Aligned discontinuous fibers come of age

One of the greatest impediments to efficient production of high-performance composite parts is part designs that incorporate compound curves. Painstaking and tedious darting and goring often are required to hand lay parts using traditional thermoset prepreg. Depending on the areal weight and resin type, prepregs — particularly unidirectional tapes — tend to be difficult to bend, drape and form around and in radii without causing wrinkles in the laminate.

Several industry suppliers have recently proposed a solution to this problem with a new class of fiber reinforcements made with carbon fiber tows that feature discontinuous — but still aligned — fiber segments. These relatively long, overlapping fiber lengths (on the order of several inches) are held together with filaments or a binder and have the ability to move relative to each other, allowing the reinforcement to more readily and easily stretch and move for greater, wrinkle-free conformability and faster layup, while still meeting structural and strength requirements.

This concept isn’t new: Wool and cotton yarns have been made for centuries by spinning entangled, discontinuous fibers into a continuous yarn, and stretch-breaking is commonly employed with apparel fibers to create yarns and avoid damage that can occur with cutting. The idea of taking continuous carbon tows and breaking them into short “staple” fibers and then aligning and recombining them into yarns first appeared in the mid-to-late 1980s in products from Courtaulds’ Heltra Division, ICI Fiberite (Tempe, Ariz.), and DuPont (Wilmington, Del.), among others. The materials, however, were not widely accepted despite their strong interlaminar shear strength, good conformability and smooth surface finish. Says one industry insider, “There were concerns about predictability in the fiber lengths, and costs tended to be higher.”

In the last few years, aligned discontinuous fiber forms have made a comeback, in part because of the growing demand for faster and automated manufacturing. Notably, the materials’ ability to stretch and deform while still delivering good mechanicals is opening the door for automated vacuum forming and diaphragm forming of compound curvature parts. A flat preform of aligned discontinuous fabric, for example, could be pulled via vacuum into a complex tool, obviating the need for hand layup methods.

Aligned discontinuous fiber materials are now available from several manufacturers, each offering a product that differs to a degree from the others. The most prominent are Hexcel (Dublin, Calif.), Pepin Assoc. (Greenville, Maine), Pharr Yarns (McAdenville, N.C.), Schappe Techniques (Charnoz, France) and Advanced Composites Group Ltd. (ACG, Heanor, Derbyshire, U.K.).

Promise and potential

Hexcel’s aligned discontinuous fiber reinforcements are dubbed SBCF (Stretch Broken Carbon Fiber). Hexcel has been developing stretch-breaking technology for several years and has teamed with Boeing Integrated Defense Systems (St. Louis, Mo.), Northrop Grumman (Los Angeles, Calif.), Albany Engineered Composites (Rochester, N.H.) and the Applied Research Laboratory (ARL) at Pennsylvania State University to develop and demonstrate a route to lower cost composite structures via the use of a new material form. This configuration is based on stretch-broken carbon fiber tows, which consist of aligned, randomly broken filaments.

Guenther Jacobsen, senior staff scientist who heads the SBCF program for Hexcel, says high-performance composite parts of complex shapes can be made with these discontinuous fibers as long as they are kept under tension in the forming process. He explains: “The stretch-broken carbon fibers give the material a pseudoductility akin to metals that makes it much easier to form complex parts. With SBCF materials, manufacturing costs can be reduced, and the number of composite parts on an airframe can be significantly increased.”

To make SBCF, Hexcel uses its stretch-break machine, an otherwise typical textile-industry breaking machine that employs three nip roll systems that rotate at different speeds, stretching the tow and generating randomly broken filaments. The machine spreads unsized AS4 or IM7 12K or 6K carbon fiber tows to ensure that the filaments can be gripped and pulled evenly during the breaking process. Jacobsen reports that a first-generation machine, modified by Hexcel, produced stretch-broken fibers with an average filament length of 4 inches/10.2 cm. Forming trials performed from 2002 to 2004 using the first-generation materials, while promising, had limitations when increasingly complexly shaped parts were attempted. The company has since developed a second-generation stretch-breaking machine that produces significantly shorter broken filament lengths. Average broken length of fibers from the newer machine is about 2.8 inches/7.1 cm, with a narrower length distribution. According to Jacobsen, a shorter broken filament length enhances formability and improves the quality of compound curvature test parts.

