Introduction

Motivation: Circular economy of automotive fiber reinforced plastics components

Continuous carbon (CFRP) and glass fiber reinforced plastics (GFRP) are among the materials with the highest potential for lightweight structures and components due to their excellent density-specific properties. In the mobility and transportation sector, the use of CFRP and GFRP offers the opportunity of reducing the mass of vehicles. For vehicles conventionally powered by internal combustion engines, this leads to a reduction in CO2 emissions. However, lightweight construction is also an important aspect for electrically powered vehicles. The high mass of the battery increases the mass of the vehicle. This is a disadvantage not only in terms of range, but also in terms of crash safety, for example, as heavier vehicles generate greater forces. Despite these advantages of fiber-reinforced plastics (FRP), they have not yet been able to establish themselves in large-scale automotive production and are used at most in sports cars or luxury vehicles. The main reasons for this are the high price of FRP materials, but also the limited sustainable end-of-life strategies.

For these end-of-life (EOL) products of FRP materials there are currently four options: landfill, incineration, recycling into the constituent materials or direct reuse of components or structures [10, 15]. Landfilling is the easiest and cheapest way to dispose of EOL FRP material, which is why most of it is landfilled around the world today [15, 22]. However, in the European Union landfilling of untreated FRP materials is prohibited considering the high amount of organic content and it is expected, that other countries will follow this legislation [18]. Another waste strategy is the incineration of FRP EOL materials. However, around 50% of the composite waste remains as ash, which also has to be landfilled and the costs for landfilling will continue to rise. In this context, incineration also appears to be an unfavorable waste route [11, 15]. Due to these restrictions on traditional disposal routes such as landfill and incineration, there is an increasing demand for recycling technologies and reuse options for FRP. Sustainable solutions for EOL products are one of the greatest challenges for fiber-reinforced plastics [15, 19]. In addition to the challenges related to disposal issues, recycling and reuse solutions offer the possibility of reducing CO2 emissions. For example, using 50% recycled glass fibers (rGF) instead of virgin glass fibers (vGF) could save 2 million tons of CO2 [23]. New sustainable solutions and technologies open up opportunities for the transition from a linear to a circular economy in the composites industry. In conventional material and production processes, natural resources are depleted, which results to economic losses. By implementing the circular economy approach, EOL products and materials should serve as a source of raw materials or as a second life component for new products. In addition, the production of fibers, in particular from carbon, is very energy- and cost-intensive, which is why material recycling and reuse of structures and components can make economic sense in order to preserve values [7, 11]. Enabling further life cycles through recycling or reuse, the high costs can be amortized over a longer useful life. Current research and development for more sustainable solutions for EOL FRP can be divided into two categories. On the one hand, there is a very significant demand for solutions and technologies for FRP components currently in use that will soon reach the end of their life cycle (e.g. wind rotor blades). On the other hand, there is a need to develop next-generation FRP products and materials with improved recycling and reuse capabilities. This paper addresses a strategy for the development and design of next-generation reusable automotive CFRP components.

