Technical Fabrics for Aerospace Applications.pdf

Copyright © 2009 by Cubic Tech Corp.

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High Strength-to-Weight Ratio Non-Woven Technical Fabrics for Aerospace Applications

Keith McDaniels1, RJ Downs2, Heiner Meldner3, Cameron Beach4, Chris Adams5 Cubic Tech Corp., Mesa, Arizona, 85205

Flexible laminates, customized for specific performance parameters, are of interest to aerospace programs such as lighter-than-air vehicles, balloon systems, decelerator systems, flexible inflatable structures and pressure vessels. The material requirements for these applications include high strength-to-weight ratio and modulus, low gas permeability, pressure retention and the capability to survive in harsh atmospheric, marine and/or stratospheric environments for extended periods of time. Non-woven multidirectional oriented composite laminates using high performance engineering fibers are produced by Cubic Tech to meet these requirements. These flexible laminates achieve a significant weight savings over woven fabrics of similar strengths by eliminating strength and modulus loss and other structural deficiencies caused by crimping of yarns during the weaving process. The absence of crimp in non-woven fabrics results in a linear elastic response that allows for ease in predicting material properties and simplification of structural models. These flexible composites afford the ability to specify structural properties, oriented to meet any design requirement. Parts can be manufactured with complex 2D and 3D geometries with integrated structures, load patches and attachment points. Structures fabricated from these laminates can be joined using standard industry seaming techniques to produce seams stronger than the base laminates. Properly designed seams hold structural loads for extended periods without failure, slip or creep.

Nomenclature CT = Cubic Tech Corp. FAW = Fiber Areal Weight LTA = Lighter-Than-Air MDA = Missile Defense Agency PBO = Poly(p-phenylene-2,6-benzobisoxazole) PEN = Polyethylene Naphthalate PET = Polyethylene Terephthalate PVF = Polyvinyl Flouride SBIR = Small Business Innovation Research UHMWPE = Ultra High Molecular Weight Polyethylene USASMDC = United States Army Space and Missile Defense Command

I. Introduction he development of advanced materials will continue to enable previously impossible aerospace applications. Research continues on novel materials that minimize weight and improve strength, while addressing other

critical properties. Flexible composites, owing their heritage to high performance, low stretch sails used in yacht racing competitions, including around the world races, continue to be developed by Cubic Tech Corp. (CT). These materials take advantage of the newest in high strength-to-weight ratio fibers, technical films, woven fabrics, and

1 Materials Engineer, Research and Technology 2 President of Cubic Tech and Senior Engineer, Research and Technology 3 CEO of Cubic Tech 4 Materials Engineer, Research and Technology 5 Vice President of Operations, Industrial Engineer, Research and Technology

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other high performance materials. Lightweight outdoor products, medical devices, inflatable tubes, parachutes, parafoils, large kites, and most recently high altitude airships have been developed using this technology. CT received funding from the MDA (a program transferred to the USASMDC) for two phases of SBIR work on the development of “Very Lightweight High Tenacity Fabrics” for high altitude lighter-than-air systems.

This paper will introduce concepts of flexible composite technology and the potential advancement in the state-of-the-art of materials for LTA, terrestrial inflatables, space inflatables, and decelerator systems. To encompass the spectrum of potential material properties, examples of ultra-light, medium, and heavyweight materials of various constructions will be presented. The intuitive engineer may find utility in these materials for applications requiring similar weight materials or anything in-between.

II. Material Challenges Material challenges in aerospace applications have been well documented in previous publications. For high

altitude, LTA systems, strength-to-weight ratio of the material is critical to payload capability, the size of the system, and maximum operational altitude. Materials must be able to survive tough stratospheric environmental conditions, which include low temperature, intense UV radiation, and high ozone concentration; low gas transmission, through the hull and ballonet, is necessary to maintain lift for long duration missions, high tear resistance ensures the durability of the vehicle (preventing catastrophic tear propagation), and materials must have adequate bondability for durable joints. Finally, low temperature flexibility is necessary for ballonet materials, which inflate and deflate during operation.1-5 Lockheed Martin’s High Altitude Airship (HAATM) program (Fig. 1) and DARPA’s Integrated Sensor Is Structure (ISIS) program are examples of unmanned stratospheric airships requiring high performance ultra-light hull fabrics. In an early phase of the ISIS program, CT developed advanced hull material samples for an unmanned stratospheric airship. These hull fabric samples achieved critical performance parameters such as: Areal density less than 100 g/m2 for a fiber strength-to-weight ratio greater than 1000 kN*m/kg and a matrix glass transition temperature of less than minus 90°C as listed in Fig. 2. These laminates must also retain greater than 85% strength at 5 years and must incorporate coatings for environmental protection and thermal control while passing radar transmittance specifications.

