Led by Jinwen Zhang, a professor in the School of Mechanical and Materials Engineering, researchers developed a recyclable material that is as strong as commonly used carbon fibre composites and can also be broken down in very hot water within a pressure vessel. The new material could be easily substituted into current manufacturing processes. The research team, including scientists from the Department of Energy’s Pacific Northwest National Laboratory, report on their work in the journal, Macromolecular Rapid Communications.

Carbon fibre reinforced composites are increasingly popular in many industries because they are light and strong. They serve as an energy-saving, lighter alternative to metals, especially in the aviation and automotive industries. They are, however, difficult to break down or recycle, and disposing of them has become of increasing concern. Early versions of modern wind turbines made of composites from the 1990s, for instance, are now reaching the end of their lifetimes, creating a significant challenge for disposal.

While thermoplastics, the type of plastic used in milk bottles, can be melted and easily re-used, the carbon fibre composites are made from thermosets. These types of plastics are cured and can’t easily be undone and returned to their original materials.

Zhang’s team developed a composite material that uses an epoxy vitrimer as an alternative to the traditional epoxy resin. The material is hard and durable like an epoxy thermoset but can also show self-healing and malleable properties at high temperatures like a thermoplastic.

When they used their epoxy vitrimer in the composite material, they were able to degrade their material in pressurised, distilled water beginning at 160 degrees Celsius, dissolving it into valuable carbon fibre and other compounds, which can then be re-used. The recycled carbon fibre was comparable in strength to brand new carbon fibre. When they raised the temperature to 180 degrees, the material completely dissolved. The epoxy vitrimer that they developed could easily be substituted into the manufacturing process.

There is no need to change the chemistry of the process – it is just a slight modification of using the epoxy vitrimer instead of traditional epoxy. The technology is simply and readily applicable.

While the new recyclable material could be easily adopted by manufacturers, Zhang is also continuing work to improve the recycling of composites that are currently in the market. In recent years, he developed an environmentally friendly method to break down the material in a liquid or ethanol medium. Earlier this year, he received a $1.2 million Department of Energy grant for the up-cycling of the composites waste.

The research was supported through grants from the Department of Energy’s Office of Energy Efficiency & Renewable Energy and the Joint Center for Aerospace Technology Innovation.

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Wind blades containing carbon fibre weigh 25% less than ones made from traditional fibreglass materials. That means carbon fibre blades could be longer than fibreglass ones and, therefore, capture more energy in locations with low wind. A switch to carbon fibre could also extend blade lifetime because carbon fibre materials have a high fatigue resistance, said Brandon Ennis, a wind energy researcher at Sandia Labs and the principal investigator for the project.

The project is funded by DOE’s Wind Energy Technologies Office in the Office of Energy Efficiency and Renewable Energy. Partners on the project include Oak Ridge National Laboratory and Montana State University.

Of all the companies producing wind turbines, only one uses carbon fibre materials extensively in their blade designs. Wind turbine blades are the largest single-piece composite structures in the world, and the wind industry could represent the largest market for carbon fibre materials by weight if a material that competed on a cost-value basis to fibreglass reinforced composites was commercially available, said Ennis.

Cost is the main consideration during component design in the wind industry, yet turbine manufacturers also have to build blades that withstand the compressive and fatigue loads that blade experience as they rotate for up to 30 years.

Ennis and his colleagues wondered if a novel low-cost carbon fibre developed at Oak Ridge National Laboratory could meet performance needs while also bringing cost benefits for the wind industry. This material starts with a widely available precursor from the textile industry that contains thick bundles of acrylic fibres. The manufacturing process, which heats the fibres to convert them to carbon, is followed by an intermediate step that pulls the carbon fibre into planks. The plank-making pultrusion process creates carbon fibre with high performance and reliability needed for blade manufacturing and also allows for high production capacity.

When the research team studied this low-cost carbon fibre, they discovered it performed better than current commercial materials in terms of cost-specific properties of most interest to the wind industry.

ORNL provided developmental samples of carbon fibre from its Carbon Fiber Technology Facility and composites made from this material as well as similar composites made from commercially available carbon fibre for comparison.