Classification of fiber reinforced plastics

Composite materials generally consist of two or more components with different physical and / or chemical properties, which, in combination, define the properties of the composite. In the case of FRP, these components are the polymer matrix and the reinforcement fiber [15]. Therefore, a large number of different material combinations are summarized under the term fiber-reinforced plastic. These material combinations are categorized based on the type and length of fiber and the matrix system. FRP materials with thermoset matrix system represent a significantly larger amount in terms of volume compared to thermoplastic FRP. However, the production and use of composite materials with a thermoplastic matrix is increasing [25]. Depending on the matrix material used, different EOL strategies are possible and preferable. The reinforcement fiber is largely responsible for the outstanding density-specific mechanical properties of FRP. Materials with continuous fiber reinforcement achieve the highest specific stiffness and strength values among the group. In continuous fiber reinforcements, the fibers have a length greater than 50 mm. On the other hand, there are short and long fiber reinforcements with fiber lengths of 0.1-1 mm or 1–50 mm. These materials have considerably reduced mechanical properties compared to materials with continuous fibers. In addition to the fiber length, the fiber material is also decisive for the properties of the composite. The highest levels of rigidity and strength can be achieved with carbon fibers. However, these are very costly and energy-intensive. By far the largest share of reinforcing fibers in FRP in terms of volume are glass fibers. Although these have lower mechanical properties compared to carbon fibers, they are more economical due to the lower purchase price [6, 7]. Due to these very high volumes of end-of-life fiber reinforced plastic material, the greatest challenge for the thermoset polymer composite industry is the cost-effective, sustainable and environmentally friendly recovery and reuse of reinforcing fibers from production waste and EOL composites [15, 22]. Therefore, this paper focuses on novel reuse solutions for continuous fiber-reinforced plastics based on thermosetting matrix systems. In the case of FRP with thermoset matrix, waste is divided into uncured prepregs and (EOL) product applications with a cured matrix. The waste with a hardened matrix makes up the significantly larger share, which is why this group is the focus of this work [9].

Literature review: Analyses of existing end-of-life solutions and technologies for continuous fiber-reinforced plastics with thermos-set matrix

FRPs in general are heterogeneous and diverse due to the combination of two materials and therefore difficult to recycle. As a result, at present there are many different End-of-life scenarios for FRPs. As stated in the introduction, landfilling and incineration are not desired and further restrictions on these types of recovery will come. The preferred alternatives are therefore recycling technologies and reuse approaches, which are summarized in Fig. 1 under the term “circular solutions”. Recycling is defined here as the conversion of EOL products and materials into secondary raw materials. Reuse enables products, components or structures to have another life cycle. This can be done through repair, refurbishment or remanufacturing. Depending on the reinforcing fiber, the polymer matrix, the component condition, the component geometry and the material quality, different EOL strategies are preferable. The following chapters provide an overview of the current state of the art in the field of recycling and reuse of continuous fiber reinforced plastics.

Fig. 1
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End-of-Life scenarios for contious fiber reinforced plastics [15]

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Recycling

Suitable recycling strategies based on different technologies are necessary for each individual material of the composite. The right recycling process depends on the reinforcement fiber and matrix material [10, 11, 18]. Most of the recycling processes for FRP focus on the recovery of the fiber due to the higher value, especially for CFRP. In addition to the price of recovery, the main criterion for the fiber recovery process is the quality of the fibers after recovery, which enables them to be reused in high-quality composite products. However, recycling of the matrix can also be useful and advantageous, if the recycled matrix material is able to compete economically with virgin material [11, 14].The current state of technology shows that current recycling processes can primarily be divided into three groups: mechanical, thermal and chemical recycling [11]. In general, these three main groups of recycling processes are based on two principles: shredding the composite material and separating fiber and matrix.

Mechanical recycling processes are technologies that shred composite waste, which is known as recyclates. The shredding process makes it possible to roughly separate the fibers and matrix. However, a complete separation as in thermal or chemical processes is not possible. Depending on the particle size and fiber quality, the recyclates can be used either as reinforcing materials or as matrix fillers in new FRP materials [14, 15]. With mechanical recycling high volumes can be processed at very low costs. For this reason, it is mainly used for GFRP material, as the glass fibers are of lesser value and the loss in value due to the shredding is not as high as with carbon fibers [15, 18, 20]. Due to the very high losses in fiber properties the reuse of mechanical recycled fibers in new products is a challenge.

The main objective of thermal processes is to recover the fibers by separating them from the matrix [11, 14]. Thermal processes are mainly used for the recovery of carbon fibers. The most important thermal process in the field of recycling is the pyrolysis process. In addition to the high cost, which makes it economically difficult to recover glass fibers, GF lose up to 80% of their strength during thermal processes due to thermal damage to the fiber [14, 19]. In summary, it can be said that the pyrolysis processes are very well suited to obtain carbon fibers with good mechanical properties. Challenges still exist in the very high costs for system technology and the energy required, as well as in the limited capacities for processing large volumes.

