Lightweight design – less is more

^{Web special Fraunhofer magazine 4.2023}

A multitude of innovative solutions will help produce lighter components and products − and make them cheaper and more sustainable in the process

Three wheels, a sturdy frame and a large cargo box: at first glance, the white delivery bike looks like any other cargo e-bike — like the sort people use these days to get their groceries, that parents use to bring their children to preschool or that couriers use to deliver goods. What’s different about this bike, developed by the Fraunhofer Institute for
Structural Durability and System Reliability LBF, is clear as soon as you start pedaling: Wow, how can a cargo bike be so light?

Lightweight cargo e-bike

“Our goal was to develop a truly innovative cargo e-bike using lightweight construction methods – and to include as many different areas of expertise at Fraunhofer LBF in the process as possible,” explains Dr. Saskia Biehl, head of the Fraunhofer project L-LBF (“Lasten-LeichtBauFahrrad” (lightweight cargo bike)). So, they bought a commercially available cargo bike with an electric motor, fitted it with sensors, loaded it up to maximum capacity and rode it through the city as well as through fields, forests and meadows to record it in operation under as many conditions as possible, while also identifying weak points. “Then,” says Dr. Biehl, “we broke the wheel down into its individual parts to see how we could create a lighter design for the front end, with the goal of reducing the mass by at least 30 percent.” By doing so, the team aims to show the huge potential for using lightweight construction methods for micromobility.

In L-LBF, the battery system is now hidden from sight – it has been integrated into the newly designed lightweight frame, rather than being attached externally in another box. The battery system in question is tubular and was developed specially for this bike; it has double the capacity of a commercial alternative. The steel hubcaps were replaced with aluminum variants with a wave design, and the cargo box can be made from either ultralight plastic, up to 100 percent natural materials or entirely from recycled materials. There are also sensor modules that monitor the position and distribution of the load in the cargo box, while also acting as connecting elements between the box and the frame. “More functions, more power – and yet even if we just take the front end, the L-LBF is 39 percent lighter than the original e-bike,” says Dr. Biehl.

Lightweight technology

Lighting the way: The German Federal Ministry for Economic Affairs and Climate Action (BMWK) has highlighted lightweight construction methods as a key technology for the future. In July 2021, the German federal government agreed on a lightweight construction strategy. Lightweight construction is not just about making slight reductions in mass. This construction philosophy focuses on selecting materials, designing products and carrying out production processes in a way that conserves materials and makes products lighter, while also improving functionality and safety at the same time.

Heavy objects are a burden on the environment. If a car is made 100 kilograms lighter, its fuel consumption decreases by around half a liter per 100 kilometers. That doesn’t sound like much, but considering cars in Germany travel 582.4 billion kilometers per year, it quickly adds up to an enormous amount. Reducing the weight of an Airbus 320 by 100 kilograms saves almost 10,000 liters of kerosene per year. The fact is, the consumption of raw materials globally has more than tripled since 1970 and around half of global greenhouse gas emissions now stem from raw material extraction and processing. Smart lightweight construction uses less material − which helps reduce the German industrial sector’s dependence on raw material imports and lessens the financial strain on companies, as 43.2 percent of costs in the manufacturing sector come from material procurement.

“Lightweight construction technology helps to reduce many different industries’ environmental footprints and to support the transition to a more sustainable industrial sector,” says Prof. Alexander Böker. The director of the Fraunhofer Institute for Applied Polymer Research IAP believes that, when combined with digitalization and bionics, lightweight construction can open up new opportunities in emerging future markets, thus serving as a driver for increasing resource and energy efficiency.

However, for these industries, the transition to lightweight construction is not a step to be taken lightly; the transformation requires a whole new way of thinking. According to Prof. Böker, research has a vital role to play here: “In order to ensure that industry, particularly the SME sector, can harness the huge opportunities for transformation offered by lightweight construction, we need to accelerate the process of transferring scientific knowledge to practical application.”

Lightening the load with smart material choices

“The right material for the right price in the right place”: according to Prof. Holger Seidlitz, director of the Polymeric Materials and Composites PYCO division at Fraunhofer IAP, this is the formula for success when it comes to lightweight construction. The aim is to find materials and components that not only allow for lighter products, but also guarantee the required functionalities and levels of safety at an economically viable price. In this respect, carbon fibers are the “black gold” of lightweight construction. First of all, they are incredibly light and yet extremely strong − this means they can be used as a substitute for metal, greatly reducing the mass of products. They can also be used in carbon fiber-reinforced plastics (CFRP), which can be molded into almost any shape while still making any product they are incorporated into rigid and strong. The drawback is that manufacturing these fibers is a highly complex, energy-intensive process.

