Neutral lightweight engineering: a holistic approach towards sustainability driven engineering

The continuously growing social and political pressure to provide sustainable products is forcing also the lightweight industry to rethink current development and manufacturing processes. While established development approaches in lightweight engineering mainly focus on technical and economical product requirements they usually do not consider sustainability criteria. To address these challenges, a new class within the lightweight disciplines is proposed—the Neutral Lightweight Engineering. Its basic goal is to integrate sustainability criteria in all decisions along the development chain of a lightweight component. The decision makers in lightweight engineering thereby have to consider the whole life cycle of a product system from material sourcing to end-of-life part management. To implement this idea, advanced development methods are necessary, using established and emerging materials as well as efficient production and end-of-life strategies. This concept article introduces the idea of Neutral Lightweight Engineering and exemplary highlights some of its aspects before the background of scientific literature.


Introduction
The current societal dynamics towards climate change response and sustainability lead to a pressure for change not only in conventional industry but also in lightweight engineering. It is crucial to produce less waste and pollutants [1] and the emission-free operation of technical systems must be established [2]. At the same time raw materials are becoming more expensive or only available in reduced quantities [3]. Against this background there is an urgent need for clean and efficient lightweight engineering strategies, supporting the shift towards a circular economy.
The use of lightweight engineering principles to overcome resource scarcity is an established approach, especially in situations of unstable availability of materials. For example a 1988 publication during the cold war era in eastern Germany calls for the "economical use of resources and materials and the rational use of energy", for "reuse" and "regeneration of the product or component" and in this context addresses the "responsibility of the design engineer" [4]. Towards the mid 1990s the Dresden Model of "Function integrating lightweight engineering in multi-material design" with its motto "the right material at the right place" became a guiding principle throughout the lightweight community in Germany [5]. It summarizes essential aspects of lightweight engineering such as the classical technical criteria like safety, design and manufacturing as well as economical aspects like costs and quality. In addition, a key component of this design concept is the consideration of the environmental impact of lightweight systems (see Fig. 1).
Saving resources is thus a constituent element of lightweight engineering and is firmly anchored in the scientific discipline, both conceptually and methodologically. However, these capabilities have not yet been pushed to the necessary extent by politics, implemented by industry and accepted by society. To support this, the stringent implementation of sustainability criteria in the established Engineering Design Processes is proposed, which leads to a new lightweight class: Neutral Lightweight Engineering. In the context of this concept paper, the term sustainability primarily encompasses environmental aspects. Although social and economic aspects form essential pillars of comprehensive sustainability, they are not the focus of this paper. The fields of action of Neutral Lightweight Engineering are presented and discussed in the context of scientific literature.

Neutral lightweight engineering
Typically, three historically evolved classes of lightweight engineering are distinguished-Economical, Efficiency and Functional Lightweight Engineering [6][7][8]. Each class is characterised by its major development goal (see Fig. 2). The Economical Lightweight Engineering aims primarily at the reduction of resources e.g., material and energy during the manufacturing. It is common e.g. in mass production of plastic parts [9] or in civil engineering [10]. The use phase and end-of-life treatment are usually not assessed. Therefore, products that correspond to the Economical Lightweight Engineering class are rarely environmentally optimized. Products that comply with the Efficiency Lightweight Engineering class evaluate the life cycle economically and focus on the use phase. For active products in particular [11] the aim is to achieve positive effects of mass reduction during the use phase-for example, reducing fuel or energy consumption [12,13] over the life time of mobile systems or the increased payload of utility vehicles [14]. For products with high energy requirements during the use phase this approach can align with environmental interests. Functional Lightweight Engineering is driven by the fulfilment of a challenging technical objective by structures of equal or increased functionality with a required Fig. 1 The 1997 published Dresden Model with its intrinsic consideration of environment in lightweight design [5] Fig. 2 The three classical lightweight engineering classes (blue) with the proposed additional class of Neutral Lightweight Engineering (green) and their major development goals Recent research showcases the potential of proper application of the R-strategies on lightweight components [29]. Wind turbines are primarily built using glass fiber-reinforced plastics (GFRP) composites. Through remanufacturing and repurposing the components can be used as bridges, within playgrounds and as urban furniture or used for new hybrid material components. Recycling allows for the material to be used as particleboards based on crushed GFRP or improved wall paint for wood protection. Finally, the use of pyrolysis or solvolysis allows to recover some of the material to be used within e.g. concrete. Reusing the blades could further improve their environmental impact and circularity. Enabling this will require lifetime monitoring of mechanical properties [30].