Chemical recycling processes are defined as the depolymerization of polymers into monomers or oligomers. Since chemical recycling, analogue to thermal processes, is very expensive and time-consuming, it is used almost exclusively for the recovery of carbon fibers due to their higher value [15, 19]. The best known and most widely used chemical recycling process for FRP is solvolysis. An environmentally friendly alternative to environmentally harmful and hazardous solvents is the use of water (hydrolysis) or alcohol (alcoholysis) in a supercritical state [8, 15]. Chemical recycling processes offer the possibility of recovering long fibers of good quality and in some cases even reusing the matrix. Similar to the thermal processes, the challenges are the high costs, as well as the limited capacities for processing large volumes [8, 15].

The use of alternative fuel in the cement industry has now become established as an industrial recycling route for GRP. 100% of the material can be reused in the form of energy (polymer as a substitute for fossil fuel) and raw material (mineral part of the GRP as a silicate substrate). This recycling route enables the valuation of very large volumes of material. Before processing, the GFRP must be mechanically crushed and mixed with additives [11, 15]. A reuse of the glass fiber in its actual sense as a reinforcing material is just as impossible as a further reuse.[15].

Reuse strategies

In contrast to the described recycling technologies for FRP, which enable a circular flow of materials by converting the EOL components and materials into secondary raw materials, the reuse strategies are based on the preservation of products, components and structures. Remanufacturing, refurbishing and repair processes enable reuse in further life cycles. A distinction is made between the reuse of parts and structures of a component or product (structural reuse) and the complete reuse of a component or product (component reuse). While there is already some research and studies in the field of structural reuse of FRP, component reuse has not yet been explored. In contrast to conventional recycling, structural reuse does not involve separation of fiber and matrix, nor does it involve shredding, which preserves the structural integrity of the composite material. The service life of the material is extended by reusing large parts and structures or special components, and the use of virgin materials can thus be reduced. The advantages of structural reuse are lower reprocessing costs and the preservation of material quality. The structural reuse approach has already been demonstrated on rotor blades used as elements for playgrounds or in street furniture. However, the large dimensions and complex shapes and material compositions limit the reuse possibilities. The solutions developed so far are very dependent on the waste available. Joustra et al.‘s studies show ways in which the decomposition of large structures such as wind rotor blades into usable structural elements such as beams and panels can expand application possibilities. They analyze the design aspects of primary products as well as the segmentation and manufacture of structural elements from EOL products, using wind rotor blades as an example [12, 13]. It is shown that the concept of structural reuse, which is based on the production of large series of standardized construction elements, has not yet been sufficiently taken into account in the product design of primary products. Only through the recyclable design of the primary products, which enables the realization of secondary design elements, can lead to a stronger and more systematic use of the structural reuse solution [12]. Joustra et al. show into which different segments a wind rotor blade can be divided and determine the mechanical properties by a calculation based on the fiber and matrix materials and the ply structure. Compared to other construction materials, the theoretical characteristic values of the segments show very high specific strengths and comparable stiffnesses. Due to the fact that the characteristic values are only calculated and the EOL materials do not reach the characteristic values due to damage or material fatigue, safety factors are to be assumed in the design [13]. Overall, the concept of structural reuse allows components with excellent properties to be produced. One challenge is the predefined structural form, which, in contrast, can be freely selected for new materials. Nevertheless, structural reuse offers great potential and should be further developed. In particular, the design of primary and secondary products is important.