Colleagues at Montana State University measured the mechanical properties of the novel carbon fibre versus commercially available carbon fibre and standard fibreglass composites. Then Ennis combined these measurements with cost modelling results from ORNL. He used those data in a blade design analysis to assess the system impact of using the novel carbon fibre, instead of standard carbon fibre or fibreglass, as the main structural support in a wind blade. The study was funded by the U.S. Department of Energy Wind Energy Technologies Office.

Ennis and his colleagues found that the new carbon fibre material had 56% more compressive strength per dollar than commercially available carbon fibre, which is the industry baseline. Typically, manufacturers accommodate a lower compressive strength by using more material to make a component, which then increases costs. Considering the higher compressive strength per cost of the novel carbon fibre, Ennis’ calculations predicted about a 40% savings in material costs for a spar cap, which is the main structural component of a wind turbine blade, made from the new carbon fibre compared to commercial carbon fibre.

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In the new study led by Rice graduate students Lauren Taylor and Oliver Dewey, the researchers noted that wet-spun carbon nanotube fibres, which could lead to breakthroughs in a host of medical and materials applications, have doubled in strength and conductivity every three years, a trend that spans almost two decades.

While that may never mimic Moore’s Law, which set a benchmark for computer chip advances for decades, Pasquali and his team are doing their part to advance the method they pioneered to make carbon nanotube fibres.

The lab’s threadlike fibres, with tens of millions of nanotubes in cross-section, are being studied for use as bridges to repair damaged hearts, as electrical interfaces with the brain, for use in cochlear implants, as flexible antennas and for automotive and aerospace applications.

They are also part of the Carbon Hub, a multi-university research initiative launched in 2019 by Rice with support from Shell, Prysmian and Mitsubishi to create a zero-emissions future.

“Carbon nanotube fibres have long been touted for their potential superior properties,” Pasquali said. “Two decades of research at Rice and elsewhere have made this potential a reality. Now we need a worldwide effort to increase production efficiency so these materials could be made with zero carbon dioxide emissions and potentially with concurrent production of clean hydrogen.”

“The goal of this paper is to put forth the record properties of the fibres produced in our lab,” Taylor said. “These improvements mean we’re now surpassing Kevlar in terms of strength, which for us is a really big achievement. With just another doubling, we would surpass the strongest fibres on the market.”

The flexible Rice fibres have a tensile strength of 4.2 gigapascals (GPa), compared to 3.6 GPa for Kevlar fibres. The fibres require long nanotubes with high crystallinity; that is, regular arrays of carbon-atom rings with few defects. The acidic solution used in the Rice process also helps reduce impurities that can interfere with fibre strength and enhance the nanotubes’ metallic properties through residual doping, Dewey said.

“The length, or aspect ratio, of the nanotubes, is the defining characteristic that drives the properties in our fibres,” he said, noting the surface area of the 12-micrometre nanotubes used in Rice fibre facilitates better van der Waals bonds. “It also helps that the collaborators who grow our nanotubes optimise for solution processing by controlling the number of metallic impurities from the catalyst and what we call amorphous carbon impurities.”

The researchers said the fibres’ conductivity has improved to 10.9 megasiemens (million siemens) per meter. “This is the first time a carbon nanotube fibre has passed the 10 megasiemens threshold, so we’ve achieved a new order of magnitude for nanotube fibres,” Dewey said. Normalised for weight, he said the Rice fibres achieve about 80% of the conductivity of copper.

“But we’re surpassing platinum wire, which is a big achievement for us,” Taylor said, “and the fibre thermal conductivity is better than any metal and any synthetic fibres, except for pitch graphite fibres.”

The lab’s goal is to make the production of superior fibres efficient and inexpensive enough to be incorporated by industry on a large scale, Dewey said. Solution processing is common in the production of other kinds of fibres, including Kevlar, so factories could use familiar processes without major retooling.

“The benefit of our method is that it’s essentially plug-and-play,” he said. “It’s inherently scalable and fits in with the way synthetic fibre are already made.”