Besides LTA applications, high performance materials are used for re-entry airbag systems. The requirements for these materials are similar to those for LTA systems except the need for increased abrasion and puncture resistance. Low mass due to launch vehicle mass constraints and high tensile strength for high-pressure impact loads is necessary. In addition, high tear strength ensures survivability during the impact with sharp surfaces, and low gas permeability retains the inflation gas. At low temperatures the material must have adequate flexibility for retraction and maintain sufficient strength at the elevated temperatures encountered during inflation and on impact. Lastly, the material can only be minimally affected by creasing for tight packaging.6 For other decelerator systems, including parachutes and parafoils, fabrics must be flexible, unaffected by folding, and resistant to shock and tearing loads.

Figure 1. HAATM Conceptual Images (courtesy of Lockheed Martin)

Figure 2. From DARPA Briefing: http://www.darpa.mil/sto/space/pdf/ISIS.pdf


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Recent advances in high performance fibers, including Dyneema®, Zylon, and Vectran meet and exceed many of the material requirements in these applications. These materials maintain a significant advantage over traditional materials, such as those made from polyester or nylon, whose strength-to-weight ratios are too low for advanced aerospace applications.

III. Flexible Composite Overview A. Flexible Composite Implementations

CT produces flexible, multidirectional, non-woven laminates from oriented filament layers and high performance films or surface coatings. The resulting composite laminates are tailored and optimized for strength, stretch properties, and minimum thickness. These materials are easily customizable to a wide variety of weights, ranging from 0.3 oz/yd2 to over 20 oz/yd2, and may be formed in 2 and 3-dimensional parts. CT’s laminates can utilize most of the advanced engineering fiber materials available, and design is based on the required operating conditions and parameters of the specific application. Available fiber choices include, but are not limited to: UHMWPE (Dyneema®, Spectra), liquid crystal polymer (Vectran), PBO (Zylon), para-aramids (Kevlar, Twaron, Technora), carbon, glass, nylon, and polyester. A variety of surface films, foils, or coatings can be incorporated including, but not limited to: PET (Mylar), polyamide (Nylon), polyimide (Kapton, Upilex, LaRC-CP1), PVF (Tedlar), and urethane. These surface materials add customizable properties including toughness, low gas permeability, low and high temperature operating capability, and visible and UV light protection, to name a few. There are also woven backing options for improved durability and abrasion resistance, although these often add to the weight and thickness of the material.

B. Internal Reinforcement / Complex Geometries The laminated construction of composite fabrics enables the introduction of internal reinforcement patterns that

minimize weight and are structurally sound locations for attachment points. Reinforcements may include attachment loops, corner patches, radial and edge load point reinforcement, and feathered patterns. Figure 3 demonstrates an integrated 360° radial reinforcement pattern, and a large integrated structure panel.

C. Woven vs. Non-Woven Construction Non-woven fabrics have a number of technical and performance advantages over woven fabrics. Lightweight

woven products must use a multitude of low denier tows, while CT is able to produce similar, and oftentimes-lighter weight, materials from a lower number of high denier tows. This eases manufacturability, while allowing for weight and thickness optimization. Additionally, woven fabrics suffer from crimp, which is caused by fibers passing over and under each other in the weave. Tensile loading of woven fabric induces transverse loads at fiber overlap sections as crimped fibers attempt to straighten; this reduces the translation of fiber strength to fabric strength and decreases long-term fatigue and creep rupture performance. Crimp related reduction in properties is particularly pronounced with higher performance engineering fibers where optimization of axial filaments properties weakens transverse properties of the filaments. Figure 4 is a magnified image of a woven polyester fabric followed by an illustration of crimp in woven fabrics as load is applied.7,8 Non-woven oriented composite laminates are free from

Figure 3. (Left) 3D 360° Radial Reinforced Panel & (Right) Large 40ft x 40ft Integrated Structure Panel for Decelerator Applications


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these limitations and may be produced with an unlimited range of fiber areal weights having multiple oriented layers positioned at any angle. The most significant advantages of oriented laminates are the ability to optimize weight, thickness, and strengths at particular locations or along predetermined load paths. Non-woven flexible composites constructed from high modulus fibers have predictable and linear properties for engineered designs. Figure 5 illustrates this point with a stress-strain graph of a non-woven material, with no crimp, and a woven fabric having crimp. A conceptual drawing of a non-woven fabric is also included in Fig. 5.