Conclusions of the current state of the art

A variety of recycling and reuse approaches and processes for the recycle of FRP have been developed and investigated to date. This shows the necessity but also the challenges that exist due to the properties and heterogeneity of the different materials. Pyrolysis and solvolysis are already used commercially in some cases, but on too small a scale to recycle the entire volume of EOL CFRP materials. In the field of GFRP, the use as a silicate substitute in the cement industry is the only commercially successful route. The reason for this is that there are still challenges and obstacles for all solutions and technologies. In the area of recycling, these obstacles can be attributed to three reasons. Currently there are still too few products on the market which use recycled fibers. The challenge is to open up new markets for fiber recyclates and to develop innovative and value-adding products made from recycled material to be able to compete with new material. Another very important aspect that is also related to the missing products is the economic efficiency. Many processes and approaches produce recycled fibers that cannot economically compete with virgin fibers. This problem exists in particular in the area of GF, since virgin glass fibers (vGF) are inexpensive and expensive recycling processes for the production of rGF are therefore not economical. In addition, current research is focused on recycling technologies that process the EOL material into a form that reduces the value of the material. The loss of value arises from the shortening of continuous fibers through mechanical processes, which, however, are often used before thermal and chemical recycling processes. In the case of glass fibers, the mechanical properties are reduced considerably in thermal and chemical processes. It is true that rCF with good mechanical properties can be recovered through these processes. However, these recycled fibers are often in the form of non-directional fluffy tufts, which makes further processing for new FRP materials difficult.

Against the background of difficult further processing and reduction of properties due to fiber shortening, structural reuse is a good alternative. In contrast to recycling, the structural material integrity and properties remain intact. In addition, the processes for reprocessing involve less effort and thus lower costs. However, structural reuse has also not yet been brought into a commercial application. The reasons for this are the lack of design consideration in primary products and suitable secondary products. Nevertheless, structural reuse is a circular solution with high potential and should therefore be further researched. The reuse of entire FRP components has not yet been considered. This approach can also be seen as an advantageous alternative to recycling but also to structural reuse, since there is no loss of properties due to fiber shortening and no need to search for suitable secondary products. For this reason, component reuse will be considered within the scope of the present work.

Methods

The aim of the present work is to investigate the possibility of reusing FRP components and to implement this based on an example. In a first step, the basic methods and guidelines for the development of reusable components and products were analyzed on the basis of two selected studies (Chap. 3.1). Subsequently, these existing methods were examined with regard to their possible application to FRP (3.2).

General design-for-reuse methods

The basic strategies of the circular economy, which are defined by Bocken et al. in their study, include slowing down material flows so that fewer products and materials are needed and closing material loops through recycling. In the area of slowing down material flows, there are two approaches, both of which fall under the concept of reuse. These are the design of long-life products and the design that enables the extension of product lives. The design of long-life products is divided into emotional and physical longevity. While design for emotional longevity aims to ensure that products are liked and trusted by their users for longer in order to prevent the replacement of functional products, design for physical longevity aims to maintain the functionality of products over a long period of time. The choice of materials is very important, for example, to withstand wear and tear without the product breaking [3, 5, 17]. There are also several strategies in the area of design for product life extension, which are shown in Fig. 2 and briefly described below.

Fig. 2
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Design strategies to slow the flow of material

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The Design for Maintenance and Repair strategy aims to keep products in perfect condition. This involves inspection and maintenance to maintain functionality and repair to restore good condition after wear or damage [3, 16]. The design of products, which enables their use in the future under changing boundary conditions (e.g. an increase in quality) is defined under the terms design for upgradability and adaptability [3, 16]. The design for standardization and compatibility represents products with parts or interfaces that also fit other products. The last strategy, which addresses the extension of product life, is called design for dis- and reassembly. In this sense, products and components are designed to be easily separable and reassemblable. This strategy is also of decisive importance for recycling, in order to enable the sorting of multi-material products [1, 3].