Design
In Neutral Lightweight Engineering product developers shall anticipate the entire product life cycle during the development phase and systematically take environmental impacts into account. The consistent consideration of the 10 R-strategies can already contribute to this. Though, the assessment of environmental impact during product development poses challenges that are known from cost estimation [31,32]. The knowledge about a product system is usually lowest at the beginning and increases in the course of the development phase. Therefore, also the predictability of environmental impacts is lowest at the beginning of the development process. At the same time, the possibilities to positively influence the environmental effects are highest at the beginning of the development phase (see Fig. 5) [33][34][35]. Key environmentally relevant decisions are made at early stages of product development, such as the selection of materials or the R-strategies to be addressed [36]. If environmentally relevant information can be considered in the early stages of the development process, the relevant knowledge and thus the leverage for positively influencing the product system both increase [37].

3
On the one hand, a various number of product development methods, like VDI 2206 [38], VDI 2221 [39,40] or VDI 2222 [41,42] are established and address various interactive, iterative, cyclic or agile approaches to consider primarily technical and economical criteria. On the other hand, established environmental assessment methods like Life Cycle Assessment (LCA) are available [24]. Neutral Lightweight Design can be achieved by systematically integrating sustainability-oriented process steps (e.g., from LCA) into the development process [43]. For products with manageable development effort, environmental assessment can be performed at the end of each development phase, enabling an iterative adaption of the technical product features and properties [44]. Following this concept, it may be necessary to repeat previous development phases if the environmental assessment suggests fundamental changes to the product, such as the choice of materials [45]. This can lead to a decreased efficiency of the development process. For technically challenging products, such as lightweight structures in the context of complex technical systems [46,47] , this procedure rapidly reaches the resources available for product development, such as time, money, experts, software or computer technology.
To successfully develop environmentally friendly and sophisticated lightweight structures in the context of complex technical systems, sustainability aspects need to be considered within the product development process (PDP). Early examples of this are Design for the Environment Decision Support (DEEDS) [48] or the Design for Environment (DFE) [49] methodology. These procedures highlight individual aspects of integrating ecodesign in the PDP such as the evaluation and minimisation of entropy to determine environmentally ideal disassembly sequences. To assess whether a development decision is sustainable, more recent research focuses on how to directly integrate LCA into the PDP [50]. This allows an iterative and interactive adjustment of technical, economical and environmental factors. An example of this is the introduction of the LCA into the PDP of wind turbines [51]. Through the assessment of each life cycle phase in the early phases of development it is possible to identify hotspots with a significant potential for environmental optimization (in this case a feasible reduction in Teflon usage which reduced the overall GWP by 3%). In addition, the LCA results contribute to the identification of energy and waste reduction potentials, the development of environmentally oriented supply chain management, and the prioritization of actions within the PDP and marketing.
Within NLE a similar development process is proposed which specifically aims to incorporate LCA within the framework of the development process as defined by VDI 2221 [39,40] (see Fig. 6).
In the initial process step during clarification of the problem or task, suitable environmental assessment methods and criteria need to be defined (a). A classic LCA is not applicable here, since it is usually performed on existing product systems, considering all relevant processes in the chosen system boundaries with their actual in-and outputs. During the early phases of development, much environmentally relevant information is not even known, for example the materials, the quality and quantity of energy used for manufacturing or the transportation distances between specific suppliers and customers. Hence it is unavoidable to make numerous assumptions and simplifications, which implies an unavoidable level of uncertainty. Therefore, depending on the project scope and quantity of resources, it may be advantageous to limit the environmental assessment to specific impact categories of interest. Subsequently, environmental requirements for the product need to be specified (b) based on e.g., market demands or legal compliance. For example, today often greenhouse gas emissions are of primary concern for which the CO 2 -equivalents are a suitable requirement criteria [52]. The objective and scope of the environmental assessment need then to be determined (c). As the development proceeds, successively more information are available and environmentally sensible decisions can be made on an increasingly solid data basis. It can be beneficial to define a narrow scope, which can then be successively expanded as the process advances and data reliability improves. Based on this, the environmental assessment of the concept (d), preliminary design (e) and overall design (f ) can be carried out. Depending on the specific setting it can be advantageous to apply tools which allow for screening LCA's within the earlier evaluations [53,54], as these significantly reduce the resources required for a first basic environmental assessment of concepts or preliminary designs. Once a product design has been elaborated, the environmental manufacturing and utilisation optimum can be determined by means of a sensitivity analyses (g). Environmentally ideal settings for parameters such as process temperatures or times need to be compiled [55]. As part of the validation phase, a final Life Cycle Assessment is carried out (h) with all available information to determine whether the requirements defined at the outset can be met by the designed product.
