Aligned discontinuous fibers come of age
Discontinuous but aligned carbon fibers are proving formable and formidable in high-performance, compound-curvature applications.
By Sara Black, Technical Editor | March 2008
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.
Source: Schappe Techniques
Examples of aligned discontinuous carbon fiber reinforcement forms. Schappe Techniques’ products, shown here, are commingled forms that combine spun stretch-broken carbon with thermoplastic filaments to form a “dry prepreg” marketed as TPFL.
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
Source: Schappe Techniques
This award-winning golf caddy is made with Schappe Techniques’ TPFL stretch-broken carbon/thermoplastic braid material.
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.
Source: Pepin Assoc.
This schematic shows a Pepin-produced CD tow, with cut segments held together with a core and an outer filament.
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.
