldahl, 2014] (where the standard test method is written in parenthesis) include:

- Mechanical tests: tensile tests (Test method ASTM D-822-91) to characterise the polymers ultimate tensile strength, yield point at 3%, stress to produce 5% of elongation, ultimate elongation, tensile modulus, and Poisson’s ratio (average three samples elongated at 5%, 7%, 10%); folding endurance cycling testS (ASTM D-2176-89); tear-strengh-initiation test (ASTM D-1004-90); tear strength-propagating test (ASTM D-1922-89); density test (ASTM D-1505-90); impact strength; and low temperature flex life test (IPC TM 650, Method 2.6.18). - Thermal tests: melting point test (ASTM E-794-85); thermal coefficient of linear expansion (ASTM E-794-85), coefficient of thermal conductivity test (ASTM F-433-77); specific heat test (Differencial Calorimetry), dimensional stability (% shrinkage) test (ASTM D-5214-91), thermal aging test (UL-746B).

- Electrical tests: dielectric strength test (ASTM D-149-91); dielectric constant test (ASTM D-150-92); dissipation factor test (ASTM D-150-92); and volume resistivity test (ASTM D- 257-91).

- Chemical tests: moisture absorption test (ASTM D-570-81); vapour permeability test (ASTM E-96-92).

- Radiation tests: exposure to electron, protons, and UV radiation in vacuum and its effect on the main mechanical, thermal, electrical, and optical properties.

- Optical tests: absorptance test (ASTM E-490 or ASTM E-903); emittance test(ASTM E- 408); transmittance test; and specularity test.

- Vacuum tests: Outgassing test for collected volatile condensable material (CVCM), recovery mass loss (RML), total mass loss (TML), and water vapour regained (WVR) (Micro-VCM according to ECSS-Q-70-02a).

Other additional tests to be performed on polymer thin film coatings, adhesives and tapes used on gossamer sail systems are:

- Tests for coatings and overcoatings: adhesion tests (ASTM D-1000), abrasion tests (ISO 9211 method 01), optical properties tests (ASTM E-490 and ASTM E-408), surface resistivity test (ASTM D-257).

- Tests for adhesives and pressure sensitive adhesive (PSA) tapes: adhesion tests (ASTM D- 1000), peel strength and shear force test (ASTM D-903).

Thin-film material development

The USA has led the research and development of high performance polymers in thin film forms and their qualification for space applications. In industry Dupont has lead the way devel- oping polymers with famous trade names such as Kapton R_{, Mylar} R_{, Teflon} R_{, Teonex} R_{, and} Kevlar R_{. In the late 1970s NASA recognised the need to advance polymers tailored for space} environment requirements, in particular with reference to resistance to atomic oxygen, ultra-high vacuum, electrons, protons, ultraviolet radiation, and with high transmittance to visible light. A resin development and materials characterization programme was undertaken at Langley Research Center that has resulted in a number of products: polyimides such as LaRCT M _{CP-1 and CP-2} (now commercialised by SRS Technologies), that are 80-100% transparent in the visible spectrum (colourless) and are heat sealable; and Atomic Oxygen (ATOX) resistant products, such as TORT M from Triton Systems Inc, that have over 15 to 100 times the AO resistance of uncoated Kapton R_. In the last decade, two new polyimide-based products appear in the market: (POSS) Kapton R _by SRS Technologies, that has increased the AO resistance of Kapton; and GORET M _{films reinforced} with PTFE nanofibres by W. L. Gore & and Associates Inc, that can be casted into polymers such as Kapton R_{, LaRC}T M _{CP-1 or Upilex-S} R _{for an increase of tensile strength and tear initiation} and propagation.