The systematic character of sustainability-oriented product development on the one side, in combination with the increased scope and complexity on the other side, predestines it for implementation as a digitally linked development process. The approach is based on established methodologies, which divide the development process into phases and steps. Feldhusen [56] proposes an approach where each individual step consists of a model, a method and the necessary input data to generate output data and information for subsequent steps. Generally valid data and information are transformed into product information by a continuous sequence. This approach can be transferred to lightweight structures [57,58]. In such a digitally linked development process, the document-centred way of working of classical engineering processes is transformed into a model-centred approach, whereby a certain analogy can be drawn with Model Based Systems Engineering.
Such a development process is predestined as a basis for integrating models and methods of environmental assessment in the individual development phases, steps, methods and models.

Material
The second major field of action in Neutral Lightweight Engineering concerns the used materials. It is well known, that the lightweight potential of standard engineering materials is often limited due to its density related mechanical properties [59]. Hence, the classic development goal in lightweight engineering-a low component mass-often cannot be reached by using just one single material. The resulting increased mass of a component can cause higher emissions in the use phase of typical lightweight products [60]. Against this background lightweight design usually combines different materials in a structure, each with its beneficial specific material characteristics [61]. Also on the material level hybridisation is applied, which often offers a significantly expanded range of properties compared to mono materials [62]. For example, due its strong anisotropy, continuous fibre reinforced composites enable a load adapted material design, saving unnecessary mass [63].
It is evident that today multi-material systems as well as plastic-based materials and fiber composites lag far behind other materials like metals in terms of recycling rates [64][65][66]. At the same time, the use of petro-based raw materials is increasingly rejected [67]. Moreover, the technology and energy resources to massively produce e.g., CO 2 -based raw materials are limited for the foreseeable future [68]. Therefore, concerning the materials, Neutral Lightweight Engineering currently has to focus on two aspects. First, the retention of materials and structures in the technosphere (Fig. 7 right) must be one central ambition of all development efforts [69]. It represents the main pathway to achieve circularity. The second aspect touches the exchange of technosphere with bio-and geosphere. If additional materials are demanded to fulfill human needs, the extraction rate must not exceed the recovery rate of the natural resource [70].
Regarding the first aspect, the material usage in the technosphere mandatory has to consider the R-strategies (see Fig. 4). This basically involves slowing the material flow [23] and increasing material efficiency (R2: reduce), which represents the classic lightweight engineering approach [73]. Moreover, it is undisputed today, that recycling (R8) will play the major role in the material life cycle [74]. The strategy of energy recovery by incineration (R9), which is the most widely pursued today [75], must not play a role in Neutral Lightweight Engineering concepts.
In an ideal circular economy, the technosphere is cradle, stock and grave of the material at the same time [22]. In this context, it is of particular importance to examine how previous life cycles and recovering processes affect the material properties [76]. The effects of individual phases in the life cycle need to be understood, the causes of property changes must be determined and correlations must be investigated across life cycles [77]. On this basis, a prediction of the material and component properties in the coming life cycle must be enabled [78]. This also means, that material models are to be developed which consider the cross-life-cycle behaviour.
The cross-life cycle property changes must be taken into account in engineering [77], and materials, technologies and design must be modified in such a way that the expected changes are minimal and the material can be kept in the cycle as long as possible [79]. From this engineering perspective, the occurring volatility of the material flows in terms of availability and quality is especially challenging. Here, fast and efficient strategies to provide material data for process and structural simulation are the key factor to enable high prediction accuracy, high lightweight degrees and low resource impacts. For this, a combination of established characterisation methods [80] and new measurement approaches like soft sensors in recycling and manufacturing [81] can provide valuable information. Tracking techniques like markers [82,83] can help documenting and merging distributed data sources to enable a continuous understanding [84]. How big data technology [85] and material tracing [86] can support circularity is under research [87]. But even if cross life-cycle behaviour is known, circularity is usually hindered by downcycling [22], contamination [88] and entropy increase [89]. While downcycling is well known for thermoplastic polymers [90] and their composites [91], the effect is also present for metals [88,92], although often unconsidered in public discussion [93]. Engineering can help slowing degradation and enhancing material efficiency [94]. However, sooner or later a specific amount of energy is necessary to upgrade the material [95] to keep it in the loop. Exemplary, depolymerisation strategies like pyrolysis [96][97][98], solvolysis [98] or chemolysis [97] are under intensive current research. In future, the energy required for recovery could help to decide, which route is the most beneficial for the specific material stream and aspired application [99]. Beside downcycling, contamination of material streams is inevitable due to limited sorting grades [100], adhesion [101] or multi-material systems [29]. The continuously increasing mixing rate is going along with rising process effort and energy demand for recovery [97]. The lightweight design engineer can mitigate this, e.g. by integrating disassembly concepts or design for recycling strategies in his development decisions [102]. Additionally, the amount of different materials or material grades with different additives should be reduced to minimize contamination [103]. For example, hybrid lightweight structures can be designed from different semi-finished products like organosheets, braided tubes and long fibre thermoplastics with the same fiber-matrix combination, enabling a single variety waste stream during recovering [104]. But generally, a distinct material loss is not avoidable [105].