The tow of broken filaments that exits the machine is sprayed with a water-based epoxy sizing, dried and wound on a spool. The SBCF can be handled like ordinary continuous tow in sub-sequent prepregging or weaving processes. According to Jacobsen, an SBCF prepreg relies on sizing to hold the filaments together. To allow the filaments to move relative to each other, heat is used to reduce resin viscosity and solvate the sizing, minimizing filament-to-filament frictional forces. Hexcel has focused its SBCF development efforts on unidirectional prepreg tape, made with the company’s AS4 12K, IM7 12K and 6K fiber and either 8552 or M73 toughened epoxy systems.

A battery of tests has been run on the SBCF materials, and several test parts have been made. Laminates subjected to tensile, compression and interlaminar shear tests have properties almost equivalent to continuous fiber forms of the same material, says Jacobsen. “The tensile property retention is greater than 90 percent when comparing Generation 2 SBCF material to continuous filament materials, independent of the resin system,” he notes. For compressive properties, no difference can be seen between the Generation 2 materials and continuous filament systems.

Northrop Grumman has trialed the material in tests that simulate automated double-diaphragm forming methods. Uniaxial and biaxial testing of AS4 and IM7 fiber systems, prepregged with Hexcel M73 epoxy resin, show that all material forms permit significant extension along the axis of the reinforcements. Northrop Grumman project engineer Magdy Barsoum says that, in terms of drapability, processing temperature is a key variable, depending on resin type.

ARL/Penn State research engineer Gregory P. Dillon succeeded in the process development of a full-scale demonstration component, a bead-stiffened panel aimed at replacing sandwich panel designs. At Albany Engineered Composites, engineers Jon Goering and Michael McClain demonstrated that SBCF-based Pi preforms can be formed into curved shapes without darting. These performs can be formed either in the dry state for use in resin transfer molded (RTM) components, or can be infused with resin and used in prepreg layups.

All parties involved in this work believe that the SBCF material systems offer the potential for implementation of new and more cost-effective processing technologies for aerospace composites. Improved formability, associated with the fiber axis stretch mechanism enabled by SBCF materials, may benefit a wide range of established and emerging lower-cost composites fabrication technologies. Work is continuing to optimize the stretch-break process and forming technologies for a variety of composite parts. The development work was discussed in a series of papers presented at the 2007 SAMPE conference and the American Society for Composites 22nd Technical Conference.

At the forefront

Schappe pioneered stretch-broken reinforcements, having created the technology in the late 1940s for apparel fibers. In the mid-1980s, Schappe was the first to stretch-break high-performance fibers, including carbon and aramid. The company sells stretch-broken spun yarn materials for gasket-like seals (called gland packing) for the heavy equipment market, as well as reinforcing materials for composites, says sales manager Xavier Giroux.

The company is best known for its trademarked TPFL “dry” prepreg fabrics, unidirectional tapes, braids and multiaxials, which combine stretch-broken 12K or 24K carbon fiber slivers, typically 40 to 200 mm/1.5 to 8 inches long, that are pin drafted (i.e., aligned) and spun with similarly broken fragments of thermoplastic filaments to form fine, commingled yarns. Fiber breaking is done on a proprietary machine designed in-house, says Giroux.

The commingled yarns are wrapped with a continuous serving filament of the same polymer to stabilize the yarn for subsequent weaving processes. These yarns can incorporate polyamide, polyphenylene sulfide (PPS), polyetheretherketone (PEEK) or liquid crystal polymer (LCP), among others. The finished yarns are typically equivalent to 6K, 3K or 1K tow.

Other stretch-broken products include all-carbon (noncommingled) unidirectional tape made directly from aligned slivers and noncrimp, stitched dry carbon multiaxial fabrics. One interesting product, called MAC102, is a carbon, noncrimp, two-layer multiaxial fabric made by stitching unidirectional stretch-broken carbon tapes in a proprietary process that includes needling. This process — in which an array of needles is passed through the material several times — pulls some of the broken ends in the z direction and entangles them to create, in effect, a 3-D preform, says Giroux. The resulting material has better drapability, interlaminar shear strength and mechanical properties than similar products made with continuous fiber and is targeted to carbon/carbon applications. In friction products, tooling materials or ablatives, the high porosity of the stretch-broken fibers greatly improves densification.

The thermoplastic TPFL products are geared to an array of markets, including medical, sports, automotive, industrial and aerospace. Schappe is partnering with a network of molders and fabricators to expand applications, explains Giroux. With a resin matrix commingled with reinforcing fibers, TPFL forms can be molded readily, simply by heating the material above its melt temperature. “Compression molding, cold stamping and bladder molding are the processes that work best with the TPFL prepregs,” he notes. One example of the material’s use in a consumer product — in this case, carbon/polyamide braid — is the sleek Mantis HE electric golf caddy, manufactured by Thermoplastik Erich Müller GmbH (Dieburg, Germany), which won the 2006 JEC Composites Innovation Award in the Sports and Leisure category. Work on several proprietary aerospace contracts also is underway, including one involving the Eurofighter jet.