IV. Materials Overview CT has experience engineering laminates using a wide variety of high performance fibers that are chosen

depending on the specific application and technical requirements. Engineering fibers each exhibit a particular blend of properties and differing applications require fibers that have differing property tradeoffs. A thorough understanding of material properties along with the use of appropriate safety factors simplifies application specific fiber selection and minimizes risk.9,10 An overview of the material properties for the Dyneema®, Zylon, and Vectran fibers is presented in the following paragraphs. CT can also produce composites from UHMWPE, para-aramid, carbon, glass, nylon, polyester, and most other engineering fibers. The fiber descriptions are followed by a brief overview of typical surface coating options. For aerospace applications requiring specific surface properties, CT will work with specialized surface coating manufacturers to meet project requirements.

Figure 5. (Left) Comparative Stress-Strain of materials with and without creep. (Right) Conceptual Drawing of a non-woven material.

Figure 4. Magnified Image of Woven Polyester and Crimp Diagram.7,8 Tensile loading of woven fabric induces transverse loads at thread overlap intersections due to straightening of crimped fibers.


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A. High Performance Engineering Fibers DSM and Honeywell produce similar UHMWPE fiber under the trade names of Dyneema® and Spectra

respectively. Dyneema® has the 2nd highest strength-to-weight ratio (second only to PBO for commercially available fibers), and outperforms Zylon and Vectran in flex fatigue and resistance to moisture and UV light. Dyneema® products are often a preferred choice for flexible composites based on these properties. Performance concerns with Dyneema® include creep, which increases with temperature and load, adhesion, and its low melting temperature.2,5 This tradeoff of less than ideal creep but excellent UV performance may be easier to design for, and determine life factors from, than from materials with opposite trade-offs. Creep may be modeled from existing parameters based on temperature and load conditions11,12, while the environmental degradation of PBO or Vectran is less quantified and more difficult to monitor in situ. References 13-16 provide information on the mechanisms and test results for the creep behavior of Dyneema®.

Zylon fiber is made from rigid-rod chain molecules of PBO. It is available with slightly higher strength-to-weight ratio than Dyneema®. In addition, Zylon has a high decomposition temperature and excellent creep resistance. The fiber has good abrasion resistance, but performs significantly lower than Dyneema®. The strength of Zylon is significantly decreased by exposure to high humidity/high temperature, and UV/visible light. As a result it must be very carefully protected from these conditions.2,17 For additional information on environmental effects on the performance of Zylon see references 18-22.

Vectran offers a unique balance of properties in comparison to other high performance fibers. Vectran is a liquid crystal polymer having strength and modulus lower than Dyneema® or Zylon but other properties falling between those of Dyneema® and Zylon including: creep, abrasion, UV performance and flex/fold resistance.2,5,23,24 Vectran was qualified for use in the Mars Pathfinder landing system, due to its performance over a wide temperature range and flex fatigue resistance.6

Figure 6 compares the density, strength, and modulus for Dyneema®, Zylon, and Vectran. Specific strength and specific modulus is based on the tensile strength and modulus divided by each fiber’s density. These values provide a better point for comparison.

B. Surface Coatings Film coatings, such as Mylar (a biaxially oriented PET), are used for their high tensile strength, toughness,

chemical and dimensional stability, gas barrier properties, high temperature resistance and low coefficient of friction. For space related applications polyimide coatings and films, such as Kapton or Upilex, remain stable over a wide range of temperatures and have good electric properties. For weatherability, PVF films such as Tedlar, provide long-term durability and environmental stability. Nylon and Urethane coatings offer excellent toughness and flexibility, at a lower cost than other options, but have lower mechanical and permeability properties.

Woven coatings are often incorporated into CT laminates when high abrasion resistance is the priority. The use of woven materials on one side of composite, or on both, creates a laminate hybrid that blends the high strength, dimensionally stable flexible composite construction with the limited benefits of woven technology. Woven coatings are typically heavier than other types of surface coatings but can be the best choice in areas of the part design where abrasion resistance is the driving factor.