A promising approach to compensate the resulting gap between material loss and industrial demand is the use of renewable sources from the biosphere (e.g. from algae, wood, farming) [106][107][108]. Since biobased materials are well established in lightweight engineering, for example as matrix and fiber [109,110], there are no general reservations. They even are discussed as option to capture carbon dioxide and store it by feeding it back to the geosphere [108]. The challenge for Neutral Lightweight Engineering is to use this class of materials for high performance technical systems. Promising examples are current approaches for the production of carbon fibers from lignins [111] and cellulose which are being pursued in the high-tech sector [112]. Due to the natural limitations and the potential competition for land and use, the focus must be here on cascade use and the utilisation of co-products or residuals [113].

Technology
While the structure of a lightweight product is essentially the synthesis of the necessary geometry and the required material, manufacturing technologies [114] determine the opportunities and limitations to place the material in the desired configuration in the structure. For this challenging task both established and innovative technologies are applied, trying to fulfill the technical and economical boundary conditions [115,116]. From the Neutral Lightweight Engineering point of view, manufacturing technology is decisive for the circularity of materials and the overall life cycle impact of lightweight components in different ways. Firstly the direct environmental effects of the manufacturing process itself have to be considered [117,118]. Here, energy consumption, consumables and material utilisation rate often play a major role. Secondly the applied manufacturing technologies have indirect effects on circularity [119] by influencing the material degradation and the composition of material flows during recycling. Third, new technologies like 3D-printing, non-destructive testing and digitalization can help extending the product life time via repair or use material streams more efficiently.
The direct contribution of part manufacturing on the overall impact of a lightweight product is often of minor importance compared to the material production and the use phase [60]. However, production technology needs to be considered to complete the picture and to allow companies to evaluate hot spots in their production [120]. For many classic technologies used to manufacture lightweight components the major environmental impact is caused by the used energy mix during production [121]. Although the environmental footprint of energy (in particular electricity) production is available in regional and temporal resolution [122,123], data quality is a major challenge for evaluating the actual technological impact [124]. So, even for established polymer technologies such as injection molding, the available data is often limited to general information not considering different (more or less energy efficient) machine types or process routes [122]. For newer lightweight technologies in particular, there are limited or no data sets in the relevant databases [125]. Here, engineers have to resort to empirical values or rough estimations [126].
Beside energy, many typically applied technologies in lightweight engineering cause significant environmental impacts due to consumables [127]. For example, Vacuum Assisted Resin Infusion applied for wind blades [128] or autoclave technology for aviation structures [129] require extensive process auxiliaries to build the vacuum bag, which is usually used once and then disposed [127]. The amount of consumables used is strongly dependent on the specific part, and process setup and therefore mostly estimated [130].
To overcome these uncertainties in technology evaluation, lightweight manufacturing processes must be systematically investigated concerning their environmental impact. Therefore, on the one hand typical production scenarios and new technological developments are to be equipped with energy measurement systems [131]. Relevant process input and output streams including waste and rejects needs to be systematically recorded during production. This data can then be examined e.g. at various operating points [131]. To provide consistent, reproducible and comparable results, this evaluation should follow standardized proceedings, like ISO 14040 for LCA [24] or ISO 14064 for greenhouse gas evaluation [132][133][134]. The LCA relevant data obtained can then help evaluating the environmental impact of the product in different stages of the product development cycle (see Fig. 6). Above that, these results can be used to identify energy efficiency potentials or evaluate approaches like process coupling to reuse thermal energy demand [135].
Another direct impact of technology is the material utilisation rate in production [119] where sprues, runners, cut-offs and production rejects decrease material efficiency [118]. If the material is not recycled-e.g., like most textile cut-offs in classic composite manufacturing -the waste material and its accompanied environmental impact is imposed on the produced part [136,137]. Since this effect is also economically disadvantageous, optimisation solutions like runner-less injection moulding [138] is established. In composites manufacturing nesting software [139] or novel patching processes that use thermoplastic composite residues and cut-offs increase material efficiency [140].