“There’s a notion that carbon nanotubes are never going to be able to obtain all the properties that people have been hyping now for decades,” Taylor said. “But we’re making good gains year over year. It’s not easy, but we still do believe this technology is going to change the world.”

Co-authors of the paper are Rice alumnus Robert Headrick; graduate students Natsumi Komatsu and Nicolas Marquez Peraca; Geoff Wehmeyer, an assistant professor of mechanical engineering; and Junichiro Kono, the Karl F. Hasselmann Professor in Engineering and a professor of electrical and computer engineering, of physics and astronomy, and of materials science and nanoengineering. Pasquali is the A.J. Hartsook Professor of Chemical and Biomolecular Engineering, of chemistry and of materials science and nanoengineering.

The U.S. Air Force Office of Scientific Research, the Robert A. Welch Foundation, the Department of Energy’s Advanced Manufacturing Office and the Advanced Research Projects Agency-Energy supported the research.

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The strength of a composite component is dependent on the correct alignment of the fibres from which it is made; fibres which are laid incorrectly can result in a range of defects that affect the structural integrity of the final part. While quality checks during the lay-up process are vital they can take 70 per-cent or more of total machine time and is a huge cost.

Detecting such defects, from gaps and overlaps to the presence of foreign objects and debris, at the relatively early lay-up stage of production is much more efficient and cost effective than identifying unacceptable weak points in the material once it is part of a completed component.

The Composite Centre at the AMRC has researched what systems were being successfully employed in related applications which identified the capabilities of the Absolute Arm and its laser scanner options, products developed by Hexagon’s Manufacturing Intelligence division.

Hexagon has developed a composite inspection system especially for measuring fibre orientations, which we believed could be a potential candidate for solving some of our inspection problems. They were able to bring the system to the AMRC to perform a case study – an Absolute Arm with RS5 Laser Scanner and a Vision System 3D.

The Vision System 3D is a camera-based sensor that can accurately detect the orientation of composite fibres using pixel-based algorithms. The system uses a metrological Absolute Arm for position referencing and, combined with scans made using the arm’s laser scanner and camera functionalities, this fibre orientation data can be mapped onto a three-dimensional model of the part being inspected using the dedicated Explorer 3D software platform.

The system lets us validate the design and simulation work that we do at our desks to make sure our design intent is being manufactured, so it becomes a good validation step for our design and manufacture process.

The researchers are primarily using the system for weaving, braiding and preforming processes. Once they perform that initial manufacturing process, they can bring the part to the workstation and use the new inspection system to do a scan of the part to generate the 3D profile of the part.

Using some of the advanced algorithms that are built into the software, they are able to determine fibre orientations that can give an indication of some of the defects that are present in the part.

That information is then taken back into the design and analysis software to update the models with the data from the as-manufactured part in order to perform an analysis which can then be compared against the as-designed part. This provides valuable information for comparing both ‘real’ and ‘virtual’ environments.

This system is one of a number of solutions for composite fibre inspection that has been developed by Hexagon’s Vision and Composites product group in Aachen, Germany. The next stage of the project is to investigate how the technology can be further developed to make them robust enough to pick up some of the more complex defects that we’re hoping to achieve solutions for in the near future.

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In a new study, Texas A&M University researchers have used a natural plant product, called cellulose nanocrystals, to pin and coat carbon nanotubes uniformly onto the carbon-fibre composites. The researchers said their prescribed method is quicker than conventional methods and also allows the designing of carbon-fibre composites from the nanoscale.

Composites are built in layers. For example, polymer composites are made of layers of fibre, like carbon fibres or Kevlar, and a polymer matrix. This layered structure is the source of the composites’ weakness. Any damage to the layers causes fractures, a process technically known as delamination.

To increase strength and give carbon-fibre composites other desirable qualities, such as electrical and thermal conductivity, carbon nanotubes are often added. However, the chemical processes used for incorporating the carbon nanotubes into these composites often cause the nanoparticles to clump up, reducing the overall benefit of adding these particles.

“The problem with nanoparticles is similar to what happens when you add coarse coffee powder to milk—the powder agglomerates or sticks to each other,” said Dr Amir Asadi, assistant professor in the Department of Engineering Technology and Industrial Distribution. “To fully take advantage of the carbon nanotubes, they need to be separated from each other first, and then somehow designed to go to a particular location within the carbon-fibre composite.”

