Research publications

Introduction

The size of the CF-CFRP (carbon fiber-reinforced plastics) market was estimated at USD 21.12 billion in 2023. It is projected to grow from USD 22.57 billion in 2024 to USD 38.4 billion by 2032, with a CAGR of approximately 6.86% during the forecast period (2024–2032) [1]. Due to their high specific stiffness and strength, CFRPs are widely used in the automotive, sports, leisure, and aerospace industries [2]. However, CFRP components are brittle under impact loading, which can result in catastrophic failure and severe splintering [3]. This brittleness raises concerns for the use of thermoset CFRP structures in safety-critical components such as wind turbine blades or automotive B-pillars.

Current hybridization concepts aim to combine materials with high stiffness, strength, and ductility [4]. Existing approaches integrate carbon fibers (CF) with stainless steel fibers (MF) or elastomer fibers (EF) using metal or elastomer films in fiber-metal laminates (FMLs), such as CARALL [5–8], or in elastomer-based laminates, such as KRAIBON [9–14]. Metal films offer higher energy absorption due to their plastic deformability and elongation at break of up to 20%, surpassing CFRP and carbon/aramid hybrid composites [15–17]. Elastomer films reduce hazardous splintering under dynamic loading due to their elastic deformation behavior [9]. While such multilayer systems improve impact and splinter resistance, they also carry a high risk of delamination [18]. Moreover, there is a lack of cost-effective and sustainable composites with enhanced impact and splinter properties that fully utilize the benefits of their individual components.

Objective

The goal of this research project was the simulation-based development of novel three-component hybrid yarns with micro-scale hybridization using three distinct material concepts. These yarns were then used to produce functional composite structures for sustainable lightweight applications. By strategically combining ductile metal fibers (MF), highly elastic elastomer fibers (EF), and high-stiffness, high-strength recycled carbon fibers (rCF), scalable composites with tailored mechanical properties were developed.

The developed hybrid yarns form the basis for application-specific composites with high energy absorption capacity and improved damage resistance.

 

Hybrid Yarn Structures and Related Composites: Development and Characterization

Development and Production of Hybrid Yarns Using Flyer Spinning Technology

Starting from the selected and characterized rCF and EF fiber materials with an average fiber length of 80 mm and defined blend ratios, the fibers were prepared using mechanical pre-opening and blending units. The pre-opened and pre-mixed fibers were processed using a lab-scale carding machine to produce card slivers of rCF and EF. Characterization of these slivers revealed a CF damage level of 10–25%, while EF fibers showed no length reduction.

To avoid damaging the stainless steel fibres during carding, card slivers were firstly produced that were either 100% rCF or a blend of rCF and EF. These were combined with 100% MF slivers to develop sandwich-type structures (rCF/MF or rCF/EF/MF slivers), which served as feed material for the drafting process. The slivers were drafted multiple times to enhance fiber blending and homogeneity. These drafted slivers were then used to produce hybrid yarns.

The ITM’s specialized flyer spinning machine was modified to optimize drafting mechanics, sliver feed, and machine settings to avoid fiber misalignment. Based on experimental investigations, optimal settings were determined, and hybrid yarns with a yarn count of 1500 tex and twist levels ranging from 40 to 150 T/m were produced. These yarns were characterized in accordance with DIN EN ISO 13934-1, evaluating unevenness, yarn structure, and tensile behavior, and were subsequently used to produce composite.

Manufacturing of Recycled Carbon Fiber-Reinforced Composite

Using the developed hybrid yarns, unidirectional (UD) composites were produced via the resin transfer molding (RTM) process. The hybrid yarns were wound under constant tension onto a frame and consolidated under optimized parameters. The resin system consisted of Hexion RIMH 135 and hardener Hexion RIMH 137.