C. Bondability & Seaming Regardless of the strength of an aerospace quality material, the strength or dimensional stability of joints used to

construct the structure is often a limiting factor in structures made using flexible materials. Due to the high strength, low elongation and homogenous distribution of fibers within the laminate, flexible composite laminates can be

Fiber Type Density (g/cm^3)

Tensile Strength

(GPa)

Specific Strength

(Strength / Density)

Tensile Modulus

(GPa)

Specific Modulus

(Modulus / Density)

Zylon AS 1.54 5.8 3.8 180 116.9 Zylon HM 1.56 5.8 3.7 270 173.1 Dyneema® 0.97 3.4 3.5 111 114.4 Vectran HT 1.41 3.2 2.3 75 53.2 Vectran UM 1.4 3.0 2.1 103 73.6

Figure 6. Fiber Properties Comparison Table12,15,21


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constructed with seams that are actually stronger than the base materials and are capable of carrying high structural loads for long periods of time without failure, seam slippage, seam distortion, or creep. Depending on the composite’s design, most standard forms of seaming are suitable for fabrication including sewing, adhesive bonding, heat welding, ultrasonic welding, and laser enhanced bonding.

V. Example Materials One of the unique characteristics of the CT flexible composite manufacturing method is the ability to produce

uniform materials, with engineered and multidirectional tailored properties, in weights ranging from 0.3 oz/yd2 to over 20 oz/yd2, while utilizing a wide variety of engineering fibers and coatings, but only a single process technology. Listed below are several classes of implementations using various engineering fibers that are suitable for a wide range of applications.

A. Test Methods Materials were tensile tested using the ASTM D3039M test method and also a non-standard test method for

tensile testing. ASTM D3039M utilizes tabbed specimens to relieve gripping stresses caused by mechanical gripping forces. For more rapid testing without the need for tabbed specimens, self-tightening Bollard grips (Fig. 7) are often used. The Bollard grips show utility for tests up to a few hundred pounds but may not provide optimal gripping for tests reaching higher loads. Most of the tensile tests in this paper were conducted using the Bollard grip test method at an approximate strain rate of 30%/min. Instron 5567 and Instron 5568 load frames with appropriate load cells, Instron Bluehill software, and Instron clip on extensometers were used.

The slit tear method is specified by Mil-C-21189 Method 10.2.4, and similarly FAA-P-8110-2, both specifically developed as airship design criteria. The slit tear specimen is 4in by 6in having a 1.25in long cut in the center of the specimen (Fig. 8). Ref. 1 is also a good resource on appropriate tear test methods for airship materials.

B. Lightweight Flexible Composites Lightweight oriented multidirectional composite laminates may offer a higher performance alternative to nylon

or polyester woven materials in applications such as parachutes, parafoils, balloons, and other applications where very thin, lightweight, strong, and tear resistant material is required. Compared to silicone coated nylon of similar weight and thickness, CT’s laminate is 80% stronger, has 10 times higher modulus, and 4 times higher tear strength. For a comparative strength to the silicone coated woven nylon, an ultra-light CT material (CT0.3HB UHMWPE Composite), weighing 0.5 oz/yd^2 is included in the table below (Fig. 9). While woven fabrics have very little strength in the bias directions, the CT0.3HB and CT1.5HB UHMWPE composites have been designed with quasi-isotropic properties. As a result, the shear properties of CT composites significantly outperform woven fabrics.

Figure 8. Slit-Tear Specimen (Mil-C-21189 10.2.4). Pictured during tear testing using Pneumatic/Hydraulic grips.

Figure 7. Bollard Grips. Pictured during tensile testing with loaded specimen and attached Instron extensometer.


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Although there is some strength contribution from coatings and films, which is not accounted for, high conversion efficiency means that the fiber strength is successfully transferred to the strength of the composite. The theoretical strength of the CT1.5HB UHMWPE Composite, based on fiber density alone, is 75 lbf/in. In comparison to the tested tensile strength of 87 lbf/in, this represents a conversion efficiency of 116%. The theoretical strength of the CT0.3HB UHMWPE composite is 28 lbf/in, when compared to a tested tensile strength of 41 lbf/in, the CT0.3HB conversion efficiency is 146%.

