In order to store hydrogen in fuel-cell electric vehicles, composite pressure vessels are one promising lightweight alternative to state-of-the-art tanks made of metal. These wound composite structures are often manufactured using continuous carbon fiber rovings in automated tape placement or filament winding processes. The market standard is to use composites with a thermoset matrix. However, the use of a thermoplastic matrix can lead to advantages in the manufacturing process and moreover it offers new end-of-life possibilities for value retention and circularity engineering [1]. Carbon fibers are a high-value material and especially long and well oriented fibers are crucial to create composites with excellent mechanical properties. The recovery of endless and oriented carbon fibers at the end of life of a product can help to increase the circularity of composites and reduce the environmental impact of new composites if recovered fibers are used [2].

One method to obtain oriented carbon fibers with preserved length from end-of-life products is to recover entire patches with a mechanical peeling process based on layer separation [3,4]. In case of wound composite structures the manufacturing process can theoretically be reversed by unwinding the tapes. In this way, the fibers stay embedded in the polymer matrix. In case of a thermoplastic matrix the recovered tape could ideally be used directly as feedstock for the manufacturing of new composite structures and therefore close the material cycle for unidirectional composite material in thermoplastic wound composite structures. This binational DFG/ANR funded work focuses on composites with a thermoplastic PA11 matrix with the goal to identify conditions which lead to a peeling process which enables tape recovery without reducing the material properties by maintaining fiber length and orientation. The tape should be recovered in a continuous manner which requires a continuous interlaminar crack propagation between the tape layers. 

A common phenomenon for interlaminar crack propagation in fiber-reinforced composites is fiber bridging which means that fibers are forming bridges between both crack surfaces. The occurrence of fiber bridging depends on the microstructure and crack propagation paths within the composite [5]. Fiber bridging can be beneficial in the use phase of the composite structure by hindering delamination. However, it is a challenge in the mechanical tape recovery process since fiber bridging leads to fiber breakage and loss of load-bearing fiber material during the value-retention process (VRP) by peeling. The development of fiber bridging during the VRP is illustrated in Figure 1. As the peeling process proceeds, fiber bridging increases which results in loose fibers on the tape surface and a loss of tape cohesion. At a peeling length of less than 1 m the material quality is already insufficient due to a reduction of the Young’s modulus and tensile strength of more than 50 % compared to reference tape material. This highlights the necessity to inhibit fiber bridging as much as possible.

Figure 1. See attached pdf file.

With fiber bridging being a major challenge in peeling-based tape recovery, this work focuses on identifying the reasons of fiber bridging and methods for fiber bridging prevention. By analyzing specimens which are manufactured at different temperatures (180 °C, 200 °C and 240 °C) and with different winding patterns as well as applying different conditions in the peeling process, this work helps to identify manufacturing conditions and peeling conditions which are beneficial for tape recovery. 

Experimental methods in this work include a video analysis of the peeling process, mechanical tests on the recovered material in order to assess the quality of the recovered material and microscopic analyses of the crack surfaces and crack propagation paths. In order to check if the composite structures which are manufactured at different temperature levels have acceptable in-service properties, a mechanical characterization of the composite material is also performed. This work gives insights into interlaminar crack propagation and identifies methods to avoid fiber bridging. 

References

[1] M. Schäkel; H. Janssen; C. Brecher, "Process analysis of manufacturing thermoplastic type-iv composite pressure vessels with helical winding pattern."  Soc. Adv. Mater. Process Eng, 2021, 955-967, doi:10.24406/publica-fhg-417015

[2] B. Ragupathi; M. F. Bacher; F. Balle, "First efforts on recovery of thermoplastic
composites at low temperatures by power ultrasonics," Cleaner Materials, 2023, 100186, doi:10.1016/j.clema.2023.100186

[3] M. Imbert; P. Hahn; M. Jung; F. Balle; M. May, "Mechanical Laminae Separation at
Room Temperature as A High-Quality Recycling Process for Laminated Composites,"
Materials Letters, 2021 130964, doi:10.1016/j.matlet.2021.130964

[4] S. Strahringer; M. Imbert; M. May; F. Balle, “Impact-Induced Interlaminar Crack Initiation in Notched Thermoplastic Composites for Single-Layer Recovery Purposes: Role of the Notch Geometry.” Journal of Dynamic Behavior of Materials, 2025, 1-14, doi:10.1007/s40870-025-00480-y

[5] F. Naya; G. Pappas; J. Botsis, “Micromechanical study on the origin of fiber bridging under interlaminar
and intralaminar mode I failure” Composite Structures, 2019, 210, 877-891, doi:10.1016/j.compstruct.2018.11.064