Composite characterization followed standardized test methods. Tensile specimens were prepared based on DIN EN ISO 527-5/A/2, with tensile testing conducted according to             DIN EN ISO 527-4. The flexural properties were evaluated in accordance with DIN EN ISO 14125 and impact resistance was assessed using DIN EN ISO 179-2 (Charpy method). The compression-after-impact (CAI) performance was measured following DIN ISO 18352. Additionally, a custom test rig was developed to analyze splintering behavior using a ZwickRoell HTM 5020 high-speed testing machine. Puncture resistance was evaluated according to DIN EN ISO 6603-2.

Selected Results and Discussion

Fig. 1 presents the relationship between flexural strength and modulus for various twist levels in hybrid yarn-based composites at a constant fiber volume content of 50 vol%. Both a CF-filament-based reference composite and three UD composites made from rCF/MF hybrid yarns (90 wt% rCF / 10 wt% MF) were investigated, differing only in yarn twist (40, 80 and 120 T/m). The reference composite achieved 725 ± 35 MPa flexural strength and a modulus of 74 ± 8 GPa. Notably, the T40 hybrid variant surpassed these values, reaching 806 ± 18 MPa and 83 ± 4 GPa, respectively.

However, increasing the yarn twist (80 and 120 T/m) led to a continuous decline in flexural properties. The intensified helical structure reduces fiber alignment in the load direction, which weakens load transfer and overall flexural performance.

Fig. 2 shows the impact strength of composites made from rCF/MF hybrid yarns at varying yarn twist levels. Results indicate a trend of increasing impact strength with higher twist (40 → 120 T/m), from 85 kJ/m² to 117 kJ/m². This improvement is attributed to a more compact yarn structure, enhanced fiber cohesion, and improved energy absorption during impact. Additionally, the tighter fiber arrangement enhances load transfer and structural integrity by reducing the number of loose fiber ends, resulting in greater resistance to sudden loads.

Summary

As part of the IGF research project 01IF22916N, a complete, industry-ready process chain for producing three-component hybrid yarns from rCF, MF, and EF was successfully developed at the ITM of TU Dresden. The process chain comprises fiber preparation, carding, and drafting to form slivers, followed by modified flyer spinning to produce hybrid yarns.

Proof of concept was provided through the production of hybrid yarns with defined fiber volume contents and a functional demonstrator. Fig. 3 illustrates the full process chain from fiber preparation to demonstrator production from rCF, MF and EF at ITM. The resulting yarns ranged from 1500 to 3500 tex and were successfully processed into textile preforms. The resulting composites demonstrated excellent mechanical performance: a maximum flexural strength of 806 ± 18 MPa, flexural modulus of 83 ± 4 GPa, and an impact strength of up to 117 ± 17 kJ/m².

The results show that yarn twist significantly influences composite mechanical properties: moderate twist enhances flexural behavior, while higher twist improves impact resistance. By adjusting the yarn twist level, the mechanical performance of hybrid composites can be effectively tailored.

These novel hybrid yarns are particularly suited for producing cost-efficient, high-performance thermoset composites with complex geometries. Their application-specific performance and process-integrated production offer high innovation and market potential, especially in the fields of materials engineering, lightweight design, sustainability, and resource efficiency. For small and medium-sized enterprises (SMEs) in the textile industry, this technology provides opportunities to develop advanced fiber-reinforced products and establish themselves as key suppliers in sectors such as automotive, mechanical engineering, wind energy, aerospace, medical technology, and sports equipment.

Acknowledgements

The IGF project 01IF22916N of the research association Forschungskuratorium Textil e.V. was funded via the DLR within the framework of the program for the promotion of industrial collaborative research and development (IGF) by the German Federal Ministry for Economic Affairs and Climate Action, based on a resolution of the German Bundestag. We thank the aforementioned institutions for their financial support.