Product Silicone Coated Woven Nylon

CT1.5HB UHMWPE Composite

CT0.3HB UHMWPE Composite

Weight (oz/yd^2) 1.3 1.2 0.5 Thickness (in) 0.002 0.002 0.001

Tensile Strength (lbf/in) 48 87 41 Theoretical Strength @ 23°C (lbf/in) -- 75 28

Conversion Efficiency @ 23°C (%) -- 116 146 Modulus ((lbf/in)/(in/in)) 237 2774 1670

Strain to Failure (%) 33.0 3.2 2.9 Slit Tear Strength (lbf) 26 108 38

Bias Tensile Strength (lbf/in) -- 87 41 Bias Modulus ((lbf/in)/(in/in)) -- 2774 1670

Helium Gas Permeability (L/m2/24hrs) -- <0.2 <0.2 Figure 9. Properties of Silicone Coated Woven Nylon and CT Lightweight Composites

C. Medium Weight Flexible Composites The properties of medium-weight laminates constructed from Vectran and Aramid fibers are included in the

table below (Fig. 10). Materials of these weights may be utilized for LTA or other inflatable structure applications. In addition to low gas permeability, these composites have excellent low temperature performance and pressure retention. The high conversion efficiencies reflect the excellent transfer of fiber mechanical properties to laminate mechanical properties that allows for predictable and repeatable material elastic properties.

Product CT35HB Vectran

Composite CT35HB Aramid

Composite Weight (oz/yd^2) 4.1 4.7

Thickness (in) 0.006 0.005 Tensile Strength @ 23°C / -60°C (lbf/in) 523 / 670 587 / 650

Theoretical Strength @ 23°C (lbf/in) 501 546 Conversion Efficiency @ 23°C (%) 104 107

Modulus @ 23°C / -60°C ((lbf/in)/(in/in)) 18951 / 22285 26556 / 28645 Strain to Failure @ 23°C / -60°C (%) 2.6 / 2.8 2.2 / 2.2

Slit Tear Strength @ 23°C / -60°C (lbf) 205 / 229 131 / 125 Figure 10. Properties of CT Vectran and Aramid Medium Weight Composites

D. Heavyweight Flexible Composites CT’s heavyweight composites may be used in tension structures having high strength, low elongation, linear

stress-strain behavior, tear and damage resistant requirements, and/or application requiring multi-axial structural reinforcement. Examples of potential applications include: large-scale heavy lift airships, inflatable structures or beams, tension structures and flexible pressure vessels. Examples of these material’s characteristics are included in the table below (Fig 11).

Tabbed specimens were tested in addition to Bollard grip specimens when the Bollard grip method resulted in low strength results. It is likely that the Bollard grips are not sufficient to provide adequate gripping and may cause stress concentration and edge effects at high loads. Using Bollard grips, the conversion efficiencies for the


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CT155HB UHMWPE Composite and CT135HB PBO composite are only 75% and 72% respectively. However, tests using the ASTM D3039M method using tabbed specimens resulted in an improvement in conversion efficiency of 90% for the CT155HB UHMWPE Composite, and 85% for the CT135HB PBO Composite.

Product CT155HB UHMWPE

Composite CT135HB PBO

Composite Weight (oz/yd^2) 13.0 10.3

Thickness (in) 0.016 0.011

Bollard Grip Method Tensile Strength @ 23°C / -60°C (lbf/in) 1813 / 2122 1618 / 1924

Theoretical Strength @ 23°C (lbf/in) 2402 2250 Conversion Efficiency @ 23°C (%) 75 72

Modulus @ 23°C / -60°C ((lbf/in)/(in/in)) 85820 / 109965 72794 / 81682 Strain to Failure @ 23°C / -60°C (%) 2.1 / 1.9 2.0 / 2.4

Tabbed Specimen Method (ASTM D3039)

Tensile Strength @ 23°C (lbf/in) 2167 1934 Theoretical Strength @ 23°C (lbf/in) 2402 2250

Conversion Efficiency @ 23°C (%) 90 86

Slit Tear Strength @ 23°C / -60°C (lbf) 313 / 504 250 / 423 Helium Gas Permeability (L/m2/24hrs) <0.2 <0.2

Figure 11. Properties of CT UHMWPE and PBO Heavyweight Composites

VI. Conclusion CT is developing materials to meet the challenges of current and future aerospace applications. High

performance fibers and surface coatings are enabling this technology along with a unique manufacturing process that produces customizable flexible composites with optimum strengths and weights (weights ranging from 0.3 to over 20.0 oz/yd2). Material properties of these materials can be tailored for strength and modulus in multiple arbitrary directions and can be produced in seamless two-dimensional flat or three-dimensional complex curved structures exceeding 40 ft in width and exceeding 100 ft in length. Materials can be joined with seams that are stronger than the base laminate materials and capable of carrying structural loads for extended periods without failure, slippage, or creep. Laminates can be engineered for pressure retention, low gas permeability, environmental resistance and abrasion resistance.

Acknowledgments The authors thank intern John-Paul Deitz for testing the hundreds of samples in support of this paper.

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