Many of these basic strategies are also reflected in the work of van den Berg and Bakker to develop a circular product design model. The model is complemented by design guidelines for circular products. Product reuse is supplemented by the reuse of components from a product. In the context of the study, this principle is referred to as “remake” [23]. Both studies also address design for recycling strategies to close material loops. However, this is not considered further here, as the focus is on reuse. The design guidelines are divided into 5 superordinate categories and corresponding subcategories from which the recommendations for action ultimately result. Due to the scope, only the main categories and the first level of structure are listed in Table 1. In addition, the main category “recycling” was also omitted due to the focus of the present work. In Chap. 4.1, the concept of a reusable assembly is reviewed using the design guidelines.

Table 1 Outline of the design for reuse guidelines [23]
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FRP component reuse strategy

As already evident from the prior part, component reuse of FRP parts is a good complement to structural reuse. The advantages of structural integrity through the avoidance of fiber shortening and the separation of fiber and matrix are also present in the component reuse approach. In this chapter, the basic material properties and design methods of fiber-reinforced plastics are compared with the requirements for general design-for-reuse methods. Based on the guidelines used in the study by van den Berg and Bakker, advantages of FRP for reusable components as well as challenges will be analyzed. In terms of durability, aging and corroding materials must be avoided. To assess aging, the fatigue strength of various structural materials is analyzed (Fig. 3). This shows that the fatigue strength of glass fiber reinforced plastics is in a similar range to that of titanium or aluminum and slightly worse than that of steel. CFRP materials, on the other hand, can have significant advantages in terms of fatigue strength and can therefore be described as more durable.

Fig. 3
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Tensile strength reduction through fatigue cycles for different materials [21]

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With regard to corrosion resistance, contact corrosion can occur with carbon fibers in combination with metals. However, the fibers are usually protected by the matrix, so that there is no direct contact between metal and fiber. There are no corrosion problems at all with GRP.

In the area of non-destructive dismantling, the joints must be selected in such a way that simple and rapid separation is possible. In this context, welding and bonding between subassemblies must be avoided, so that internal components and subsystems requiring it can be accessed for testing before and after refurbishment. Fiber-reinforced plastics are challenging in terms of joining. Reversible joints such as bolts or rivets are generally avoided, as they cut through the continuous fibers and create potential weak points. In addition, screwing or riveting of CFRP materials results in contact between the metal and the carbon fiber, which can promote contact corrosion. For this reason, FRP is favored for bonding, which is in contradiction to the guidelines. The repair option, which is important for long service life, is also a challenge with FRP. On the one hand, damage is not always easy to detect, since delamination and fiber breaks, for example, can be located inside the material. Broken fibers also represent an interrupted load transfer. Established repair methods are based on milling out the damaged structure and then patching it with a new laminate. Nevertheless, a continuous fiber cannot be replaced. Therefore, repaired areas represent a weak point in the component. On the other hand, FRPs offer very good possibilities for integrating sensors that can detect damage in the material or structure. In terms of lifetime prognostics in the field of maintenance, this can be a very important advantage. Although such sensors are associated with high costs, they can still be economically viable due to their longer service life.

From the analysis of the guidelines for circular and reusable products and the properties of FRP materials and products, it can be seen that the reuse of components can be particularly useful for CFRP products and components due to their high fatigue strength. In the case of FRP, no advantage can be seen with regard to this aspect. Furthermore, CFRP materials have a higher economic value, which is why a greater effort with regard to reuse is economically justifiable. On the other hand, there are still technical challenges that make reuse difficult in the area of disassembly and repair. However, there are already many different research projects, so that an improvement is to be expected.

Results and discussion

Application example: Reusable CFRP base frame of a vehicle platform

As part of the European research project FiberEUse, the approach of durable and reusable components and structures made of CFRP is already being implemented at demonstrator level by the consortium partners Fraunhofer IWU, EDAG Engineering GmbH and INVENT GmbH. The focus of the work was to demonstrate the technical feasibility of such a solution using a reusable CFRP-based vehicle frame for an electric vehicle platform. In addition, not only the advantage of reuse but also the generation of a weight advantage should be achieved. As a result, a basic frame for an electric vehicle was created, consisting of two straight and two curved pultruded profiles and four connecting corner elements. A carbon fiber-reinforced bulk-molding compound (BMC material) was selected for the implementation of the four corner elements. A complex layer structure of unidirectional carbon fibers and multiaxial carbon textiles was calculated and defined for the profiles to meet all the load requirements of a vehicle. The concept of the vehicle platform with the CFRP base frame is shown in Fig. 4.