To facilitate the even distribution of carbon nanotubes, Asadi and his team turned to cellulose nanocrystals, a compound easily obtained from recycled wood pulp. These nanocrystals have segments on their molecules that attract water and other segments that get repelled by water. This unique molecular structure offers the ideal solution to construct composites at the nanoscale, said Asadi.

The hydrophobic part of the cellulose nanocrystals binds to the carbon fibres and anchors them onto the polymer matrix. On the other hand, the water-attractive portions of the nanocrystals help in dispersing the carbon fibres evenly, much like how sugar, which is hydrophilic, dissolves in water uniformly rather than clumping and settling to the bottom of a cup.

For their experiments, the researchers used a commercially available carbon-fibre cloth. To this cloth, they added an aqueous solution of cellulose nanocrystals and carbon nanotubes and then applied strong vibration to mix all of the items together. Finally, they left the material to dry and spread resin on it to gradually form the carbon nanotube coated polymer composite.

Upon examining a sample of the composite using electron microscopy, Asadi and his team observed that the cellulose nanocrystals attached to the tips of the carbon nanotubes, orienting the nanotubes in the same direction. They also found that cellulose nanocrystals increased the composite’s resistance to bending by 33% and its inter-laminar strength by 40% based on measuring the mechanical properties of the material under extreme loading.

“In this study, we have taken the approach of designing the composites from the nanoscale using cellulose nanocrystals. This method has allowed us to have more control over the polymer composites’ properties that emerge at the macroscale,” said Asadi. “We think that our technique is a path forward in scaling up the processing of hybrid composites, which will be useful for a variety of industries, including airline and automobile manufacturing.”

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“Frontal polymerisation doesn’t use an autoclave at all, so it doesn’t require that huge upfront investment,” said Bliss Professor Philippe Geubelle in the Department of Aerospace Engineering at the U of I. “It’s a chemical reaction sustained by the release of heat as the front propagates. It can save a lot of energy and it generates much less carbon dioxide, so that’s an environmental benefit.”

Geubelle said they began comparing the two methods by looking at the thermo-chemical equations in order to model the two polymerisation processes. In that way, they could compare the methods for a variety of composite materials, and particularly, the time duration each method takes to manufacture the same part.

“The key contribution from the theoretical point of view is we’ve rewritten the reaction-diffusion equations to extract the two most important non-dimensional parameters,” Geubelle said. “Using just these two parameters allowed us to look at a wide range of chemical parameters, such as the activation energy and the heat of reaction, and at the impact of the initial temperature of the resin.”

Geubelle said this method helped to compare the composite manufacturing processes based on bulk and frontal polymerisation in terms of the time it takes to manufacture a part. The study found that there were instances when one or the other was faster.

“Imagine you want to make something that is one meter long. Frontal polymerisation will be able to do complete the task before bulk polymerisation starts to kick in,” Geubelle said. “On the other hand, if you want to make something that is 10 meters long, then bulk polymerisation may actually take place before the front reaches the other end of the part. It’s the competition between these two processes that we analysed in this study.”

He went on to say there are several ways to speed up the process for frontal polymerization: start the front at both ends so it goes twice as fast, or heat it from the bottom by using a heated panel beneath it. “That process is so fast, we refer to it as flash curing,” Geubelle said, “but it does use more energy than for a single front.”

Manufacturing composite parts using frontal polymerization instead of bulk polymerization has a lot of advantages.

“With frontal polymerisation, you don’t need the large capital investment of the autoclave, making it a very attractive option,” Geubelle said. “The time it takes to cure a composite part is also much shorter and the environmental impact is substantially reduced.”

The study, “Frontal vs. bulk polymerization of fibre-reinforced polymer-matrix composites,” was written by S. Vyas, X. Zhang, E. Goli, and P. H. Geubelle. It is published in Composites Science and Technology.