Proven for parts

Pepin Assoc.’s product, trademarked DiscoTex, is a fabric made with aligned discontinuous fiber tows developed in a process that cuts rather than breaks fibers into discrete segments. Pepin uses a proprietary machine into which two tows are fed. Rollers spread and nip the tows and then a knife cuts each one into 1.3-inch/32-mm lengths, but the cuts are staggered so that the segments overlap. At this point, a thin, continuous axial core filament is placed between the two cut tows, and a single, continuous binder filament (of the same material as the core filament) is wrapped around the overlapped segments. Both fugitive filaments hold the tow together during subsequent weaving operations. According to company president John Pepin, the resulting “continuous discontinuous” (CD) tow can be manufactured with carbon, glass or ceramic fibers.

When woven and prepregged, the stretchable and conformable DiscoTex material, according to Pepin, readily adheres to compound curves and mold surfaces to form complex parts that can’t be produced efficiently using conventional materials. As an example, an undulating fairing panel employs nine plies of DiscoTex prepreg that are heated for greater pliability (depending on the resin system), vacuum bagged and placed in the female mold. The flat material simply stretches and deforms under vacuum to pull into and conform to mold contours. According to Pepin, the fairing panel made with traditional prepreg takes two technicians four hours to lay up in the tool; with DiscoTex, one technician can lay up and vacuum-form the panel in 30 minutes (the part was subsequently autoclave-cured).

Further, Pepin’s materials can be used dry to simplify preforming. Rather than build a preform to the exact part shape, a molder can stack plies of dry DiscoTex, enclose them in a vacuum bag and use VARTM to form a flat infused panel. The bagged, infused stack is then placed in the part mold, with the edges of the preform sealed against the tool. A vacuum pulls the flat preform into the mold and forces it into the desired shape. Pepin points out that no pressure other than vacuum is necessary to deform the material. Unlike ordinary unidirectional prepregs, the material reportedly can be stretched in any in-plane direction, including the primary reinforcement direction.

The company is participating in the Defense Acquisition Challenge Program, authorized by federal legislation, which provides opportunities for small- and medium-sized companies to qualify their innovative technologies for U.S. Department of Defense (DoD) programs. Pepin, with Boeing Integrated Defense Systems, is working to qualify DiscoTex fabric to a new Boeing specification for use in structural airframe components with compound curvatures. These would include shear panels, which are bulkhead-like structures that stiffen an aircraft’s fuselage to support bending and shear loads. Shear panels often are designed with a sine-wave-shaped surface to provide buckling resistance. Says Pepin, “It’s an efficient shape for a stiffness-driven part, but it’s expensive to make with traditional methods and prepregs.”

For the Boeing qualification program, DiscoTex, made with 3K carbon, is prepregged by Cytec Engineered Materials Inc. (Tempe, Ariz.) using a common epoxy resin system. Pepin produces test panels at its facility, made with unstretched fabric and fabrics at 30 percent stretch. Coupon testing is being done at Wichita State University (Wichita, Kan.), and Boeing personnel are providing engineering support and evaluating the results. Preliminary data show that there is some knockdown in properties when compared to continuous material but, in general, the retained mechanical properties are relatively high. “With this material, designers can conceive of new parts that they ordinarily wouldn’t make because it would be too expensive,” notes Pepin. “You can achieve structural parts that are repeatable and affordable, with any fiber type.”

A good spin on spun yarns

Another approach to aligned discontinuous fiber reinforcement is that developed by Dr. Jim Hendrix and marketed through fiber manufacturer and spinner Pharr Yarns. Hendrix’s patented method starts with continuous, large-tow carbon fibers, typically 24K to 80K, which are flattened and stretch-broken on a textile stretch-break converting machine equipped with several rollers. The resulting product comprises a random array of short staple fibers or slivers ranging from a few millimeters to 180 mm/7 inches long, with an average fiber length ranging from 5 to 6 inches/130 to 150 mm. The slivers are collected in round containers that feed directly into a spinning apparatus without any secondary handling steps to diminish the fiber’s alignment, explains Hendrix. The process allows the fiber segments to overlap, yet slide or “draft” past each other as a new, thinner yarn is created by spinning. The resulting material is called “balanced ply-twisted yarn,” so named because two slightly twisted yarn ends are combined and twisted in opposite directions to eliminate torque. The slight twist holds the fibers together for subsequent handling during weaving, and no chemical binder is used.