Fig. 4
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Concept of a reusable CFRP base frame of an electric vehicle platform

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The approach follows the current trend toward platform-based vehicle construction. Compared to fully metallic platforms, a weight advantage of 40 kg could be realized. The front and rear sections of the vehicle platform were realized in metal construction, mainly in aluminum, and connected to the CFRP base frame. The connection is made by means of bolts and is designed in such a way that the front and rear sections can be easily and quickly dismantled in front of the base frame. In the event of a crash, the front and rear sections absorb some of the energy and release it again in the form of plastic deformation. They also serve to accommodate the electric drive axles. To protect the CFRP base frame in the event of a side impact, crash-absorbing aluminum profiles are also attached to the outside of the pultruded profile. The aluminum profiles are designed so that, in the event of a Euro NCAP side impact, the aluminum profiles are plastically deformed and the CFRP profiles are plastically deformed. On the one hand, this provides optimum protection for the occupants, the battery and, at the same time, the CFRP frame so that it can be reused in the event of an accident. In this case, there is a separation of functions, with the aluminum profiles absorbing the energy in the event of a crash and the CFRP frame representing the structural integrity of the platform. Without the profiles, even minor accidents would require replacement of the entire frame. The aluminum and pultrusion profiles are joined by adhesive bonding For the straight glued pultruded profiles, a CNC saw is used, which separates the profiles exactly in the middle of the 3 mm thick adhesive layer. More complex glued geometries can be separated, for example, by grooving thermally expanded sections [2].

Various design-for-reuse aspects were taken into account in the construction of the CFRP frame and the entire platform concept. The following Table 2 shows which of the guidelines from van der Berg and Bakker were taken into account in the design and how they were implemented.

Table 2 Analysis of the reusable vehicle platform with regard to the design for reuse guidelines
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The vehicle platform solution presented is intended to achieve an advantage in terms of greenhouse gas emissions (CO2). A life cycle analysis was carried out to verify this objective. As part of this analysis, a conventional steel vehicle platform was compared with the reusable lightweight platform made of aluminum and CFRP. Due to the high energy demand in the production of carbon fiber and aluminum components, the production of the lightweight platform results in a CO2 equivalent of 1617 kg CO2 compared to 716.5 kg CO2 for the steel variant. Accordingly, in the use phase and in the area of reuse, this deficit originating from production must be compensated. A life cycle of the platform was defined with a mileage of 180,000 km, which corresponds approximately to a service life of 4 years. After these 4 years, the steel platform has reached the end of its life, while the complete lightweight platform completes another life cycle of 4 years. Only after these 4 years the aluminum components have to be replaced. The CFRP frame, on the other hand, will continue to be used. In the analysis of CO2 emissions over the service life, the beak-even point at which the lightweight platform achieves an advantage over the steel solution is after 4 years, i.e. at the time of the first reuse. However, this advantage can only be realized through reuse. The weight saving of 40 kg alone would not have been sufficient [4].

To implement the concept of the reusable vehicle platform, further processes, must be developed. This also includes non-technical issues such as the logistics of reusable parts as well as new business models. The condition of the components at the end of a vehicle’s life is crucial for reuse. Only undamaged components can be reused in another vehicles. For this reason, a condition assessment is essential. Repair or refurbishing may be necessary to make the basic frame suitable for reuse. In this context, logistical aspects and new operator models play an important role. It must be ensured that at the end of the product’s life, the vehicles are returned to the manufacturer or special remanufacturers to be dismantled and the reusable components made ready for a further life cycle.