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Fibre-reinforced polymer composites have many useful properties, but their big drawback is they are typically complex and expensive to manufacture. In recent years, three-dimensional (3D) printing of composites has been successfully demonstrated using thermoplastic polymers and discontinuous fillers, but the resulting 3D-printed composites often have poor mechanical properties and low service temperature due to the limitations of the constituent properties. Consequently, 3D printing of composites using continuous carbon fibres and thermosetting polymers is expected to offer exceptional mechanical properties and thermal stability as well as featured design flexibility, low cost, reliability, and repeatability. However, no additive manufacturing technique has ever been reported to process continuous carbon fibres and thermosetting polymers for direct 3D printing of the finished composite.

Now, a team of engineers from the University of Delaware has developed a 3D printing technology that enables low-cost, flexible production of items made of fibre-reinforced polymer composites using continuous carbon fibres and thermosetting polymers. Their results were recently published in the journal Matter.

This is believed to be the first time anyone has achieved such 3D printing of continuous carbon fibre and thermosetting composite

Continuous carbon fibres and thermosetting resins are very important to make strong and lightweight composites, and they are widely used in many applications, such as aerospace, automotive, and sports products,” said Kun (Kelvin) Fu, “3D printing could reduce labour and tooling cost, and fabricate composite in a more energy-efficient, rapid, and reliable way with minimum defects.

The team developed an approach called localised in-plane thermal assisted (LITA) 3D printing, which allows the user to control the thickness and degree of curing of liquid polymer that solidifies into the desired shape.

In LITA 3D printing, the researchers carefully manipulate the temperature of the carbon fibers, aiding the flow of liquid polymers into channels between the carbon fibers. Then, the polymers are cured, solidifying into a three-dimensional structure. No post-curing is needed in LITA 3D printing, which could save a large amount of energy compared to the conventionally fabricated composites requiring tens of hours of post-curing.

The team developed a robotic system that includes a unique printing head and automated robot arm. This customized 3D printer allows the group to print a variety of shapes and structures.

LITA 3D printing could provide many industries with a rapid, energy-efficient method to make composite components in a variety of shapes using a variety of combinations of polymers and fibers.

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Japan currently produces a wide range of composite materials but require a lot of time and expense to develop. This new R&D, conducted under the Japanese government’s Cross-ministerial Strategic Innovation Promotion Program (SIP), aims to reduce the cost and time needed to develop composite materials for next-generation aircraft by up to 50 per cent.

To achieve this, the team will create an integrated system capable of digitally developing CFRP for aircraft structures using simulation tools developed by Tohoku University, and NEC’s SX-Aurora TSUBASA vector supercomputer.

Specifically, by implementing simulation codes on a supercomputer that can analyse mechanical responses from a molecular level to the aircraft’s wing and fuselage, the processes of material selection and design can be performed at high speed and at multiple scales.

With this system as a common platform, it is expected that tailor-made material development can more effectively meet the demands of airframe manufacturers.

Using the results from scientific research in composite materials, Tohoku University has already developed a variety of simulation tools for CFRP, together with companies participating in Japan’s Cross-ministerial Strategic Innovation Promotion Program. The integrated systems developed in this R&D will be applied to aircraft as well as a wide range of other vehicles in the future.

Simulation programs for materials integration systems will be vectorised and parallelised for the SX-Aurora TSUBASA vector supercomputer in order to significantly reduce the execution times of the simulation programs. This research emphasises cooperation with participating companies in order to accelerate programs.

The SX-Aurora supercomputer consists of vector engines that perform high performance simulation programs and a vector host that performs a wide variety of processes. This R&D utilises NEC’s supercomputer technologies and system construction know-how to systematise simulation programs with the SX-Aurora supercomputer. The materials integration system will combine both data science and optimal material design.

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Key to the strength and versatility of these hybrid, layered materials in high-performance applications is the orientation of fibres in each layer. Recent innovations in additive manufacturing (3D printing) have made it possible to finetune this factor, thanks to the ability to include within the CAD file discrete printer-head orientation instructions for each layer of the component being printed, thereby optimising strength, flexibility, and durability for specific uses of the part. These 3D-printing tool-paths (a series of coordinated locations a tool will follow) in CAD file instructions are therefore a valuable trade secret for the manufacturers. 