At the Carbon Lab Factory Lusatia, a joint initiative by the German states of Saxony and Brandenburg that involves close collaboration by the Fraunhofer Institutes IAP and IWU, the Brandenburg University of Technology Cottbus-Senftenberg and the Chemnitz University of Technology, the plan is to develop and produce “green” carbon fibers in the future. Could these fibers be spun from local, renewable raw materials? And how far can renewable energy sources go toward improving the environmental footprint of CFRP production? According to Prof. Seidlitz, the circular economy approach could also help transform carbon fibers from the “black gold” to the “green gold” of lightweight construction: “The longer we can keep carbon fibers in the cycle, the better it is for the environment.”

As part of the TransHyDE project MUKRAN, Fraunhofer IAP is collaborating with five partners from science and industry to examine ways of optimizing the circular economy for CFRPs. Led by Prof. Seidlitz, the researchers have developed two types of spherical storage tanks for hydrogen. Just their shape alone helps to save mass: Due to the laws of physics, the walls of conventional cylindrical tanks need to be twice as thick as the spherical tanks. “Tanks made from pure steel also have many times the mass of those made from CFRPs,” explains Prof. Seidlitz. This high mass also increases fuel costs, as the chassis and brakes of the vehicles that transport these tanks must be reinforced. And this will eventually raise the question as to whether hydrogen is actually an environmentally friendly solution.

To avoid using unnecessary quantities of expensive carbon fibers, researchers in the MUKRAN project are searching for ways to make hydrogen storage containers that are light, yet sturdy. To assist with this, the research team at Fraunhofer IAP develops technologies such as sensors that can be applied to the fiber-reinforced composite parts using printed electronics – “layer by layer, where possible, so that we can find out more about what happens inside the material when it is under stress,” says Prof. Seidlitz. This information will be crucial for designing and constructing the containers efficiently. Later on, when the high-pressure tanks are put into operation, the sensors will also be useful for condition monitoring.

The tanks are being tested at locations such as the ZenaLeb (Zentrum für nachhaltige Leichtbautechnologien, “center for sustainable lightweight technologies”), a project group that involves collaboration between Fraunhofer IAP and the Brandenburg University of Technology Cottbus-Senftenberg, and is funded by the German state of Brandenburg. The aim of ZenaLeb is to develop efficient lightweight structures and bring them into mass production.

Another innovation has already made the leap from the lab to implementation: As part of a wide range of aviation projects funded by the BMWK, researchers at the Fraunhofer Institute for Casting, Composite and Processing Technology IGCV have collaborated with the aviation supplier Premium Aerotec in Augsburg to develop new technologies for efficiently manufacturing structural components from CFRPs. The team ultimately succeeded in replacing the titanium frame of the door in an Airbus 350 with one made from CFRPs. “The CFRP door frame is not only significantly lighter, it’s also more economical because we use automated manufacturing processes that make efficient use of materials, such as automated fiber placement,” says Kevin Scheiterlein, group manager for Fiber Placement and Composite Molding at Fraunhofer IGCV.

“Lightweight construction is an interdisciplinary field,” says Prof. Seidlitz: In order to make the technology available for a wide range of industries, research findings from all along the value chain must be pooled and the relevant synergies must be harnessed. This approach has been wholeheartedly adopted by the Fraunhofer Research Field Lightweight Construction, a consortium of 14 Fraunhofer institutes. The consortium acts as an expert point of contact and support for industry, with services ranging from developing new materials and material combinations to efficient, automated manufacturing and joining technologies right through to sustainable construction methods and suitable testing processes.

The institutes participating in the Fraunhofer Research Field Lightweight Construction include the Fraunhofer Institute for Mechanics of Materials IWM, which focuses on testing technology – this sub-area of the lightweight construction field is key in determining how much material can actually be saved without risking functionality and safety. “The more I learn about a material and the more comprehensively I can characterize it, the closer I can get to the limit,” explains Dr. Jörg Hohe, group manager for Composite Materials in the Component Safety and Lightweight Construction business unit at Fraunhofer IWM. His team is studying polymer, ceramic and metal matrix composite materials to determine their operational properties, with the aim of reducing costs in material and component development.