In addition, the Small Business Innovation Research (SBIR) programme funded several industry lead projects managed by NASA centres in the late 1990s and early 2000s [Garner et al., 1999]. For example, Astral Technology Unlimited Inc, that specialises in metallisation of a variety of films to below 1 µm thickness produced Aluminium and Chromium coatings on PET and PEN films of just 0.9µm; Triton Systems Inc produced TORT M _{and CP class films that are now rip-stop reinforced} and electrically conductive in order to reduce electrostatic charge buildup; and SRS Technologies produced 1.5 µm thick CP-1 films with embedded fibers and rip-stops, and developed methods to produced continuous reinforced rolls and manufacture adhesiveless seamed full-size sails. Integral shear-compliant borders were produced for ATK’s 20 m solar sail demonstrator [Laue et al., 2005]. Also, carbon-filled CP-1 with high emissivity and special additives for de-wrinkling membranes have been investigated for near-Sun solar sail missions [Talley et al., 2002].

Recently thin-film damage tolerant films have been explored using polymer additive manufac- turing. In [Belvin, 2012] a fused deposition modelling (FDM) process was used to build directly on the thin film, lightweight, hierarchical complex geometries for integral rip-stops. Tensile testing of specimens with an initial tear revealed that these reinforced membranes had a higher tear resistance than neat films of equivalent mass.

In Europe the Solar Sail Materials project funded by ESA was aimed at making progress on solar sail materials, their assembly, and integration for future European solar sail missions [Dalla-Vedova et al., 2011, Dalla-Vedova et al., 2014]. It consisted of the identification of a suitable film material for European projects, thinning of the reference film material/s to 3 µm or less, identification of suitable film coatings (reflective and emissive) and protective overcoats, identification of an environmentally resistant assembly concept and related adhesive/s, and selection of a preferred sail-boom fastening concept. Also, Italy has investigated techniques for photolysation of sail films in space, that would remove the plastic substrate in order to obtain an ultralight all-metal solar

sail [Scaglione and Vulpetti, 1998].

JAXA’s IKAROS 7.5 µm thick sail membrane used a new material, ISAS-TPI, jointly developed by JAXA and Kaneka Corporation, using a nonstandard synthesis process. Rather than relying on the current methodology of selecting COTS materials and fitting them into a nonstandard application, such as solar sailing, the IKAROS team developed a material and process to fit the application [Bryant et al., 2014]. The ISAS-TPI polyimide is a gel cast film, that is UV stable, creep resistant with excellent thermal stability below Tg, is heat heat sealable, and is soluble in the

imide form. Following this impetus to include polymer synthesis to optimise material properties for a specific application, NASA/LaRC is currently developing a new process to produce ultra-thin high-performance films [Bryant et al., 2014]. This process consists of moving away from gel cast films, that normally have a thickness production limitation, onto belt and melt-blow forming. The latter process would allow the production of continuous ultrathin film membranes from COTS resins or from new purposely developed ones for a specific application, such as solar sailing.

Sail material life testing

Extensive experimental studies have been carried out to assess the survivability in space of thin film material candidates, since the launch of the balloon-type structures ECHO I, ECHO II and Explorer IX in the 1960s, that demonstrated the utility of polymeric materials for space applications. There are many environmental factors that can affect thin films in space, degrading their performance or even producing catastrophic failure. These mainly include: Atomic Oxygen (ATOX), radiation (mainly ultraviolet/UV), ultra-high vacuum, temperature extremes, high and low energy protons and electrons, charging, and micrometeoroids. Normally tests are performed to study the synergistic effects of several of these environmental factors [Dever et al., 1992], as in space a combination of them will occur. For example when thermal effects, resulting from the ratio of absorptance/emittance, are combined with UV radiation, additional mechanical failure modes, such as material ablation (radical fragmentation and static discharge), chemical hardening (radical cross-linking), and physical failure (cracking and tearing) can occur.

The effects of vacuum UV radiation on thin (12.7-25.4 µm) polyimide films proposed for use on the JWST sunshield was studied in [Dever et al., 2001]. Materials included in the screening test included Kapton R_{E, Kapton} R_{HN, Upilex} R_{S, CP1, CP1 with vapour deposited aluminum} (VDA) on its back surface, and CP2 with a VDA coating on its back surface. Samples were exposed to approximately 1000 equivalent sun hours (ESH) of VUV radiation. Changes in the solar absorptance were observed for some materials, and, additionally, significant changes in spectral reflectance were observed in the ultraviolet to visible wavelength region for all of the polyimide materials tested. Changes in the ultimate tensile strength and elongation at failure were within the experimental uncertainty for all samples, which indicate that the fraction of the polymer thickness affected by VUV was not enough to cause changes in the bulk mechanical properties.