 

References

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Introduction, problem definition and aim of the project

Carbon fiber-reinforced plastics (CFRP) are increasingly used in lightweight applications due to their high stiffness and strength as well as low density, especially in aerospace, transportation, wind energy, sports equipment or construction. Global demand of CFRP is predicted to increase to 197,000 t/a by 2024, almost tripling compared to 2011. This shows an urgent need for solutions to recycle the high quality carbon fiber (rCF) in terms of the circular economy. This is necessary not only due to strict legal regulations, but also for ecological and economic reasons. In recent years, numerous research institutes and companies developed solutions for the reuse of rCF in the fields of nonwovens, injection molding or as hybrid yarns. However, the majority of these works involve the use of rCF in combination with thermoplastic fibers for thermoplastic composites. In the field of rCF-based thermoset CFRP, mainly rCF nonwovens made of 100% rCF have been so far developed. Since the fibers in the nonwovens mostly have a limited length and a low orientation and process-related additional high fiber damage occurs, with these materials only maximum 30% of the composite characteristic values of CFRP components made of carbon filament yarns can be so far achieved.

Currently, the matrix systems used in the field of high mechanical loaded CFRPs are predominantly thermoset. Such components exhibit high dimensional stability, high stiffness and strength as well as are suitable for the implementation of complex component geometries due to low-viscosity matrix systems. However, primary carbon filament yarns are particularly used for these components due to the insufficient properties of rCF. In addition to low sustainability, the utilization of these filament yarns result in at least 200 % higher cost. The production of primary carbon filament yarn requires a high-energy demand of about 230 MJ/kg with a CO2 emission equivalent to 20 kg CO2/kg CF. Here, a significant improvement of the CO2 balance is required to make a substantial contribution to the envisaged climate protection goals of the Federal Republic of Germany and the EU. For this reason, the focus of the project work is the development of novel, sustainable rCF heavy tows made of recycled carbon fibers (rCF) and associated manufacturing technologies for the implementation of cost-effective thermoset composites with high mechanical performance.

Acknowledgments

The IGF project 21612 BR of the Research Association Forschungskuratorium Textil e.V. was funded by the Federal Ministry of Economics and Climate Protection (BMWK) via the AiF within the framework of the program for the promotion of joint industrial research and development (IGF) on the basis of a resolution of the German Bundestag. We would like to thank the above-mentioned institutions for providing the financial resources.

Abstract
Building in a resource-saving way and still exploiting a high performance potential, is that even possible? At the Institute for Textile Machinery and High Performance Material Technology (ITM) at the TU Dresden, such composite optimized profiled textile reinforcements for concrete applications and the related manufacturing technology were developed as part of the research project IGF 21375 BR. On the basis of braiding and forming technology, a new generation of profiled reinforcement yarns was developed with the help of simulation-based investigations. Like ribbed steel reinforcements, these profiled yarns have a very high bond with the concrete matrix, but despite the profiling they almost fully exploit the performance potential of the carbon fibers in terms of tensile properties. In this way, the bond length required for complete force transmission between the textile reinforcement and the concrete can be reduced to just a few centimeters, and up to 80 % of the component-dependent oversizing of the textile reinforcement can be saved. The further development of the multiaxial warp knitting technology for the requirement-based and fiber-friendly processing of the profiled yarns into grid-like reinforcement structures enables the production of profiled textile reinforcement structures with the highest bond properties for use in carbon-reinforced concrete components with maximum material and resource efficiency.

Initial situation and problem definition
As is generally known, climate change is the greatest challenge of the 21st century, which can only be successfully overcome by consistently saving resources and CO2 emissions. Since the construction industry, with a share of approx. 38 % of global CO2 emissions, has made a significant contribution to global warming to date, in particular due to the enormous cement consumption [1], a change to more energy and resource efficiency as well as a growing awareness of sustainability is absolutely necessary. In the course of this, a resource-efficient carbon concrete, consisting of a corrosion-resistant textile reinforcement in combination with a significantly reduced concrete cover, is established in the construction industry as a convincing alternative to conventional steel reinforced concrete [2,3].