In addition to direct effects, technologies affect environmental impact indirectly due to their influence on material quality. For example, polymer degradation in thermoplastic injection moulding is strongly influenced by dwell time or high shear velocities which can occur at disadvantageous process setups [141]. To support circularity, degradation should be as low as possible [142]. The process engineer therefore must know, which parameter settings influence degradation and potential conflicts with economically driven process settings need to be harmonized [143]. New technological developments can help increasing circularity by reducing processing induced degradation of materials. For instance, in injection moulding of lightweight parts, physical foaming strongly reduces melt viscosity leading to improved flowability and enabling less shear induced degradation of the polymers [144].
Manufacturing and especially joining technology also influence the repairability (R4) of a system [145], whereas detachable joints usually allow the simple exchange of a damaged component [146]. If replacement is not economically or environmentally reasonable (according to LCA), repairing can lengthen the component use phase. For high value lightweight products the damage can be identified non-destructively e.g. by computed tomography [147], ultrasonic testing [148] or thermography [149]. According repair structures have to be designed, which compensate the damage without limiting the function. Material-specific repair technologies must be provided to implement the replacement or reinforcement structure like patching [150] or 3D printing [151]. For early damage detection and repair, structural health monitoring is established e.g., at wind turbines [152]. This can potentially lengthen the use phase of a lightweight product [153].
The choice of manufacturing technologies has also influence on the composition of material flows during recycling (R9). Also the joining technologies determine how different components can be separated after the use phase [154]. While mechanical joining usually allows a good separation of different material fractions [155], adhesive bonding is accompanied by agglomerations of different materials, which are difficult to separate [156]. Furthermore, manufacturing processes for hybrid structures strongly affect the contamination of materials during recycling [157]. In addition to the basic components, hybrid lightweight structures often contain additional adhesion promoters [158], which later appear as impurities in the recyclate stream [159]. The amount of these adhesives can be reduced e.g., by plasma coating [160]. With new technologies like laser structuring a bonding can also realised without additives by microscopic form locking [161]. In combination with the end-of-life and recycling technologies the joining methods often determine the quality of the received material streams and the energy required for recovery [162].
The variety of possible manufacturing steps and process technologies and the strong interaction of technology, material and design (cf. Fig. 3) make it challenging to achieve Neutral Lightweight Engineering. To overcome this, digitization can help gaining relevant information along the material and component life [85]. If the measurements are assigned to the specific part, a successively completing data set containing both quality and sustainability relevant information emerges [163]. Above that, in the future this digital twin could help Neutral Lightweight Engineering at the end of the use phase to gain information about the environmental backpack [164]. An example of this would be the automated collection of operational data of wind turbines to assess whether or not a turbine still fulfills the technical requirements of being reused after its initial life cycle [30].

Conclusion
In the context of climate change, increasing scarcity of resources and challenging political and societal objectives, the demands on lightweight engineering are becoming increasingly complex. In this context and regarding the introduction of a true circular economy, the concept of Neutral Lightweight Engineering is proposed as a new lightweight engineering class. Neutral Lightweight Engineering aims at the holistic minimisation of the environmental footprint of a product system. Although not practically feasible, its ideal is a resource-neutral circular economy.
The associated shift in focus changes the way engineers think and act in the future by taking the entire life cycle into account. The objective of Neutral Lightweight Engineering results in four fields of action for the lightweight engineering community and in particular the research and development engineer: a sustainability-oriented development process, the technical application of recycled and renewable materials, and manufacturing technologies for high-tech products from these materials with low resource consumption and minimal environmental impact. In parallel, methodical competence must be developed to consider the impact of design, materials, technology and their interactions on the entire life cycle of the product. Life Cycle Assessment methods and the consideration of the 10 R-strategies are proposed to achieve the principles of Neutral Lightweight Engineering in all four fields of action.
The consideration of the proposed lightweight class by companies could enhance awareness in the engineering departments and encourage them to produce demonstrably sustainable lightweight products. In addition to advantages in marketing, efficient use of resources support a more circular economy. This plays an increasingly important role due to the growing political pressure on businesses to offer sustainable solutions. Furthermore, geopolitical conflicts and supply chain issues pose less risk as dependency on imported primary resources is reduced. A comprehensive application of the concept could lead to companies not only making selective environmental progress, but to the introduction of sustainability KPIs into their corporate management in addition to the classic cost and lightweight engineering metrics.
The concept paper refers to a large number of examples in the proposed fields of action and places them in the context of Neutral Lightweight Engineering. It can be seen that single objectives have already been partially implemented or are