However, a team of researchers from NYU Tandon School of Engineering led by Nikhil Gupta, a professor in the Department of Mechanical and Aerospace Engineering showed that these tool-paths are also easy to reproduce — and therefore steal — with machine learning (ML) tools applied to the microstructures of the part obtained by a CT scan. 

Their research, Reverse engineering of additive manufactured composite part by toolpath reconstruction using imaging and machine learning, published in Composites Science and Technology, demonstrates this method of reverse engineering of a 3D-printed glass-fibre reinforced polymer filament that, when 3D-printed, has a dimensional accuracy within one-third of 1% of the original part.

The investigators, including NYU Tandon graduate students Kaushik Yanamandra, Guan Lin Chen, Xianbo Xu, and Gary Mac show that the printing direction used during the 3D-printing process can be captured from the printed part’s fibre orientation via micro-CT scan image. However, since the fibre direction is difficult to discern with the naked eye, the team used ML algorithms trained over thousands of micro CT scan images to predict the fibre orientation on any fibre-reinforced 3D-printed model. The team validated its ML algorithm results on cylinder- and square-shaped models finding less than 0.5° error.

Gupta said the study raises concerns for the security of intellectual property in 3D-printed composite parts, where significant effort is invested in development but modern ML methods can make it easy to replicate them at low cost and in a short time. 

Machine learning methods are being used in the design of complex parts but, as the study shows, they can be a double-edged sword, making reverse engineering also easier. The security concerns should also be a consideration during the design process and unclonable toolpaths should be developed in future research.

Nikhil Gupta, Professor in the Department of Mechanical and Aerospace Engineering

The study is supported by the National Science Foundation grant from the Secure and Trustworthy Computing program. 

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TuFF was patented in June 2020 with 32 claims. According to the U.S. Patent Office, the claim(s) within a patent application clearly define the invention, its scope and what aspects are legally enforceable.

TuFF represents a paradigm shift in composites design and opens the door for composites to replace metals in a variety of applications in the automotive, aerospace, infrastructure, electronics industries and more. Many common products, from kitchen appliances to smartphones and more, are now made with stamped sheet metal, and manufacturers might someday use TuFF instead.

TuFF is a low cost, can be made quickly, and is recyclable. Instead of expecting the metal manufacturers to redesign metal parts like aeroplanes, we decided to create a new material that can be designed and processed like metals using their existing manufacturing equipment – while still providing 40-70% weight savings

Jack Gillespie, director of CCM

While transforming existing industries, TuFF could enable the development of new products, such as flying cars, said John Tierney, senior scientist at CCM. “For urban air mobility, you need aerospace performance at automotive rates, which is exactly what TuFF provides,” he said.

Researchers at CCM started working on TuFF in 2016 when they received a $14.9 million, three-year cooperative agreement from the Defense Advanced Research Projects Agency (DARPA) for the Tailorable Feedstock and Forming (TuFF) Program. The objective of the TuFF program was to develop new composite materials with properties equivalent to previously used materials and develop a single-step manufacturing process that enables the use of the advanced materials for small parts weighing less than 20 pounds at costs competitive with aluminium. The project also included CCM faculty alumni collaborators at Clemson, Drexel and Virginia Tech universities.

About four decades ago, scientists theorised that if they could align these short carbon fibres precisely, they could make composites with desirable properties, but no one achieved this feat in practice until now. It took a few years, but after trying several different alignment mechanisms, the team at CCM figured out how to bring everything in line. The process can now use any type of fibre (or combinations) with nearly all polymers (thermoplastics and thermosets).

The Composites Centre has established a semi-automated pilot plant incorporating new control systems and inline sensors for quality control. TuFF product forms range from 20-inch wide rolls, tailored blanks for forming parts and narrow and steerable tapes for additive manufacturing processes. The team has demonstrated the feasibility and scalability of novel technologies developed through this program and are looking to supply TuFF material to designated industry partners for evaluation, prototype development and scale-up.

Researchers are now conducting additional experiments, including modelling and simulation, to further understand the behaviour of TuFF so that they can tailor it for more applications.

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