The problem is, even within the same batch, no two components are 100 percent identical  small fluctuations always arise during production. On top of that, many composite materials and solid foams have a particularly irregular microstructure and so their material properties can vary widely. However, for safety-critical components such as those used in the aerospace sector, the automotive industry, construction and hydrogen transportation, when it comes to selecting materials, researchers can’t only look at the mean values – the whole scatter plot must be taken into account. So is it better to use a bit more material to be on the safe side? “That usually leads to ‘oversizing’ – and means the potential of lightweight construction is not being exploited to the fullest,” says Dr. Hohe.

In order to predict the variance of material properties in a more accurate and cost-effective way, researchers at Fraunhofer IWM have developed processes for creating numerical simulations of composite materials and components. These simulations allow them to predict the expected range of the fluctuations. Dr. Hohe explains: “With the simulation, we obtain data that we can’t directly measure though experiments – in inaccessible areas where we can’t take measurements, for example.”

Natural-fiber-reinforced plastics

However, anyone who works with natural-fiber-reinforced plastics (NFRPs) can only look on with envy at the range of fluctuation for CFRPs and the discussion around how far they can go in replacing petroleum-based plastics. It is difficult to apply existing models and processes to NFRPs for multiple reasons: “For one thing, they are less homogeneous, and for another, they also have special properties in terms of moisture absorption and thermal stability,” says Dr. Christian Beinert, head of the Polymer Processing and Component Design department at Fraunhofer LBF.

In the BMWK-funded COOPERATE project, Fraunhofer LBF researchers are investigating how biopolymer composites can be used in lightweight construction. “The main focus of our research right now is motor vehicle components that are put under high levels of mechanical stress,” explains project manager Georg Stoll. “We want to reduce CO₂ emissions – first of all, by replacing petroleum-based polymers with biogenic plastics, i.e., plastics made with linseed oil or other renewable raw materials.” The researchers’ second tactic is to use as little of this plastic as possible. “By combining these two approaches – substituting materials and using less material – we expect to see a reduction of 75 percent in our carbon footprint.”

The researchers hope that newly developed digital modeling methods will help here: “We not only look at the component in its final form, but also how the component is filled with material during the production process, the direction of the fibers and the local material properties that occur as a result,” explains Mr. Stoll. The design and production processes will be set up so as to make the material as sturdy and as rigid as possible − by ensuring that the fibers are positioned in the right way in the areas that experience the highest levels of stress. Developing the models required for this will be crucial for lightweight construction, adds Dr. Felix Dillenberger, deputy head of the Polymer Processing and Component Design department, who is in charge of the Mechanics and Simulation research field at Fraunhofer LBF. “The more effectively we can digitally map and optimize the production process and material properties, the more material we will ultimately be able to save.”

As part of the collaborative project LowCarboVan, Fraunhofer LBF is working on a very practical use for NFRPs: in the cladding of conventional motor homes. “As demands for comfort are increasing, these vehicles are getting bigger and their designs are becoming more elaborate,”
explains project manager Dr. Dillenberger. “So they’re gradually heading toward the limit of what is allowed under a regular driver’s license. That’s why this industry is very interested in developing a lightweight version.”

The aim of the research project is to develop technical solutions to reduce moisture absorption by the flax fibers used in the exterior cladding that is exposed to the elements.
This development will be important for a multitude of other potential NFRP applications. The researchers’ goal for 2024 is to create a prototype motor home that is lighter than conventional models and has natural fibers in its cladding − and to get it on the way to series production.

Lightening the load with innovative production processes

How does a lightweight material become a lightweight component – i.e., a component that helps the manufacturing industry to easily swap its standard materials and processes for alternatives? One possible option is pultrusion, a process that can be used to efficiently and cheaply produce continuous fiber-reinforced plastic profiles that are light and very robust. This process involves impregnating glass or carbon fibers with plastic, before pulling them through a heated tool and then curing them. “It’s a highly automated process that we can use to produce large batches, so it’s also suitable for series production,” says mechanical engineer Elisa Ruth Bader, a research scientist at the Fraunhofer Institute for Machine Tools and Forming Technology IWU.