The benefits of spun yarns include greater in-plane shear properties because the woven yarns’ slivers effectively grab and help hold the laminate together. Hendrix reports that while strength and tensile modulus are about 10 to 15 percent lower when compared to continuous filaments, a 30 percent increase in in-plane shear strength compared to continuous fiber tows has been measured in sample parts, which improves the structure’s impact resistance.

Hendrix’s principal interest, and the impetus behind his research, is to convert commercial large-tow carbon into finer yarns — he has produced 6K, 3K, 1K, 0.8K and even smaller tow. “When I was technical director for a company that had a need for fabrics woven with 1K carbon yarns, the materials were very expensive if available at all,” says Hendrix. “I spent the next five years developing this method to create them economically.”

Spun fibers are more economical than continuous filament tows at these fine tow sizes, he notes. Fabrics woven with fine spun-carbon yarns are less “sleazy” than those made with filament yarns, meaning they maintain better fiber alignment. Another advantage is faster wetout due to higher porosity. To overcome the knockdown in properties, mechanical performance can be improved by using a parent tow with higher flex and shear properties. Further, a variety of commingled hybrids can be created by combining carbon with other types of fibers to impart specific properties, such as aramid or thermoplastic fibers to toughen parts and make them more impact resistant.

Pharr Yarns’ spun yarns have been used in a range of sporting goods applications, including hockey sticks and bicycle frames. Racing bicycle manufacturer Asia Seiko (Tantzu, Taiwan) employs spun carbon fabric woven by Barrday Inc. (Cambridge, Ontario, Canada) to create its frames and rims. According to Asia Seiko, spun carbon in the bicycle’s tubular frame has improved stiffness compared to continuous filament carbon tubing. Says Hendrix, “Spun yarns give the composites manufacturer the flexibility to specify a reinforcement that meets the specific needs of the application and the part to be produced.”

The long and the short of it

ACG has recently introduced its own aligned discontinuous material: DForm Deformable Composite System Technology. Rather than adapting textile technology like stretch-breaking, ACG starts with standard continuous-fiber 48-inch/1200-mm wide unidirectional prepreg tape. It selectively slits the material in a discrete pattern, with the slits at a 45° angle to the axis of the tape. Each slit or cut is approximately 50 mm/2 inches long, effectively creating a web of discontinuous fibers, says ACG’s technical marketing manager, Dr. John Nixon.

“The prepreg tape retains its overall integrity and fiber alignment, but the slits allow intraply movement,” says Nixon. “When the tape is layed up, the slits open slightly, allowing stretch and conformability while good strength is maintained.” The degree of conformability can be modified by changing the cut density and the fiber length between cuts; the latter can vary from 20 to 60 mm/0.79 to 2.4 inches.

Currently, DForm consists of four layers of slit tape (oriented 0°/90°/0°/90°)that form a single ply, with a total areal weight of about 800g. Other formats will be introduced as applications grow. The current market focus is tooling, reports Nixon, so the idea was to create an initial product similar in weight to prepreg tooling cloth — six DForm plies (or 24 prepreg tape plies) are typically enough for a standard tool. The benefits of using DForm for tool construction are many, he notes. Layup time is reduced 20 to 30 percent because the stack can be layed up quickly and doesn’t require debulk cycles. Tests show that the material stretches and conforms to the required shape in one autoclave cycle. Further, the directional nature of the material’s uni fibers helps maintain dimensional accuracy and performance predictability in the tool. A key point is that fabric print-through is eliminated, which is an issue with standard woven tooling cloth prepreg.

Tests performed in cooperation with the University of Nottingham (U.K.) have demonstrated how DForm, with drape and handling similar to a woven fabric prepreg, significantly reduces laminate wrinkling in parts that feature compound curves. Mechanical tests have shown some drop off in properties, as might be expected, when compared to continuous unidirectional prepreg. However, comparisons to woven fabric prepreg are closer. Nixon notes that the longer fiber-length format performs best: “Clearly this is a technology that lends itself to the production of high-stiffness components where high strength is not the prerequisite.” Additionally, DForm readily flows under pressure and can be used in matched-die compression molding, possibly for semi-automated processes in the automotive industry.

The material is currently undergoing sampling trials with customers, including Formula 1 race teams, for autoclave-cured composite tooling. Nixon believes the product is a clear choice over random short fiber or infusion methods for making tooling.

All of these materials have the potential for translating high-performance carbon fiber into more complex parts, with reduced touch labor costs and faster throughput. Concludes Pepin, “What we’re creating with these materials is greater manufacturability for composites.”