Advantages, disadvantages and challenges of reusing FRP components

In principle, the savings in CO2 emissions and primary raw materials, as well as the weight advantage of 40 kg achieved, show that the concept of the reusable CFRP frame makes sense and can generate advantages. In addition, the approach presented represents a solution for making the use of CFRP in automobiles more attractive, as the higher costs can be amortized over longer periods of use. In addition, a sustainable use of the materials is demonstrated. However, these advantages are offset by a few disadvantages and challenges. The solution represents a very limited area in which the concept of reusable structural components made of CFRP can be used. By reusing the frame, only the basic dimensions of the vehicle are already predefined for subsequent vehicle generations, although certain changes are possible, for example in the design of the front and rear end. This makes it possible to develop a wide variety of vehicles based on the platform by freely designing the entire vehicle body and, for example, the interior. Transferring the component reuse concept to other areas of the vehicle, such as the body, would restrict this scope for design. Another critical aspect is the high design and production engineering effort required to implement this solution. Further challenges or potential to make component reuse even more attractive exist, among other things, in the condition determination of the CFRP frame after several life cycles. Non-destructive testing methods such as ultrasound or active thermography can reliably detect damage. However, these methods are time-consuming and cost-intensive. It will therefore not be possible to scan each individual frame completely using these methods, as this would take too much time. An alternative would be, for example, online structural monitoring using sensors that immediately display the component status. Other aspects include improved and alternative solutions in FRP repair and rapid disassembly of adhesive bonds. There are already research approaches here and technological progress can be expected, as well as in the further reduction of CO2 emissions in the production of carbon fibers.

In principle, the component reuse strategy can be applied to all applications of CFRP. However, the materials in classic CFRP applications such as aviation or in wind rotor blades are exposed to much greater stresses. However, these products also have a very long service life of their own (approx. 20 years). When applying the component reuse strategy for these products, they would have to be designed accordingly and a separation between capping and long-life reusable structures would have to be realized. An example implementation would be a segmentable wind rotor blade with an internal durable support structure that can be repaired and an external planking that is replaceable. In this case, the joining technology would also be very important. However, there are already initial approaches for separable laminate layers, for example [24].

In the concept of the ideal circular economy, there is no unsustainable consumption of resources, since the entire cycle is closed via several circuits. However, this is only possible if the system is based on renewable energies and there is no loss of quality in the recycling of materials. If this is not the case, the time aspect must be taken into account and the consumption of resources and the decrease in material quality must be slowed down as much as possible. One possibility is the cascade utilization. For fiber-reinforced plastics, an ideal circular economy is currently not possible due to the nature of the material, which is why cascade use to slow down resource consumption offers a very good opportunity. The component reuse strategy offers the opportunity to integrate a further cascade into the use of FRP and thus to further slow down the consumption of resources. Figure 5 shows an example of how such a cascade use can be supplemented with the CRS.

Fig. 5
figure 5

Example of cascade utilization of CFRP material with component reuse strategy

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Conclusions

The component reuse strategy is another approach for the sustainable use of EOL FRP, which is particularly suitable for CFRP due to its high fatigue strength. Based on known design for reuse guidelines, the component reuse for a CFRP frame of a vehicle platform was demonstrated. The use of CFRP and its reuse resulted in a weight advantage and a reduction in CO2 emissions. The reuse of CFRP components is an approach that has a high potential, since, analogous to structural reuse, no complex fiber matrix separation is carried out and the fibers are retained in their length. However, the approach has so far only been demonstrated on the vehicle platform solution presented. It must be proven that other applications are also useful. This will require further technical developments in the field of separable joints, condition assessment of EOL components and the repair of CFRP. Furthermore, this essay only highlights and discusses the engineering advantages and current technical drawbacks of a reuse strategy. The exemplary mentioned approach of reusing a CFRP base frame could and should open up new business models for selling cars, because of high initial cost and amortization over time. In conclusion reusing structural (car) components could be an excellent way to reduce the need of raw materials.