Stretch-broken Carbon Fiber: The New Material Of Choice?
Fiberforge (Glenwood Springs, Colo.) is testing stretch-broken carbon fiber (SBCF) for use in some of its more complex part designs, because it offers better drapeability without compromised properties. So what is stretch-broken carbon fiber? Stretch breaking was conceived as an antidote for the limited formability of con-tinuous fiber prepreg, which has restricted its use in automated manufacturing processes. Recently, Hexcel (Dublin, Calif.) and Schappe Techniques (Charnoz, France) have independently developed SBCF systems that reportedly achieve the formability of random discontinuous or chopped fiber-reinforced systems without the large mechanical per-formance knock-downs that must be figured in when using the latter. During the stretch-breaking process, the fibers in a carbon tow are pulled, causing a more or less regular pattern of breakage. The individual filaments, still quite long and aligned with one another, have some freedom to move independently. As a result, individual tows in a uni tape, for example, can be conformed to a contour or wrapped around a corner more easily, because fibers on the tow's outside radius can separate at the breaks, allowing the heated material to be conformed to deep draws and complex contours.
Schappe's SBCF process places 24K continuous carbon fiber in a pro-prietary stretch-breaking machine, which pulls the fibers until they break at what the company terms "natural weak points." Schappe sales manager Michel Jev says that the process does not lengthen or thin the tows. The stretched tows are blended with nylon or other thermoplastics resins. The yarns achieved are then woven, braided or made into uni tape. Schappe cites not only increased drapeability and drawability as SBCF advantages, but also low void content, high mechanical properties and reduced cycle times.
Hexcel's process starts with unsized AS4 or IM7 12K or 6K carbon fiber tow. Tows are fed into Hexcel's proprietary stretch breaker. The process differs from Schappe's in that the tows are spread out and broken in two stages. The first stage randomly breaks fibers at about 2 percent strain, into 10.16-cm (4-inch) lengths, on average, and stretches the tow by 11 percent. The second stage randomly breaks remaining continuous fibers and stretches the fibers another 4 percent. The tow is then sprayed with a water-based epoxy sizing, dried and wound on a spool for subsequent use in a standard thermoset prepreg process, to produce unidirectional and woven fabric prepreg. Trial tests have shown high conformability with no wrinkling or less-defined wrinkling than is seen with traditional carbon fiber prepreg tape and fabric.