In [Edwards et al., 2004a, Edwards et al., 2004b] the first study of space environmental effects on samples of solar sail material candidates was carried out. Significant degradation of mechanical properties after radiation exposure was confirmed. However, the thermo-optical properties did not degrade much, and thus propulsion performance of the solar sail should not be significantly affected. In [Murphy, 2007] the results of sail membrane material life testing carried out on 2.5

and 5 µm aluminised CP-1 film samples in shown. Testing at the NASA/MSFC SEE laboratory included electron, proton, UV, and micrometeroid exposure. Optical and mechanical properties were measured before and after exposure. The conclusion was that the VDA CP-1 membrane passed the requirements for a variety of near-term solar sail missions.

Different atomic oxygen sensors designed for low Earth orbit experiments have been proposed, such as the one shown in [Osborne et al., 1999]. The small power and mass, reusable sensor unit developed is designed to be affixed to an exterior spacecraft surface that experiences AO impingement. The effects of atomic oxygen on several hydrocarbon-based polymers, which is the chemical composition of the majority of thin film materials, was studied in [Allegri et al., 2003]. Kapton R_{100 HN and PM-1E samples were exposed to the space environment during LDEF and} MIR “Komplast” mission. Mass and thickenss loss data as well as variations of optical properties were assessed. Anisotropic superficial texturing and roughening of the exposed polymers was also pointed out. For unexposed and inner placed flown specimens, no significant alteration of optical properties was shown. While the solar absorptance of outer surfaces was strongly increased, thus implying a sensible reduction of both transmittance and reflectance. On the contrary the infrared emittance varied only slightly from unexposed to outer exposed specimens, pointing out how the effects of surface texturing are remarkable only for visible light wavelength. Issues that can affect the atomic oxygen protective coatings making them ineffective in some cases yet still effective in others was studied in [Banks et al., 2004], by observing in-space examples of failed and successfully protected materials using identical protective thin films. Tests on coated Kapton R _{samples revealed} that VDA films are not as protective as sputter-deposited silicon dioxide films because of a greater number of pin window defects. Also, computational modelling was conducted, which indicated that atomic oxygen trapped between the front and back surface of double-aluminised films caused accelerated undercutting damage, as shown on the ISS’s thermal blankets. The atomic oxygen erosive process in unprotected Kapton R _{films in LEO can be as high as 0.1mm/year as calculated} in [Dever et al., 1992].

Hypervelocity impact tests on VDA coated Kapton R _{films at cryogenic (40 K) and elevated (420} K) temperatures was conduced in [Wells, 2006]. The materials were impacted with 40-100 µm soda lime spheres utilising a plasma drag gun to accelerate the particles to velocities between 5 and 12 km/s. The two test conditions resulted in significant differences in the nature of the impact damage, as was anticipated by [Myers et al., 2003] that carried out impact tests with velocities up to 7 km/s, and studied temperature effects on the bumper hole diameter. The tests at cryogenic temperatures produced impact damage characteristic of sheer forces. At elevated temperature impacts produced domed structures with possible subsurface damage characteristic of vaporization and flow processes. The elevated temperature test also resulted in delamination around the deformed area of some of the impact sites. In [Edwards et al., 2004b] hypervelocity impact tests were carried out, on both pristine and radiation aged solar sail thin-film material candidates. The preliminary tests indicated that the sail material does not posses a tendency to rip as a result of micrometeroid impact.

Static testing

Static testing of gossamer sail structures normally involve the determination of the structure’s shape under static loading conditions. The results are then normally used for numerical model validation, fine-tuning or redefinition purposes. Generally, measurements of the gravity-induced sail sag are taken. Measurements of boom deflection under axial, bending, and torsional loads are also taken.

• Measuring techniques

Traditional measuring equipment used on gossamer structures include force transducers to meas- ure and validate the applied forces, and strain gauges to measure strain in the loaded booms