Mechanical characterization of recycled carbon staple fiber yarn for load-bearing applications

The use of fiber reinforced components for lightweight constructions has been increasing rapidly over the last decades. The rising amount of waste per year leads to an increasing requirement to develop new techniques to recycle and reuse carbon fibers. To fulfill environmental and economic goals, upcycling of end-of-life products and production waste into structural components must be achieved. Recycling of carbon fiber reinforced plastics (CFRP) is challenging both in isolating the fibers from the matrix and in reusing the receiving fibers in a technical or constructive application with load-bearing requirements. Subsequent continuous fiber reinforcement, as in components made of new fibers, is not possible due to the shortening of the fibers by cutting processes in the recycling process. The shortened recycled carbon fibers (rCF) have to be converted into other types of semi-finished products like woven or nonwoven fabrics. An interesting option is to ply them into a continuous staple fiber yarn. One commercially available product is an rCF roving from Wagenfelder Spinnerei GmbH (Germany). This product (see Figure 1) stands out due to the fixation by a binding yarn and the variation of fiber composition by adding PA 6 fibers into the stable yarn (from 30/70% (rCF/PA6) up to 90/10%). In previous projects, the rCF roving was further processed into components using a bio-based matrix. By using this resin, which consists of 40% renewable raw materials, it was possible to further reduce the ecological footprint of the manufactured components. But in addition to the environmental aspects, the mechanical performance of the material must also be suitable for industrial use.

Various tests on unidirectional components made from rCF rovings have shown good mechanical performance of this material. For example, the tensile properties parallel to the fiber orientation are close to those made of new fibers. With a Young’s modulus of > 100 GPa it is possible to design load-bearing structures and to “upcycle” the recycled carbon fibers. The strength of 850 MPa and a fracture strain of 1% is also comparable to composites with standard carbon fibers.

An important point for designing components is not only the knowledge of the mechanical behavior but also about the damage behavior. The characterization of the mechanical properties through standardized tests (tensile, compression, etc.) is well established. More complex tests have to be performed to analyze the damage behavior. In-situ X-ray microscopy experiments are an excellent method to perform mechanical tests and to detect the structural changes in the meantime. Especially for rCF fibers it can be an important gain of information, e.g. onthe exact fiber orientation, the crack initiation and propagation, the pull-out and post-failure behavior. Such tests have been performed at IVW using the X-Ray microscope Zeiss Xradia 520 Versaincluding the mechanical in-situ test module Deben CT 5000 ( Figure 2 ). In transverse tensile tests on rCF samples, a benign failure behavior of the sample can be detected. Individual fiber bundles bridge the resulting crack after failure and thus ensure structural cohesion of the specimen (Figure 3).

Further studies on processing, mechanical performance and failure behavior are planned in the future to further advance the usability and use of recycled carbon fibers in load-bearing components and thus contribute to the achievement of global ecological as well as economic goals.

Contact:

Christian Becker, M.Sc.
Design of Composite Structures
Leibniz-Institut für Verbundwerkstoffe GmbH
Erwin-Schrödinger-Str. 58
67663 Kaiserslautern
E-Mail: christian.becker@ivw.uni-kl.de
Telephone: +49 631 2017 206