Due to the high load-bearing capacity of the textile reinforcement with the smaller concrete cross-sections required, the bond between the textile and the concrete is extremely important. So far, R&D has focused on the development of impregnations and impregnation systems for improved material bond with the concrete matrix [4]. However, only small forces with a shear flow of about 5 - 40 N/mm can be transferred, an efficient utilization of the textile reinforcement is not possible. Solutions with profiling of the yarn surface promise significant improvements in the transmission of bond forces [5]. Therefore, new technologies for the continuous and reproducible production of profiled textile high-performance fiber yarns and their further processing into reinforcement structures were developed within a research project at the ITM of the TU Dresden. These innovative, profiled reinforcements are characterized by their ability to transmit significantly higher bond forces in concrete [6,7]. In particular, this was realized by a form-fitting effect between the textile and the concrete, that meets the specific requirements of a stiff and symmetrical surface profile of the reinforcement yarns in order to guarantee a constant and high force transmission. To generate the yarn profiling, solutions based on braiding technology and forming processes were developed and implemented with the help of simulation-supported studies. The premises were a permanently stable textile structure and a profile with a symmetrical structure. The realization of grid-like reinforcement structures, consisting of the profiled reinforcement yarns, was carried out using the multiaxial warp knitting technology. This was developed further on a modular basis with regard to the existing processes (yarn feeding, weft yarn insertion, knitting process, impregnation and winding) in accordance with the necessary adaptation measures for the fiber-friendly and requirement-based further processing of the profiled reinforcement yarns into grid-like structures.

Development of the innovative profiled reinforcement yarns
For the development of bond optimized profiled reinforcement yarns for concrete applications, a simulation-supported yarn development was carried out on the basis of braiding and forming technology. In particular, the main challenge was to realize profiled yarns with minimal structural elongation, so that, an initial force transmission of the textile reinforcement is possible and the concrete crack widths are minimized [3] if the concrete matrix fails at approx. 0.2 % elongation. For this purpose, a new type of varying braiding structure was developed. Moreover the braiding technology was further developed to enable a low-undulation and pre-stabilization of the braiding yarn structure during the braiding process, yet still ensuring further textile processing. As a result, it is now possible to implement novel vario braiding yarns as well as conventional packing braided yarns, consisting of carbon fibers with nearly eliminated structural elongation, minimal fiber damage and the required pre-stabilization of the yarn structure (see Table 1).

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Performance potential of the new profiled reinforcement yarns
The newly developed profiled reinforcement yarns are characterized by nearly unchanged tensile properties, yet up to 500 % higher bond properties compared to carbon rovings without profile or rovings extracted from reference textiles (see Figure 1). In addition, they do not show any noticeable structural elongation, so that an initial force transmission is possible without additional crack opening after the failure of the concrete matrix. However, an increase in bond strength of more than 500 % from approx. 20 N/mm of the carbon rovings without a profile to over 100 N/mm of the profiled reinforcement yarns was achieved, which is accompanied by a significant increase in material efficiency (see Figure 1). The vario braiding yarns in particular are characterized by very high bond stiffness, which is of particular interest for an initial force transmission. The packing braiding yarns and the profiled rovings with tetrahedral geometry have almost the same bond properties. The bond stiffness is marginally lower compared to the vario braiding yarns, whereas their production is more productive than the vario braiding yarns.

Development of the multiaxial-warp knitting process
To process the newly profiled reinforcement yarns into a grid-like reinforcement structure, a biaxial warp knitting machine Malimo 14022 at the ITM and the corresponding sub-processes (yarn feeding, weft yarn insertion, knitting process, impregnation and winding) were adapted and further developed so that on the one hand the pre-stabilized braiding yarns and the consolidated tetrahedral-shaped profiled rovings can be processed further. For this purpose, the weft thread laying process in particular was modified by developing a new type of weft thread guide for the laying of the pre-stabilized braiding yarns. Since the rigid profiled rovings could not be processed with the conventional weft laying process, a new type stick placement system consisting of a stick magazine and a shaft with profile rollers was developed (see Figure 2). The pre-cut sticks were individually inserted via the stick placement system into a transport chain modified with new fixing elements.