In addition to manufacturing straight profiles, researchers at Fraunhofer IWU are now also able to produce curved profiles (radius pultrusion). These can be used to create many different shapes, from solid profiles to complex multichamber hollow structures. Fraunhofer IWU researchers are also looking into producing hybrid profiles – combinations of fiber-reinforced plastics and other materials – and incorporating shape-memory alloy wires that can measure strain into the profiles, thus upgrading them by including sensor technology. Ms. Bader believes that “pultrusion is a process that holds great potential for lightweight construction.”

In the flagship project ALBACOPTER, which is coordinated by the Fraunhofer Institute for Transportation and Infrastructure Systems IVI, six Fraunhofer institutes are bringing their expertise to the table to develop a particularly lightweight, sustainable, aerodynamic drone. As part of this project, the Fraunhofer Institute for Chemical Technology ICT has developed pultrusion profiles made from fiber-reinforced thermoplastic that will be used in the drone’s frame. The researchers have chosen to use a mono-material sandwich structure for the drone’s cargo box. “This enables the box to be recycled to an excellent degree at the end of its service life, as although the box is made up of several layers, including foam layers, we’re only using a single type of plastic,”explains Michael Wilhelm, group manager for Structural Composites at Fraunhofer ICT. The idea is that this type of aircraft could some day perform tasks such as autonomously delivering goods in cities.

Mr. Wilhelm is also working on further advancements to the pultrusion process. His research is mainly focused on the monomer caprolactam, which is used to make polyamide 6. “This monomer is as fluid as water,” says Mr. Wilhelm. “That means reinforcing fibers such as glass or carbon fibers can easily be soaked in it and impregnated to the maximum possible extent.” In addition, the thermoplastic matrix allows parts to be shaped or thermally joined later. “And when the parts reach the end of their service life,” Mr. Wilhelm adds, “the material can simply be ground down and put to use in a new application through injection molding. One advantage of this is that the properties of the material are almost as good as those of virgin injection-molded material. That allows us to create a very simple closed material loop.”

However, this process comes with a challenge: Caprolactam’s low viscosity means special tools and processing machinery are required. In the CaproPULL project, a consortium consisting of Fraunhofer ICT and several industry partners developed in-situ pultrusion as a method, whereby the monomer can be processed to form stable, durable profiles made from continuous fiber-reinforced thermoplastics. Mr. Wilhelm believes these types of profiles could be used to provide localized reinforcement for components in the automotive industry, for example. However, the construction sector could also be another potential field of application: “Unlike steel, the fiber-reinforced plastics don’t corrode,” the expert explains. “This could significantly extend the life span of structures such as bridges.”

And what about 3D printing – could this be a gamechanger for lightweight construction?

After all, additive manufacturing is well suited to producing complex forms with hollow internal structures, which saves material and reduces mass. Along with other industry partners, researchers from the Fraunhofer institutes LBF and IWU are investigating this approach as part of the joint research project ECO2-LInE. The goal of the project is to develop natural fiber-reinforced plastic components that could replace metal structures and that are suited to SEAM, a particularly fast method of additive printing. “In this way, we want to manufacture sustainable components to be used in internal and external vehicle structures – for cars, buses and trains alike,” says Dr. Dillenberger of Fraunhofer LBF. “3D printing allows us a great deal more freedom in designing components than injection molding does, for instance.”

Multi-material components

However, sometimes there’s no way around using metal. Does that have to mean a lot of mass? Since January 2023, Fraunhofer IGCV has been working with an international team of experts on the EU-funded project MADE-3D (Multi-Material Design using 3D Printing); the team is researching methods of manufacturing multi-material components that are strong but light.

“We want to join metals together using an additive process so that we can exploit their special properties to the highest extent possible,” explains Christopher Singer of Fraunhofer IGCV. The concept of creating alloys, i.e., making materials from at least two elements, is crucial to lightweight construction, as the individual ingredients’ “talents” can still be utilized, but with less weight. This means that expensive metals such as titanium need only be incorporated into products in places where their properties are needed. “In terms of processes, we are mainly looking into laser beam melting and laser material deposition – these are both additive technologies that can be used to manufacture unusual structures and that can also help to incorporate the individual metals into the alloy in a targeted way.”

The use of machine learning will help researchers to design the ideal materials for specific areas of application. By using lighter metals and less material and customizing the material’s functions, the weight of individual components could be reduced by up to 50 percent.

