Effect of process variables on the uncured handleability and formability of stretch broken carbon fiber

Abstract

Carbon fiber is a high-performance reinforcing material used extensively in aerospace composites. Although carbon fiber is used in both continuous and discontinuous form, the continuous carbon fiber is limited by its inability to stretch due to its low strain to failure during manufacturing structures with complex geometries. Stretch broken carbon fiber (SBCF) is a type of discontinuous and aligned carbon fiber which has the potential to solve this limitation of inextensibility of its continuous counterpart. The discontinuous nature of SBCF enhances its stretchability making this material a prime candidate for manufacturing parts with complex curvatures. SBCF is generated by stretching the fibers using a pair of differentially driven rollers, which breaks the fibers at their intrinsic flaws. Although SBCF can be stretched due to being discontinuous, it compromises the tensile strength due to the lack of fiber continuity. Therefore, a polymeric coating known as sizing is applied to the SBCF to reconstruct its tensile strength. In the context of SBCF production, sizing serves two important functions. Firstly, sizing provides uncured carbon fiber the desired handleability and back-tension ability. Secondly, sizing enhances the formability of SBCF, which is a defined as the ease at which a material can be formed into a desired shape without failure. The goal of this work is to investigate the effect of process variables on the generation of stretch broken carbon fiber with consistent and repeatable material properties. The stretch broken carbon fiber research group at Montana State University (MSU) has developed a stretch breaking machine known as 'Bobcat' to generate single tow MSU SBCF. The noteworthy process variables related to MSU SBCF production includes sizing deposition on the tow, stretch ratio, nip force, line speed, fiber length distribution, and tow tenacity. Target amount of sizing deposition on MSU SBCF tow was achieved by choosing an appropriate sizing bath. A temperature-controlled tow tenacity result suggests that MSU SBCF possesses adequate handleability, back-tension ability and formability. MSU SBCF also shows a narrow fiber length distribution and relatively short mean fiber length which indicate improved formability. Reproducibility of these results were observed in the replicate batches of MSU SBCF. Suitable stretch ratio and nip force regimes were identified to optimize MSU SBCF production.