In order to guarantee textile processing, the pre-stabilized braiding yarns were impregnated and consolidated after the warp knitting process, contrary to the rigid profiled rovings, which do not require any further impregnation.. On the basis of extensive production tests, a new type of impregnation system was developed based on the kiss coater process with an additional coating roller for applying an impregnation agent to both sides of the pre-stabilized braiding yarns. Various reinforcement structures were manufactured and characterized with the implemented system technology. Figure 3 shows a new type of profiled textile reinforcement consisting of prefabricated profiled rovings with tetrahedral shape.

Acknowledgments
The IGF research project 21375 BR of the Forschungsvereinigung Forschungskuratorium Textil e. V. is funded through the AiF within the program for supporting the „Industriellen Gemeinschaftsforschung (IGF)“ from funds of the Federal Ministry for Economic Affairs and Climate Action on the basis of a decision by the German Bundestag.

The complete publication is available as download.

Introduction, problem definition and aim

Fiber-reinforced plastic composites are designed according to required stiffness and strength or impact and crash properties. Complex, overlapping load scenarios are only taken into account to a very limited extent. There are first practical approaches for realizing composite components, e.g. the B-pillar of an automobile [1]. In which composites (e.g., carbon fiber prepregs) are combined with metallic components (e.g. steel sheets) in order to achieve the necessary damage tolerance along with high weight-specific stiffness and strength. In such concepts, hybridization takes place at the macro (structural level) or meso (yarn level) level and requires extremely complex and cost-intensive manufacturing processes [2-4]. Furthermore, these components also have highly pronounced interlaminar interfaces, where complex stresses generate high shear stresses. As a result, premature structural failures occurs due to delamination [5-8]. In order to overcome these disadvantages and for use in future developments, a concept is developed and implemented in the project presented here. The approach provides the design of the combination of various fiber components by hybridization at the micro-level (within a yarn/fiber level), thus maximizing their property potentials. The use of recycled high-performance fibers also results in significant advantages over conventional composites in terms of sustainability, resource efficiency and cost-effectiveness.

The project aims to create a new three-component class of materials hybridized at the micro level for thermoplastic lightweight applications. By combining the reinforcing fibers such as carbon and aramid, it is possible to combine high stiffness and strength with high crash and impact properties by varying the reinforcing fiber proportions and fiber makeup in a way appropriate to the load case. Fig. 1a schematically shows the properties of state-of-the-art CF/AR hybrid composites (Fig. 1a bottom, highlighted by an ellipse) according to state of the art, from engineered yarns to be developed (top, area within the dashed lines) and the theoretical material potentials (top, colored lines), each depending on the fiber volume fractions. The systematic investigation of the influence of the material-specific fiber volume fractions for a scalable composites design was carried out in five stages (CF/AR or rCF/rAR: 50/0 %; 40/10 %; 25/25 %; 10/40 %; 0/50 %).

The development work focused on three main areas. The first focus was the further development of the process technology so that the composites based on engineered yarns exhibit high strength and stiffness due to low fiber damage, high uniformity and high fiber orientation. The second focus was the first-time implementation of the homogeneous blending of three fiber materials at the micro-level. The third focus was designing the engineered yarns so that outstanding, scalable stiffness, strength, crash and impact property combinations can be set explicitly for a wide range of requirements (Fig. 1a).

For the concrete realization of the desired goal, CF/AR/PA6 or rCF/rAR/PA6 hybrid yarns were developed using two material concepts (Fig. 1b) based on two yarn formation technologies (Fig. 1a) for the production of thermoplastic composites with outstanding, scalable stiffness, strength, crash and impact property combinations. The interrelationships between process parameters and material-yarn composite properties were analysed. A sound knowledge for the material-dependent design of the engineered yarns could be achieved. Furthermore, the best possible material and process parameters for specific applications was derived and a process guide was prepared for the control of the manufacturing processes for the SMEs. A detailed description of the development work can be taken from the final report.

Acknowledgement

The IGF project 21004 BR/1 of the Forschungsvereinigung Forschungskuratorium Textil e. V. is funded through the AiF within the program for supporting the „Industriellen Gemeinschaftsforschung (IGF)“ from funds of the Federal Ministry for Economic Affairs and Climate Action on the basis of a decision by the German Bundestag.