Joining metals to fiber-reinforced plastics

The Fraunhofer Institute for Material and Beam Technology IWS is focusing on joining metals to fiber-reinforced plastics. Simply gluing the two materials together is the wrong move, as Jana Gebauer, a researcher at Fraunhofer IWS, explains: “Not only do adhesives take a long time to harden, but they also age, which shortens the components’ life spans.” For this reason, researchers at Fraunhofer IWS have been working on alternative solutions for joining metals and FRPs. One of these methods involves applying a protective, functional layer of metal to a plastic composite using thermal spraying. According to Ms. Gebauer, the trick is to pre-treat the plastic: “A rough surface is not enough for components that are put under this kind of complex stress. We need to create structures with a laser so that we can make a reliable join between the hot metal and the cold FRP.” First of all, pulsed laser radiation is used to very precisely the fibers in the plastic matrix, but without damaging them. The sprayed particles stick to these fibers. Additional indentations are then made that act like a clamp or a hook-and-loop fastener.

“As the thermal spraying process is suitable for a multitude of materials, this technology can be scaled up to be used for other applications and sectors,” says Ms. Gebauer. In the CHIMERA project by the German Federal Ministry of Education and Research (BMBF), this process is currently being used to develop battery housing with electromagnetic shielding. The HPCi® technology for combining metals and plastics using direct thermal joining was also developed by Fraunhofer IWS. This technology has already been put into use as a joining tool in the manufacture of car bodies and aircraft.

Laser roll welding

Fraunhofer IWS is also responsible for another lightweight construction method: laser roll welding. “This enables us to manufacture lightweight panels quickly and cheaply – and they are especially durable and don’t use any adhesives, which makes them easier to recycle,” explains Andrea Berger, a researcher at Fraunhofer IWS. At the core of the sandwich panels, there is a light, metal structure with hollow chambers and thin metal sheets are fixed to each side. The internal structure is then rolled between two rollers, while a scanned laser beam heats the surface of the metal between the rollers from above and below; this is done at lightning speed and with high precision. The rollers press the sandwich elements together so tightly that they are permanently joined. “Because this involves a roll-to-roll process, we can achieve very rapid production,” says Ms. Berger. “In addition to its low energy requirements, this speed makes the process very attractive to the manufacturing industry – for shipbuilding or metal building construction, for example.”

Lightening the load with smart design

The batteries in electric cars are heavy: In small cars, they weigh about 250 kilograms, while large limousine batteries can weigh as much as 700 kilograms. Larger batteries guarantee longer ranges, but they come with another problem: The bigger a traction battery, the larger its environmental footprint will be. Fraunhofer IWU wants to use smart design methods to unlock the potential for optimization here in terms of sustainability. In collaboration with the Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM, the Institute for Surface Engineering and Thin Films IST and the Institute for Wood Research, Wilhelm-Klauditz-Institut, WKI, the IWU is working to design a new battery housing as part of CoolBat − a project funded by the BMWK. They are aiming to “accommodate all the required functions with fewer individual components and fewer joins − and the joins that are used will be easy to dismantle,” explains project coordinator Rico Schmerler, a research scientist at Fraunhofer IWU. To do this, they will take steps such as integrating the temperature control channels into the support structures. Aluminum foam in the base plate, used in combination with phase-change material, assists with cooling the battery. At the same time, it provides protection if any accidents occur, as it can absorb the energy from the impact.

The goal of CoolBat is to increase the range and charging speed of batteries “while saving 15 percent of the CO₂ emitted per battery housing over the entire life cycle,” explains Mr. Schmerler. The holistic approach is key to this project: “We test and assess all our ideas to see what CO₂ savings we can potentially achieve by making better choices in terms of materials, technologies, and manufacturing methods.”

Summary

“The potential of lightweight construction technologies is far from being fully unlocked,” says Prof. Alexander Böker, director of Fraunhofer IAP. “Developing new materials, manufacturing methods and applications can hugely increase efficiency and significantly reduce the environmental footprint.” For some industries, the transformation toward lightweight construction will bring enormous challenges, such as high costs, safety requirements and the need to adjust existing production processes. “Regardless, lightweight construction is an important step toward a sustainable future,” insists Prof. Böker. “And most sectors will need to move in this direction if they’re going to stay competitive.”